Browsing by Person "Cadisch, Georg"

Now showing 1 - 20 of 33

Abundance and diversity of total and nitrifying prokaryotes as influenced by biochemical quality of organic inputs, mineral nitrogen fertilizer and soil texture in tropical agro-ecosystems
(2016) Muema, Esther Kathini; Cadisch, Georg
Tropical agro-ecosystems are limited in nutrient resources as a consequence of i) being composed of highly weathered soils, ii) low native soil organic matter (SOM) content due to conversion of natural forests to arable lands and iii) continuous cropping without replenishing soil nutrients. Recovery of SOM by use of organic residues is faced with other competing uses like animal fodder. Moreover, existing SOM is further reduced by increased turnover rates due to favorable climatic conditions in the tropics. Incorporation of residues is therefore a justified means to restore SOM and to provide crop nutrients through microbial mediated activities like nitrification. Nitrification is a central step of the nitrogen (N) cycle, whereby ammonia is converted into nitrite and then to nitrate by bacteria and archaea through production of the amoA gene encoding the alpha-subunit of the enzyme ammonia monooxygenase. In order to better understand the impact of organic residues of contrasting biochemical quality (i.e., high quality Tithonia diversifolia (TD; C/N ratio: 13, lignin: 8.9 %, polyphenols: 1.7 %), intermediate quality Calliandra calothyrsus (CC; 13, 13, 9.4) and low quality Zea mays (ZM; 59, 5.4, 1.2)) on nutrient provision, effects of residue quality on dynamics of relevant decomposer microbial communities were studied. In addition, mineral N fertilizer was used to compensate for mineral N limitations especially in case of low and intermediate quality residues. Since N is one of the most limiting crop nutrients in the tropics, this study therefore focused on ammonia-oxidizing prokaryotes, using DNA-based quantitative PCR (qPCR) and terminal restriction fragment length polymorphism (TRFLP) techniques. In addition, soil physicochemical properties were measured and linked to the dynamics of microbial communities. The study hypothesized that soil type due to differences in structure and nutrient background, as well as seasonality, which influences soil moisture, would shape the response of the studied communities to biochemical quality of residues. Overall, the results of this PhD research revealed specific responses of dynamics of AOB and AOA to quality of organic residues and their combinations with mineral N fertilizer. They also revealed effects of interrelations between quality of residues and soil texture as well as seasonality particularly precipitation on dynamics of microbial communities. Future investigation of active microbial communities with the use of RNA-based approaches need to be considered to further improve our understanding of quality of SOM on soil nutrient dynamics.
Adaption to rainfall and temperature variability through integration of mungbean in maize cropping
(2021) Khongdee, Nuttapon; Cadisch, Georg
Climate change has threatened global agricultural activities, particularly in tropical and subtropical regions. Rainfed cropping regions have become under more intense risk of crop yield loss and crop failure, especially in upland areas which are also prone to soil erosion. In Thailand, maize is one of the important economic crops and mostly grown in upland areas of northern regions. Maize yield productivity largely depends on the onset of seasonal rainfall. Uncertainty of seasonal rainfall adversely affects maize yield productivity. Therefore, coping strategies are urgently needed to stabilize maize yields under climate variability. In order to identify suitable coping strategies, early maize sowing and maize and mungbean relay cropping were tested on upland fields of northern Thailand. The specific aims of this thesis were (i) monitoring growth and yield performance of maize and mungbean under relay cropping, (ii) testing early maize sowing and maize – mungbean relay cropping as coping strategies under rainfall variations (Chapter 2), (iii) testing effects of relay cropping on growth and yield of mungbean under weather variability (Chapter 4), (iv) determining suitable sowing dates under erratic rainfall patterns by using a modelling approach (Chapter 3), and (v) developing a technique for diagnosis of crop water stress in maize by thermal imaging technique (Chapter 5). Specifically, in Chapter 2 early maize planting or relay cropping strategies were assessed for growth and yield performance of maize under heat and drought conditions. Maize planted in July showed, regardless of sole or relay cropping, low grain formation as a consequence of adverse weather conditions during generative growth. However, July-planted maize relay cropping produced higher above ground biomass than July-planted maize sole cropping and early planting of maize in June. Despite unfavourable weather conditions, maize was, at least partly, able to compensate for such effects when relayed cropped, achieving a higher yield compared to maize sole cropping. June-planted maize sole cropping, however, was fully able to escape such a critical phase and achieved the highest grain yield (8.5 Mg ha-1); however, its associated risk with insufficient rain after early rain spells needs to be considered. Relay cropping showed to be an alternative coping strategy to cope with extreme weather as compared to maize sole cropping. However, relay cropping reduced maize growth due to light competition at young stages of maize before mungbean was harvested (Chapter 2). This negative impact of relay cropping is partly off-set by considering of land equivalent ratio (Chapter 4). Land equivalent ratio indicated a beneficial effect of relay cropping over maize and mungbean solecropping (LER = 2.26). During high precipitation, mungbean sole cropping produced higher yield (1.3 Mg ha-1) than mungbean relay cropping (0.7 Mg ha-1). In contrast to the period of low precipitation, mungbean relay cropping used available water more efficiently and was able to establish its plant, while mungbean sole cropping could not fully withstand severe drought and heat. Mulching effects of maize residues conserved soil water which was then available for mungbean to grow under extreme weather condition. WaNuLCAS modelling approaches can be used to support the decision of maize sowing date in northern Thailand to cope with climate change as indicated by goodness of fit of the model validation (R2 = 0.83, EF = -0.61, RMSE = 0.14, ME = 0.16, CRM = 0.02 and CD = 0.56) (Chapter 3) using forty-eight-year of historical rainfall patterns of Phitsanulok province. Only 27.1% of rainfall probability was classified as a normal rainfall condition. Consequently, maize in this region had faced with high rainfall variability. From long term simulation runs, the current maize sowing date led to strong maize yield variation depending on rainfall condition. Early maize sowing i.e. 15 and 30 days before farmers and staggered planting produced higher yield than current farmers’ practice (mid of July) in most conditions (91.7%). Simulations revealed that water was the most limiting factor affecting maize growth and yield while nutrients (N and P) had only limited impact. Results of the WaNuLCAS model could be used to identify optimal maize planting date in the area prone to soil erosion and climate variation of northern Thailand; however, the model cannot fully account for heat stress. Thermal imaging technique is a useful method for diagnose maize water status. As presented in chapter 5, the developed Crop Water Stress Index (CWSI) using a new approach of wet/dry references revealed a strong relationship between CWSI and stomatal conductance (R2 = 0.82). Our study results established a linear relationship to predict final maize grain yield and CWSI values at 55 DAS as follows “Yield = -16.05×CWSI55DAS + 9.646”. Both early planting of maize and/or relay cropping with legumes are suitable coping strategies for rainfall variability prone regions. The positive response of early planting and legume relay cropping offers the opportunity of having a short-duration crop as sequential crop, providing an additional source of protein for humans and fostering crop diversification on-site. This leads to a win-win situation for farmers, food security and the environment due to an enhanced sustainability of this cropping system.
Benefits and trade-offs of legume-led crop rotations on crop performance and soil erosion at various scales in SW Kenya
(2021) Koomson, Eric; Cadisch, Georg
Soil erosion and land fragmentation threaten agricultural production in large parts of the Western Kenyan Highlands. In Rongo watershed, maize–common bean intercropping systems, which dominate the agricultural landscape, are vulnerable to soil degradation, especially on long slope lengths where ground and canopy cover provision fail to protect the soil from the disruptive impact of raindrops. The inclusion of soil conservation measures like hedgerows, cover crops or mulch can reduce soil erosion, but compete with crops for space and labour. Knowledge of critical slope length can minimise interventions and trade–offs. Hence, we evaluated maize–common bean intercrop (MzBn) regarding runoff, erosion and crop yield in a slope length trial on 20, 60 and 84 m plot lengths, replicated twice on three farms during one rainy season in Rongo, Migori County. Additionally, we investigated systems of MzBn (farmers’ practice), MzBn with 5 Mg ha-1 Calliandra calothyrsus mulch (Mul), Arachis hypogaea (Gnt), Lablab purpureus (Lab) and Mucuna pruriens (Muc), regarding their impact on infiltration, runoff, soil loss, soil C and N loss during three rainy seasons (long and short rains, LR and SR, 2016, and LR 2017). Measured field data on soil, crop, spatial maps and meteorology were used as input datasets to parameterize and calibrate the LUCIA model. The calibrated and validated model was then used to simulate agronomic management scenarios related to planting date (planting with first rain vs baseline) and vegetation cultivar (short duration crop) to mitigate water stress. Based on the measurements, groundcover was most influential over rainfall intensity (EI30) and plant canopy cover in predicting soil loss. Dense groundcover of Mul at the beginning of the rainy seasons was decisive to significantly (p<0.05) lowering overall seasonal average runoff by 88, 87 and 84% over MzBn, Lab and Gnt, respectively, whereas, soil loss under Mul was reduced by 66 and 65% over Gnt and Lab, respectively. The high proportion of large soil aggregates (> 5mm) in the topsoil under Mul at the end of SR 2016 significantly (p<0.05) increased infiltration rates (420 mm hr-1) in LR 2017 compared to Lab (200 mm hr-1) and Gnt (240 mm hr-1). Average C and N concentrations in eroded sediments were significantly reduced under Mul (0.74 kg C ha–1, 0.07 kg N ha–1) during the LR 2016 as compared to MzBn (3.20 kg C ha–1, 0.28 kg N ha–1) and Gnt (2.54 kg C ha–1, 0.23 kg N ha–1). Likewise, in SR 2016 Mul showed significantly lowered C and N losses of 3.26 kg C ha–1 and 0.27 kg N ha–1, respectively, over Lab (9.82 kg C ha–1, 0.89 kg N ha–1). Soil loss over 84 m slope length was overall significantly higher by magnitudes of 250 and 710% than on 60 and 20 m long plots, respectively, which did not differ significantly among each other (p<0.05). For runoff, 84 m plot length differed significantly from 60 and 20 m, but in the opposite trend as for soil loss. Across all three farms, slope gradient and slope length were the variables with highest explanatory power to predict soil loss. At the individual farm level, under homogeneous slope and texture, slope length and profile curvature were most influential. Considering results of slope length experiments, plot lengths less than 50 m appear to be preferential considering soil loss, sediment load, and soil loss to yield ratio under the given rainfall, soil and slope conditions. Our results call for integrating slope length options and cropping systems for effective soil conservation. We recommend planting Mucuna and Calliandra–hedgerows as buffer strips below the critical slope length, and legume cash crops and maize uphill. Such approaches are critical in the backdrop of land fragmentation and labour limitation in the region to sustainably maximise land area. In the modelling exercise, crops planted one and three weeks after the baseline planting date increased Maize and Muc grain yield over the baseline during the three cropping seasons, the three weeks treatment in particular. This could be due to more favourable weather conditions during the shifted vegetation period. Increased grain yield corresponded to high water use efficiency (WUE). The short duration crop planted three weeks after the baseline planting date (PD3WL+SDC10) showed the highest grain yield after PD3WL (three weeks late plaing with BL variety). The use of cultivars with short growth cycle offers the flexibility of planting again where crops failed due to crop water stress or where the rains delay, ensuring completion of the growth cycle before the season ends. Given that short growth duration crops produce less grain yield compared to their counterpart full season crops, due to the length of their cycles, breeding programs must prioritize traits that can enhance the size of the grain-filling sink. At the plot level, management systems that reduce evaporation and retain soil moisture, e.g. mulching, application of farmyard manure etc., must be promoted to reduce evapotranspiration.
Combined effects of drought and soil fertility on the synthesis of vitamins in green leafy vegetables
(2023) Park, Taewan; Fischer, Sahrah; Lambert, Christine; Hilger, Thomas; Jordan, Irmgard; Cadisch, Georg
Green leafy vegetables, such as Vigna unguiculata, Brassica oleraceae, and Solanum scabrum, are important sources of vitamins A, B1, and C. Although vitamin deficiencies considerably affect human health, not much is known about the effects of changing soil and climate conditions on vegetable vitamin concentrations. The effects of high or low soil fertility and three drought intensities (75%, 50%, and 25% pot capacity) on three plant species were analysed (n = 48 pots) in a greenhouse trial. The fresh yield was reduced in all the vegetables as a result of lower soil fertility during a severe drought. The vitamin concentrations increased with increasing drought stress in some species. Regardless, the total vitamin yields showed a net decrease due to the significant biomass loss. Changes in vitamin concentrations as a result of a degrading environment and increasing climate change events are an important factor to be considered for food composition calculations and nutrient balances, particularly due to the consequences on human health, and should therefore be considered in agricultural trials.
Compound-specific 13C fingerprinting for sediment source allocationin intensely cultivated catchments
(2018) Brandt, Christian; Cadisch, Georg
The loss of fertile topsoil due to soil degradation and erosion not only threatens crop productivity, but also induces sedimentation of aquatic systems and leads to social-, economical-, and environmental problems in many regions of the world. The abandonment of shifting cultivation in favor of intensive mono-cultural cropping systems on sloping land accompanied by rainfall detachment and surface runoff induced soil erosion is one of the most pressing environmental and agricultural problems in the highlands of Southeast Asia. Informed soil management strategies require knowledge on the main sediment sources in a catchment. Compound-specific stable isotope (CSSI) fingerprinting, based on δ13C values of fatty acid methyl ester (FAME), allows identifying hot-spots of soil erosion, particularly with regard to assigning sediment sources to actual land uses. In this regard, we assessed the potential of the CSSI – fingerprinting approach to assign sediment sources to specific land use types in various intensely cultivated catchments. In a first step we improved the statistical procedure to identify sediment sources in a heterogeneous agricultural catchment in the mountainous northwestern region of Vietnam. In a next step we tested the CSSI-fingerprinting under different agro-ecological conditions to evaluate its global applicability, using an aligned protocol. Finally, we integrated CSSI-fingerprinting and fallout radio nuclide (FRN, 210Pbex, 137Cs) analysis to estimate past net erosion rates linked to land use types. In conclusion, the integrated Bayesian SIAR-CSSI approach was an appropriate tool to identify and assign sediment sources to actual land uses in small and heterogeneous catchments. This methodology was also suitable to identify hot-spots of soil erosion in contrasting catchments of different sizes and agro-ecological zones. Integrating CSSI-fingerprinting and fallout radio nuclide analysis to determine past sediment budgets provided insight into the impact of specific land use changes on soil retrogression and degradation. Such knowledge is of great value for informed and effective soil conservation through evidence-based land management and decision making.
Coupling pyrolysis with mid-infrared spectroscopy for the characterization of soil organic matter
(2021) Nkwain Funkuin, Yvonne; Cadisch, Georg
Soil organic matter (SOM) is known to play an important role in the global carbon cycle due to its ability to sequester atmospheric carbon dioxide (CO2) and maintenance of soil physical, chemical, and biological properties. Due to the growing need to enhance the understanding of SOM composition and dynamics as influenced by natural and anthropogenic factors, in addition to the limited ability to exist analytical techniques to provide in-depth knowledge into the constituents of SOM, a lot of research is currently focused on the development of new techniques to address the aforementioned concerns. In this study, a novel analytical technique, pyrolysis coupled with mid-infrared spectroscopy (Py-MIRS) was developed and applied to study SOM bulk chemistry in soils by measuring certain mid-infrared organic functional groups. Secondly, the developed Py-MIRS technique was applied to soil samples from different long term experiments to investigate the effects of agricultural management practices and land uses by monitoring the different functional groups. Lastly, the implications of methodological considerations of diffuse reflectance Fourier transform mid-infrared spectroscopy (DRIFTS) on specific mid-infrared functional groups and quality indices were investigated on soils from a number of long-term field experiments. Py-MIRS was developed by testing critical experimental conditions like pyrolysis temperature, heating rate, and time using a range of reference standard compounds varying in chemical and structural composition and bulk soils. As a next step in the methodological development, the suitability of the newly developed Py-MIRS was further evaluated by testing the effect of long-term management and land use on the molecular composition of SOM in bulk soils taken from long-term field experiments in Ultuna, Sweden, and Lusignan, France. The newly developed Py-MIRS technique and the evaluation of the effect of drying temperatures on peak areas obtained with DRIFTS demonstrate progress in the use of pyrolytic and spectroscopic techniques in the domain of SOM characterization. Py-MIRS revealed its potential as a rapid, reproducible, and effective technique to yield information on SOM molecular composition with minimal constraints due to mineral interferences and secondary thermal reactions. Py-MIRS also provided some insights into sustainable practices that improve SOM quality. However, the technique requires further development and testing on different clay mineralogies and land uses.
Crop yield and fate of nitrogen fertilizer in maize-based soil conservation systems in Western Thailand
(2021) Wongleecharoen, Chalermchart; Cadisch, Georg
The increase in food demand and land scarcity in high-potential lowland areas have forced cropping intensification with a transformation of land use from subsistence to permanent agriculture in remote hillside in Southeast Asia. This change and inappropriate land use are the prime cause of soil degradation by erosion, which have negatively affected the agricultural systems productivity and sustainability in Thailand. Therefore, vulnerable land in sloping terrain is classified as unsuitable for continuous production of arable crops unless conservation measures are introduced to stabilize the landscape. Even though conservation practices can stabilize sloping land, farmers have not been widely adopted the measures due to various constraints, such as crop area loss and crop-tree competition. To improve land use management, a two-year study (2010-2011) was conducted at the Queen Sirikit research station (13°28’N, 99°16’E), Ratchaburi Province, Thailand, on a hillside with a slope of around 20%. The treatments consisted of (T1) maize (Zea mays L.) mono-crop under tillage and fertilization, (T2) maize intercropped with chili (Capsicum annuum L.) under tillage and fertilization, (T3) maize intercropped with chili, application of minimum tillage plus Jack bean (Canavalia ensiformis (L.) DC) relay cropping and fertilizer application, (T4) maize intercropped with chili, application of minimum tillage with Jack bean relay cropping and fertilizer application plus perennial hedges of Leucaena leucocephala (Lam.) de Wit, (T5) as T3 but without fertilization, and (T6) as T4 but without fertilization. There was an additional plot of chili sole cropping to calculate the land equivalent ratio (LER). The first part of the study evaluated yield performance and nitrogen use efficiency (NUE) of crops using the 15N isotope technique under diverse fertilized cropping systems during the first year. Maize grain yields were lower in T2 (3.1 Mg ha-1), T3 (2.6 Mg ha-1) and T4 (3.3 Mg ha-1) than in the control (T1) (6.7 Mg ha-1). The total returns from maize and chili yields were 1,914, 5,129, 3,829, 3,900, 3,494, and 2,976 USD ha-1, for T1, T2, T3, T4, T5 and T6, respectively. Higher economic returns in mixed crop systems, by selling both maize and chilies, compensated for the maize area loss by intercropping. Maize 15NUE was highest in T2 (53.5%), being significantly higher than in T1 (47.0%), T3 (45.5%), and T4 (45.7%). Overall system’s NUE in T2 (56.8%) was comparable to T1 (53.8%) and T4 (54.5%) but significantly lower in T3 (48.6%). Minimum tillage and hedgerows (despite their positive filter effect) did not increase NUE but adversely affected maize growth during the establishment phase. The second part of the study examined nitrogen fertilizers fate and quantified partial nitrogen budgets at plot level over two cropping seasons for various maize-based cropping systems with or without fertilizer application. Overall plant uptake of fertilizer 15N applied to maize was 48.6-56.8% over the first season, while residual fertilizer 15N recovery of plants was only 2.3-4.9% over the subsequent season. The quantity of applied labelled N remaining in the soil at the end of season 1 and season 2 was 6.2-28.1% and 7.7-28.6%, respectively. Thus, 60.0-76.0% in season 1 and 12.7-31.3% in season 2 of the applied fertilizer 15N were accounted for within the plant-soil system. Consequently, 24.0-40.0% and 12.9-16.1% of labelled fertilizer N were not accounted for at the end of season 1 and season 2, respectively. The derived N balance over two years revealed severe soil N depletion under T1 (-202 kg N ha-1), T5 (-86 kg N ha-1) and T6 (-48 kg N ha-1), and a slightly negative N budget under T2 (-5 kg N ha-1). In contrast, T3 (87 kg N ha-1) and T4 (62 kg N ha-1) had positive N balances. The increase of N input via additional N fertilizer applied to chili and symbiotic N2 fixation of legumes, and the reduction of N losses by soil erosion and unaccounted fertilizer N (probably lost via leaching, volatilization and denitrification) were the main factors of the positive N balances under maize-chili intercropping systems with conservation measures and fertilization (T3 and T4). Maize yield decline under T1, T2, T5 and T6 in season 2 was related to negative N balances, while maize yield increase under T3 and T4 was related to positive N balances. However, maize-chili intercropping with fertilization had some advantage (LER > 1.0) relative to sole species cropping. Moreover, total returns from crop yields in season 2 of all maize-chili intercroppings (1,378-1,818 USD ha-1) were higher than chili sole cropping (1,321 USD ha-1), which pointed to its crucial role in decreasing production risk by reducing yield loss by pests and diseases observed in chili plants. The third part of the study used combined data of stable isotope discrimination and electrical resistivity tomography (ERT) to improve understanding of competition at the crop-soil-hedge interface. Hedges significantly reduced maize grain yield and aboveground biomass in rows close to hedgerows. ERT revealed water depletion was stronger in T1 than in T4 and T6, confirming time domain reflectometry (TDR) and leaf area data. In T4, water depletion was higher in maize rows close to the hedge than rows distant to hedges and maize grain δ13C was significantly less negative in rows close to the hedge ( 10.33‰) compared to distant ones ( 10.64‰). Lack of N increased grain δ13C in T6 ( 9.32‰, p ≤ 0.001). Both methods were negatively correlated with each other (r= 0.66, p ≤ 0.001). Combining ERT with grain δ13C and %N allowed identifying that maize growth close to hedges was limited by N and not by water supply. In conclusion, the results suggested a significant positive interaction between mineral N fertilizer, intercropping systems and soil conservation measures in maintaining or improving crop yields and N balances in Thailand’s hillside agriculture. Simultaneously, combining ERT imaging and 13C isotopic discrimination approaches improved the understanding of spatial-temporal competition patterns at the hedge-soil-crop interface and pointed out that competition in maize-based hedgerow systems was driven by nitrogen rather than water limitation. Therefore, sustainable agriculture might be achieved if farmers in Thailand combine soil conservation measures with appropriate and targeted N fertilizer use.
Developing indicators and characterizing direct and residual effects of biological nitrification inhibition (BNI) by the tropical forage grass Brachiaria humidicola
(2018) Karwat, Hannes; Cadisch, Georg
Nitrogen (N) losses from agroecosystems harm the environment via increased nitrate (NO3-) amounts in water-bodies and nitrous oxide (N2O) emissions to the atmosphere. Bacteria and archaea oxidize ammonium (NH4+) to NO3- under aerobic conditions. Furthermore, under mainly anaerobic conditions, microbial denitrification reduces NO3- to gaseous N forms. The tropical forage grass Brachiaria humidicola (Rendle) Schweick (Bh) has been shown to reduce soil microbial nitrification via root derived substances. Therefore, biological nitrification inhibition (BNI) by Bh might contribute to reduction of N losses from agroecosystems. The present doctoral thesis aimed at assessing the potential of the actual BNI by Bh, as well as the residual BNI effect with new developed methodologies. The overall research was based on the following major objectives: (1) characterization of the residual BNI effect by Bh on recovery of N by subsequent cropped maize (Zea mays L.) under different N fertilization rates; (2) investigate if low enzymatic nitrate reductase activity (NRA) in leaves of Bh is linked to reduced NO3- nutrition by effective BNI; (3) identify a possible link between plant delta 15N of Bh and the BNI effect of different Bh genotypes on nitrification, plant N uptake and NO3- leaching losses. The overall objective was to use and test new methodologies with a minimum of disturbance of the plant-soil system, to characterize BNI of different Bh genotypes in greenhouse and field studies. The first research study focused on the investigation of a potential residual BNI effect of a converted long-term Bh pasture on subsequent maize cropping, where a long-term maize monocrop field served as control. The residual BNI effect was characterized in terms of enhanced maize grain yield, total N uptake and 15N (labeled) fertilizer recovery. Furthermore, the impact of residual BNI effect on soil N dynamics was investigated. The residual BNI effect was confirmed for the first maize crop season after pasture conversion on the basis of lower nitrification in incubation soil, higher total N uptake and higher maize grain yields. However, the residual BNI effect did not result in higher 15N fertilizer uptake or reduced 15N fertilizer losses, nor in reduced N20 emissions. Applied N was strongly immobilized due to long-term root turnover effects, while a significant residual BNI effect from Bh prevented re-mineralized N from rapid nitrification resulting in improved maize performance. A significant residual Bh BNI effect was evident for less than one year only. In the second research study it was the aim to verify the potential of nitrate reductase activity (NRA) as a proxy for the detection of in vivo performance of BNI by selected Bh accessions and genotypes grown under contrasting fertilization regimes. NRA was detected in Bh leaves rather than in roots, regardless of NO3- availability. Leaf NRA correlated with NO3- contents in soils and stem sap of contrasting Bh genotypes substantiating its use as a proxy of in vivo performance of BNI. The leaf NRA assay facilitated a rapid screening of contrasting Bh genotypes for their differences in in vivo performance of BNI under field and greenhouse conditions; but inconsistency of the BNI potential by selected Bh genotypes was observed. The third research study emphasized to link the natural abundance of delta 15N in Bh plants with reduced NO3- losses and enhanced N uptake due to BNI. Increased leached NO3- was positively correlated to rising delta 15N in Bh grass, whereas the correlation between plant N uptake and plant delta 15N was inverse. Long-term field cultivation of Bh decreased nitrification in incubated soil, whereas delta 15N of Bh declined and plant N% rose over time. Delta 15N of Bh correlated positively with assessed nitrification rates in incubated soil. It was concluded that decreasing delta 15N of Bh over time reflects the long-term effect of BNI linked to lower NO3- formation and reduced NO3- leaching, and that generally higher BNI activity of Bh is indicated by lower delta 15N plant values. Within the framework of this thesis, a residual BNI effect by Bh on maize cropping could be confirmed for one season due to the combined methodological approaches of soil incubation and 15N recovery. The development of the NRA assay for sampled Bh leaves was validated as a rapid and reliable method linked to the actual soil nitrification after NH4+ fertilizer supply. Consequently, the assay could be used for both greenhouse and field studies as BNI proxy. The gathered data from the third study indicated that decreasing delta 15N of Bh over time reflects the long-term effect of BNI linked to lower NO3- formation and reduced NO3- leaching, and that generally higher BNI activity of Bh is indicated by lower delta 15N plant values. Consequently, it was suggested that delta 15N of Bh could serve as an indicator of cumulative NO3- losses. Overall, this doctoral thesis suggests the depressing effect on nitrification by Bh might be a combined effect by BNI and fostered N immobilization. Furthermore, BNI by Bh might be altered by different factors such as soil type, plant age and root morphology of the genotypes. Finally, future studies should consider that Bh genotypes express their respective BNI potential differently under contrasting conditions.
Development of coupled mid-infrared spectroscopic and thermal analytical approaches for the characterization and modeling of soil organic matter dynamics of arable soils
(2013) Demyan, Michael Scott; Cadisch, Georg
Soil organic matter (SOM) is a large part of the global carbon cycle both as a stock, as a source of fluxes (gaseous, dissolved, or sediments) to other stocks, and is also an important component of soil fertility and likewise plant productivity. Due to the growing need for additional data for both global studies related to climate change and soil fertility, additional information is needed not only on the total quantity of SOM, but its distribution within time and space and also its quality. In this study the use of mid-infrared spectroscopy in different applications was explored as an indicator of soil quality or composition, to measure the distribution of quality in different soils and fractions, and how these new methods could be used for SOM model parameterizations compared to other methods for both short and medium term model simulations. Firstly, certain mid-infrared active functional groups as measured with diffuse reflectance spectroscopy (DRIFTS) were studied in a long term fertilization experiment (Bad Lauchstädt) to ascertain the suitability of these different functional groups as indicators of the long term impacts of different fertilizer applications and also in various SOM fractions as separated by size-density approaches. Secondly, a coupled mid-infrared thermally evolved gas analysis was combined with in-situ monitoring of changes in vibrational functional groups to assign different qualities to different temperature ranges during a thermal oxidation experiment to 700 °C. Lastly, these two approaches were compared to traditional SOM fractionation as more rapid alternatives to parameterizing SOM pool sizes in the Century multi-compartment SOM model applied to arable soils at sites in the Kraichgau and Swabian Alb areas in Southwest Germany. In the long-term experiment (Bad Lauchstädt) it was found that certain vibrational functional groups (i.e. aliphatic (2930 1/cm) and aromatic (1620 1/cm)) in bulk soil varied (P < 0.05) according to long-term farmyard manure (FYM) and/or mineral fertilizer application. The application of 30 Mg/ha every second year of FYM increased the proportion of aliphatics as compared to aromatics, while the opposite was true for the control treatment (without any mineral and FYM fertilizer). The ratio of the aromatic to aliphatic relative peak areas were found to be positively related to the ratio of stabilized (SOC in heavy density fractions and clay size fraction) to labile SOC (light density fraction). This indicated that this peak area ratio (aromatic to aliphatic) is an indicator for the relative contribution of stabile to labile SOM as a stability index. In the next phase of the methodological development, evolved gas analysis (EGA) was used during a programmed heating of soil samples to 700°C to link EGA characteristics with SOM. An additional methodological step was the utilization of in-situ diffuse reflectance (in situT DRIFTS) measurements during heating as an indicator of the nature of SOM being decomposed at different temperatures. Thermal stability was found to be affected by experimental conditions and also sample type. The heating rate, amount of C in the sample, and volume of the sample in the heating chamber changed the rate and overall shape of the CO2 evolution curve and needed to be optimized when comparing different SOM fractions. In the long term experiment of Bad Lauchstädt, a decreasing thermal stability as measured by temperature of maximum CO2 evolution was found in the order from control > mineral fertilizer > manure > manure and mineral fertilizer. Furthermore, after a 490 day soil incubation at 20°C the thermal stability of SOC increased, but only slightly. In the in-situT DRIFTS method, the intensity of previously identified vibrational functional groups decreased (degraded) at different temperatures. The functional groups decreased in the order of aliphatic, alcoholic, and carboxylates, and at higher temperatures, also aromatic groups decreased. These findings were used as rules for fitting multiple peaks to the total evolved CO2 curve to derive SOM pools of different reactivity. Pools derived from the measured fractions of mid-infrared functional groups (aliphatic, carboxylate/aromatic, aromatic), evolved gas analysis (CO2) fitted peaks (centered at 320, 380, 540°C), and size-density fractionation (particulate organic matter, heavy density fraction, silt and clay fraction) in addition to a long-term equilibrium model run, were used to parameterize the SOM pools of the Century model as implemented in the Land Use Change Assessment tool (LUCIA) and compared to measured soil surface CO2 fluxes and soil organic carbon (SOC) contents after 2 years. The best fits for the short term study were found to be the SOM fractionation DRIFTS and EGA pool initialization methods, but the differences over two years were very small for the three different parameterization methods and generally CO2 fluxes were underestimated. A 20 year simulation, keeping all rate constants the same, on the other hand, showed large changes in both the SOC (14 Mg/ha, 0 to 30 cm) and the distribution in the pools. As compared to the 2010 baseline SOC, the DRIFTS, EGA-1, and SOM fractionation methods were closest in the Kraichgau site, while the equilibrium method was closest in the Swabian Alb. Overall, DRIFT mid-infrared spectroscopy showed its utility as a rapid assessment of the general distribution of stable to labile SOM in bulk soil. Additionally, when coupled with EGA and in-situ DRIFTS measurements, the integrated method can provide additional information during the thermal degradation of SOM during heating. All methods investigated found changes as a result of soil fertilization management, and between SOM fractions. Lastly, it was shown that such information can be used for direct SOM model inputs, although the methods should be tested on further land uses and soil types. These mid-infrared thermally coupled spectroscopic techniques represent an advance in the use of mid-infrared spectroscopy in the field of detailed SOM characterization for modeling SOM dynamics.
Environmental and farm management effects on food nutrient concentrations and yields of East African staple food crops
(2021) Fischer, Sahrah; Cadisch, Georg
Hidden hunger affects two billion people worldwide, particularly children and pregnant women. Human health and well-being are dependent on the quality and quantity of food consumed, particularly of plant-based foods. Plants source their nutrients from the soil. Essential nutrients for both, plants and humans, therefore, predominantly originate from the soil. Very little is known about the influence of environmental factors (e.g. soil types and abiotic factors, such as weather), or farm management choices (e.g. fertilisation or agrobiodiversity), on nutrient concentrations of edible crop parts. The main aim of this thesis was, therefore, to analyse the effects of soil fertility, farm management, and abiotic factors such as drought, on the quantity (yields) and quality (nutrient concentrations) of essential macro- (Mg, P, S, K, Ca) and micronutrients (Fe, Zn, Mn and Cu), of the edible parts of three East African staple food crops, i.e. maize (Zea mays L.), cassava (Manihot esculenta} Crantz), and matooke (East African Highland Banana (Musa acuminata Colla)), and discuss the resulting implications for food and nutrition security. Two research areas were selected in East Africa, one with a high fertility soil (Kapchorwa, Uganda - Nitisol) and one with a low fertility soil (Teso South, Kenya – Ferralsol). In each region, 72 households were randomly selected, and leaf and edible crop parts, and soil samples collected on three fields per household, organised by distance (closest, mid-distance, and farthest field). Maize and cassava were collected in Teso South, maize and matooke were collected in Kapchorwa. Yields, fertilizer usage and species richness (SR) and diversity (SD) were recorded per field. The total nutrient concentrations were measured in all samples collected (soils and plant parts). A drought occurring in the second rain season of 2016 provided the opportunity to analyse water stress effects on crop quantity and quality (Chapter 2). Edible part samples and yields collected in both seasons were compared. Soil chemical and physical properties, together with farm management variables, were compared to edible part nutrient concentrations and yields using a Canonical Correspondence Analysis (CCA) (Chapter 3). To understand the strength of association between the measurements routinely done by agronomists (leaf measurement) and nutritionists (edible part measurement), samples of each crop were collected, and were compared to each other and to yields, using a bivariate linear mixed model (Chapter 4). During the severe drought, nutrient concentrations in Kapchorwa decreased significantly from normal to drought season in both crops. In contrast, during the moderate drought in Teso South, nutrient concentrations increased significantly in both crops. Lacking nutrient phloem mobility is suggested to play a vital role in mobilisation of micronutrients (Fe, Mn, and Cu), as shown by their decreased concentration under severe drought in the yields of both crops in Kapchorwa (Chapter 2). Soil type had a very strong effect on food nutrient concentrations. Maize grain nutrient concentrations and yields, for example, were significantly higher for all nutrients measured on higher fertility soils. Maize grain had the highest correlations with soil factors. In contrast, corresponding correlations to management factors were much weaker (Chapter 3). Concerning the comparison of nutrient concentrations in different plant parts, low phloem mobile nutrients Ca, Mn, Fe, Zn, and Cu showed the largest differences in correlations between leaves and edible parts. In the same comparison, perennial crops (matooke and cassava) showed lower correlations between leaves and edible parts, than annual crops (maize) (Chapter 4). Environmental factors, such as drought impacted food nutrient concentrations. Severe drought caused a potential “double-burden” for consumers, decreasing both yields and nutrient concentrations, particularly of micronutrients. Considering food nutrient concentrations, apart from yield, as response variables in agronomic trials (e.g. fertilisation or soil improvement strategies) would contribute towards discounting the notion that crops growing on fertile soils always produce healthy and high-quality foods. Leaves may provide information on plant health, however, do not provide enough information to gauge both yields and food quality, particularly regarding micronutrients. The results also showed that measuring the edible part is vital to assessing food quality, particularly due to the observed effects of nutrient mobility, affecting particularly micronutrients and Ca. Ending hunger and improving food and nutrition security for all, particularly when confronted with global change issues such as degrading soils and a changing climate, requires a collaborative effort by all disciplines concerned.
Impact of land use change on soil respiration and methane sink in tropical uplands, Southwestern China
(2020) Lang, Rong; Cadisch, Georg
Land use conversion could modulate soil CO2 emissions and the balance between CH4 oxidation and production via changing soil physical, chemical and biological properties. Large areas of natural forests have been converted to rubber plantations in Southeast Asia, but its impact on soil CO2 and CH4 fluxes has not been sufficiently understood. This study was conducted in Xishuangbanna, Southwestern China, aiming to quantify the impact of this land use change on soil CO2 and CH4 fluxes and to clarify mechanisms responsible for the differences between natural forests and rubber plantations. Dynamics of soil respiration rates in two land uses were compared, and a mixed effect model was used in studying the interference of soil moisture on estimating temperature sensitivity (Q10) of soil respiration (Chapter 2). The land use change impact on the ability of soils to function as CH4 sink was firstly assessed with surface CH4 fluxes measured by static chambers, and then assessed with gas concentration profiles determined from soil probes. Confounded controlling factors and land use effects were disentangled, and the pathway of interactions between CH4 processes and mineral nitrogen was identified (Chapter 3). The concentration gradient method and one-dimensional diffusion-oxidation model were applied to quantify the vertical distribution of CH4 uptake in soil profiles, and to separate the relative control by gas diffusivity and by methanotrophic oxidation on CH4 uptake (Chapter 4). Distinct different temporal patterns of soil respiration were observed on sites during most of the rainy season: forest maintained a high soil respiration rate, while soil respiration in rubber plantations became suppressed (by up to 69%). Forest soils thus emitted the highest amount of CO2 with an annual cumulative flux of 8.48 ± 0.71 Mg C ha-1 yr-1, compared to 6.75 ± 0.79, 5.98 ± 0.42 and 5.09 ± 0.47 Mg C ha-1 yr-1 for 22-year-old rubber, rubber-tea intercropping, and 9-year-old rubber, respectively. Adding a quadratic soil moisture term into the regression model accounted for interference of moisture effect on the effect by soil temperature, therefore, improved temperature sensitivity assessments when high soil moisture suppressed soil respiration under rubber plantations. The static chamber method showed that soils under natural forest were stronger CH4 sinks than soils under rubber plantations, with annual CH4 fluxes of -2.41 ± 0.28 kg C ha-1 yr-1 and -1.01 ± 0.23 kg C ha-1 yr-1, respectively. Water-filled pore space was the main factor explaining the differences between natural forests and rubber plantations. Although soils under rubber plantations were more clayey than soils under natural forest, this was proved not to be the decisive factor driving higher soil moisture and lower CH4 uptake in the former soils. Concentration gradients method showed that CH4 consumption in 0-5 cm soil was significantly higher in natural forests than in rubber plantations, with a mean CH4 flux of -23.8 ± 1.0 and -14.4 ± 1.0 ug C m-2 h-1 for forest and rubber plantations, respectively. The atmospheric CH4 oxidized by top 10 cm soil accounted for 93% and 99% of total consumption for forest and rubber plantations, respectively. CH4 diffusivity at four sampled depths were significantly lower in rubber plantation than in forest. This reduced CH4 diffusivity, caused by altered soil water regime, predominately explained the weakened CH4 sink in converted rubber plantations. Estimated isotopic fractionation factor for carbon due to CH4 oxidation was 1.0292 ± 0.0015 (n=12). Modeling 13CH4 distribution in soil profiles using a diffusion-oxidation model explained the observations in the dry season, but suggested CH4 production in subsoil in the rainy season. In summary, converting natural forests into rubber plantations tended to reduce soil CO2 emissions, but this conversion substantially weakened CH4 uptake by tropical upland soils. The altered soil water regime and conditions of soil aeration under converted rubber plantations appear to have a pronounced impact on processes of gaseous carbon fluxes from soils. The clarified mechanisms in this study could improve the regional budget of greenhouse gases emissions in response to land use change and climate change.
Impacts of the fungal bio-control agent Fusarium oxysporum f.sp. strigae on plant beneficial microbial communities in the maize rhizosphere
(2016) Musyoki, Mary Kamaa; Cadisch, Georg
Striga hermonthica causes severe yield reduction in cereal crop production in Sub-Saharan Africa. Intergrated Striga management has been proposed as one of the best options to control striga. Along this line, the use of biocontrol agent (BCA) Fusarium oxysporum f.sp. strigae (Foxy-2) has been proven as an effective and environmental friendly management strategy. It is well established that a prerequisite for a successful BCA is sufficient risk assessment analysis. Towards this direction, Foxy-2 was assessed for its potential non-target impacts on the abundance, community structure of bacterial and archaeal nitrifying prokaryotes as well as enzymatic activities of proteolytic bacteria. Maize rhizosphere soils treated with or without Foxy-2, Striga and high quality organic residues (i.e., Tithonia diversifolia) as N source were evaluated by quantitative polymerase chain reaction (qPCR) and terminal restriction fragment length polymorphism (TRFLP). It was observed that Foxy-2 had a promoting effect on archaeal abundance under controlled conditions in sandy soils. Furthermore, crop growth stage, seasonality and soil type had a strong effect on abundance and community structure of nitrifying prokaryotes over Foxy-2 inoculation. In addition proteolytic enzymatic activities analysis showed that Foxy-2 did not affect their activities. Correlation analysis also showed that abundance and community structure on nitrifying communities positively correlated with extractable organic carbon, extractable organic nitrogen and soil pH, while proteolytic enzymatic activities correlated with extractable organic nitrogen and soil ammonium. It was concluded that Foxy-2 is compatible with nitrifying prokaryotes and proteolytic enzymatic activities.
Inter-microbial competition for N and plant NO3− uptake rather than BNI determines soil net nitrification under intensively managed Brachiaria humidicola
(2021) Egenolf, Konrad; Schad, Philipp; Arevalo, Ashly; Villegas, Daniel; Arango, Jacobo; Karwat, Hannes; Cadisch, Georg; Rasche, Frank
Brachiaria humidicola (syn. Urochloa humidicola) has been acknowledged to control soil nitrification through release of nitrification inhibitors (NI), a phenomenon conceptualized as biological nitrification inhibition (BNI). Liming and N fertilization as features of agricultural intensification may suppress BNI performance, due to a decrease in NI exudation, increased NH3 availability and promotion of ammonia oxidizing bacteria (AOB) over archaea (AOA). A 2-year three-factorial pot trial was conducted to investigate the influence of soil pH and soil microbial background (ratio of archaea to bacteria) on BNI performance of B. humidicola. The study verified the capacity of B. humidicola to reduce net nitrification rates by 50 to 85% compared to the non-planted control, irrespective of soil pH and microbial background. The reduction of net nitrification, however, was largely dependent on microbial N immobilization and efficient plant N uptake. A reduction of gross nitrification could not be confirmed for the AOA dominated soil, but possibly contributed to reduced net nitrification rates in the AOB-dominated soil. However, this putative reduction of gross nitrification was attributed to plant-facilitated inter-microbial competition between bacterial heterotrophs and nitrifiers rather than BNI. It was concluded that BNI may play a dominant role in extensive B. humidicola pasture systems, while N immobilization and efficient plant N uptake may display the dominant factors controlling net nitrification rates under intensively managed B. humidicola.
Investigating the mode of action of the mycoherbicide component Fusarium oxysporum f.sp. strigae on Striga parasitizing sorghum and its implication for Striga control in Africa
(2011) Ndambi Beninweck, Endah; Cadisch, Georg
Amongst the factors that are a threat to food security in Africa, is the parasitic weed Striga hermonthica which affects mostly cereals that constitute the staple food for subsistence farmers, thus affecting the livelihood of millions of people. Popularly known as witchweed, attack due to S. hermonthica can completely destroy the yield of cereal crops. Efforts to combat Striga have had very limited success since farmers rarely adopt control methods due to the mismatch between technologies and farmers? socio-economic conditions. Being such a severe problem, an appropriated method for Striga management adapted for African farmers is very much needed. The use of soil-borne fungi for biocontrol is now being developed as an alternative to the use of chemicals considering the specificity of such fungi and the fact that most of the damage by Striga is done before its emergence. The fungus Fusarium oxysporum f.sp. strigae has been identified and shown to be effective and specific to S. hermonthica and S. asiatica but its mode of action is not yet well known. It is required that the mechanisms underlying the mycoparasitic process of this natural antagonistic agent be well understood before its use. Thus, studies on the effectiveness, specificity and timely colonization of Foxy 2 on S. hermonthica are necessary as well as studies on the effect of Foxy 2 in Striga-host plants which should demonstrate its non-pathogenicity to food crops. The objective of this study was therefore to investigate the mode of action of Foxy 2 in its target S. hermonthica and non-target Sorghum bicolor and also to examine the safety of the use of this mycoherbicide by evaluating its ability to produce toxins. In the first part of the thesis, the ability of Foxy 2 to colonize sorghum roots and possibly shoots was investigated using light and transmission electron microscopy. The efficacy of Foxy 2 to cause death of S. hermonthica seedlings attached to Foxy 2 colonized sorghum roots was also evaluated. Microscopic investigations revealed that the intensity of root colonization by Foxy 2 increased with time and Foxy 2 could survive and colonize the sorghum rhizodermis, root hairs and cortical parenchyma up to four weeks after sowing. This behaviour is well adapted for Striga control as it corresponds to the peak of Striga seedling attachment. Hyphae were completely absent from the sorghum root central cylinder even after four weeks and also absent from the sorghum shoots up to 11 weeks after sowing indicating the non-pathogenity of Foxy 2 to sorghum. Furthermore, Foxy 2 was effective in controlling S. hermonthica by causing disease in 95% and 86% of S. hermonthica seedlings when coated on seeds of tolerant and susceptible sorghum cultivars respectively. Therefore, Foxy 2 could be combined with the tolerant sorghum variety in an integrated approach against S. hermonthica and S. asiatica. The effect of Foxy 2 on various growth stages of S. hermonthica was investigated subsequently so as to understand the mechanisms of action of Foxy 2 within S. hermonthica in the real living complex between the mycoherbicide Foxy 2, the parasite S. hermonthica and its host sorghum. Light, scanning and transmission electron microscopy were used to evaluate the pattern of colonization and control of S. hermonthica seedlings and shoots by Foxy 2. Results showed that 26 days after sowing Foxy 2 coated sorghum seeds, all tissues of the young S. hermonthica seedlings attached to sorghum roots were completely degraded and destroyed by Foxy 2 including the haustorial intrusive cells, hyaline tissue, vessels, central xylem elements and Striga cortical parenchyma. Some S. hermonthica plants which attached to areas of the sorghum root which were not yet colonized by Foxy 2 (towards the root tips), were able to outgrow the fungus and emerged. In the emerged S. hermonthica shoots, hyphae had subsequently penetrated and colonized vessels clogging them over long distances and were identified up to the top of the plants. In some vessels there was an intensive blockage of the vessels by hyphae such that spaces or gaps were rare. Ultrathin sections showed that the diseased S. hermonthica shoots reacted to Foxy 2 invasion by forming an electron dense wall coating along the secondary vessel walls probably to prevent fungal digestion of the walls. The study thus identified two mechanisms by which Foxy 2 contributed to wilting and death of S. hermonthica which included complete digestion of underground S. hermonthica seedlings and hyphal clogging of vessels in emerged S. hermonthica plants which interfered with water conduction. In order to understand the reactions of sorghum towards the presence of Foxy 2 as part of the risk assessment to ensure the safe use of this biocontrol agent, the action of Foxy 2 and a known pathogenic Fusarium species, F. proliferation, were compared in the fourth chapter. Sorghum roots were also wounded to expose the vascular system so as to investigate whether removal of the endodermal barrier could give access to Foxy 2 into the vessels which could lead to digestion resulting in wilting of the sorghum plants. The colonization processes of the two Fusaria species were quite different at all stages of growth. While F. proliferatum degraded the endodermis, invaded the central cylinder and digested the xylem parenchyma two weeks after sowing, Foxy 2 was restricted to the cortex even up to four weeks after sowing. Hyphae of Foxy 2 filled the intercellular spaces at the outer endodermal wall but could not penetrate the endodermis. Sorghum roots were observed to react to Foxy 2 invasion by reinforcing the central cylinder as seen by an increase in blue auto fluorescence especially of the endodermis. Five days after wounding and inoculating sorghum roots, Foxy 2 hyphae invaded the central cylinder very close to the cut but were completely absent from the central cylinder at a distance of 3000 µm from the cut, meanwhile F. proliferatum hyphae had digested the cells of the central cylinder at this distance. This indicated that not only the endodermis was a barrier but there could also be a physiological barrier within the central cylinder of the sorghum root which did not allow further spread of Foxy 2. Hence, exposure of the vascular system did not serve as a route for the invasion of Foxy 2 which therefore implied that it could not cause wilting of the plant. In the last part of the thesis, S. hermonthica shoots were analyzed by HPLC-MS/MS to investigate the possible production of toxins by Foxy 2 to kill the plant. Amongst the toxins tested (beauvericin, fumonisins B1, B2, B3, C and P series, enniatins A, A1, B and B1, and moniliformin), only beauvericin (BEA) was detected to be produced by Foxy 2 in S. hermonthica shoots. The concentration of this toxin increased with increased infection e.g. 60 µg BEA/kg Striga shoot tissue (dry weight) were detected three weeks after emergence rising to 720 µg BEA/kg Striga shoot tissue after six weeks in the severely diseased S. hermonthica shoots. When beauvericin was applied on S. hermonthica shoots at concentrations of 50 µM, transmission electron microscopy showed that all cell types became necrotic. However, beauvericin as well as all the other toxins were not detected in sorghum grains harvested from sorghum plants which were hosts to the S. hermonthica plants and growing from Foxy 2 coated sorghum seeds. Given that some F. oxysporum strains were previously shown to be able to produce fumonisins which are among the toxins which have been reported to be of potential risks to human and animal health, a pure culture of Foxy 2 was evaluated for its fumonisin production ability. Results from real-time PCR using two specific primer pairs for the FUM1 gene (which is the key gene for fumonisin synthesis), were negative confirming that Foxy 2 was not able to produce fumonisins and might not be of major concern for human and animal health when used as a biocontrol agent in the field, therefore safe for use as a biocontrol agent. To conclude, Foxy 2 showed potential to control S. hermonthica by completely destroying young underground stages and clogging vessels in aboveground stages, as well as producing the toxin beauvericin, both actions contributing to wilting of the plants. Its non-pathogenicity to sorghum and its inability to produce fumonisins could be seen as factors which make it well suited as a biocontrol agent. Further research needs to be done to evaluate its efficacy under field conditions and the impact of naturally occurring soil microorganisms and abiotic conditions on performance of Foxy 2 so as to understand its interactions with the environment and to optimize its efficacy.
Land use change and its impact on soil properties using remote sensing, farmer decision rules and modelling in rural regions of Northern Vietnam
(2017) Nguyen, Thanh Thi; Cadisch, Georg
After the Indo China war in 1954, a dramatic rise in population in Northwest Vietnam led to an increased demand of agricultural land for food security requirements. Slash and burn systems which existed for many hundreds of years were replaced by intense cash crop systems, particularly maize production. Maize cropping was further expanded to steeper sloping areas, resulting in a risk of soil degradation. Therefore, investigating Land Use Change (LUC) and its impact on soil properties were considered in this study. The study aimed to identify LUC in 1954, 1973, the 1990s and 2007 in Chieng Khoi commune, Yen Chau district, Son La province, Vietnam using available remote sensing data. Furthermore, a detailed land use map classification method was developed using farmers’ decision rules. Based on farmers’ crop decision rules and, food requirement and population information, a simple LUC model was developed to simulate LUC annually from 1954 to 2007. Moreover, total soil nitrogen and carbon were determined under a chronosequence of intense cultivation. Thus, developing a modelling tool had the aim to assess the impacts of LUC on soil fertility at watershed level. The first case study (Chapter 3) presented the LUC assessment, using available remote sensing data combined with farmer information. Forest areas decreased from 1954 to 2007, except in the 1990s because of policies that aimed to encourage and support afforestation programmes to increase forest land. However, planted forest has since decreased again since 1999 whereas agricultural land has increased dramatically. Agricultural land expanded to both natural forest and planted forest areas until 2007 legally (with encouragement of agroforestry) and illegally thereafter (at the border between cultivated land and forest). The establishment of an artificial lake in Chieng Khoi commune opened the accessibility to forest land surrounding the lake, with a forest area of 929 ha remaining in 2007 compare to more than 2,500 ha in 1954. Paddy rice areas did not change because of their specific location (lower and flat lands), but production increased and was intensified by two cropping seasons per year due to irrigation improvements and a continuous water supply from the artificial lake. The second case study (Chapter 4) presented the development of a LUC model, using the outputs from the first case study comprising farmers’ decision rules and food requirements for an increased population. For later periods, the influence of market orientation factor was considered. The model successfully simulated the expansion of cultivation areas and replacement of forest land by agricultural land. Simulations were at accepted level of accuracy comparing actual and simulated LUC (Goodness-of-fit – GOF values greater than 0.7 and Figure of merit - FOM values greater than 50%). The third case study (Chapter 5) demonstrated an investigation of the soil fertility dynamic under intense cultivation and the development of a simple dynamic and spatially-explicit modelling tool to assess the changes in soil fertility. The Dynamic of total Carbon and Nitrogen distribution (DyCNDis) model was constructed using field data combined with literature information. The field data showed that, under a decade of maize mono cultivation in slope areas, both nitrogen and carbon were largely depleted. Furthermore, the DyCNDis model showed an acceptable level of validation (modelling efficiency – EF of 0.71 and root mean square error - RMSE of 0.42) to simulate nitrogen and carbon under intense maize cultivation at watershed level. Additionally, the model identified hotspot areas of 134 ha (18.9% of total upland cultivation areas) that are threatened by soil degradation through intense cultivation over a long-term period. In conclusion, the combination of qualitative and quantitative approaches allowed assessing impacts of LUC on environmental services such as soil fertility through the developed DyCNDis modeling tool. The combination of improved LUC analysis with a simple spatial dynamic soil fertility modeling tool may assist policy makers in developing alternative implementation strategies for local stakeholders in regions which face data limitations. The modelling tools developed in this study were able to successfully simulate LUC and to identify locations where soil conservation methods at watershed level need most urgently to be applied to avoid soil degradation. The model tools were able to simulate the trends rather than values of agricultural area expansion and reduction of soil nitrogen and carbon. The developed approaches could be linked and coupled to other modelling tools to economically consider benefits or ecological concerns toward sustainable crop production in remote and rural regions.
Litter quality and microbes explain aggregation differences in a tropical sandy soil
(2022) Laub, Moritz; Schlichenmeier, Samuel; Vityakon, Patma; Cadisch, Georg
Soil aggregates store most soil organic carbon (SOC), but how does litter quality influence their formation? We hypothesized varying litter quality to facilitate differences in aggregate formation by altering the seasonal development of microbial biomass (MB) C and N, with MB driving aggregate development in a tropical sandy soil in Thailand. Aggregate development was studied in a long-term fallow experiment, receiving 10 Mg ha−1 annual applications of rice (Oryza sativa) straw (low N and polyphenols (PP)), groundnut (Arachis hypogaea) stover (high N, low PP), tamarind (Tamarindus indica) litter (medium N and PP), or dipterocarp (Dipterocarpus tuberculatus) leaf litter (low N, high PP) compared to a control. N-rich litter from groundnut and tamarind led to significantly higher MB, bulk soil C and aggregate C than dipterocarp, rice straw, and the control. Bulk soil C and small macroaggregates C of N-rich litter treatments increased about 7% in 30 weeks. Increasing MB N explained increasing small macroaggregate C and both, MB C or N were important covariates explaining temporal variations of C stored in themicroaggregates, in silt and clay. MB also explained temporal variations of aggregate fraction weights. With time, SMA C only increased in the N-rich groundnut and tamarind treatments, but decreased in other treatments. Connections of MB to aggregate C and weight substantiated the importance of microbial activity for aggregate formation and carbon sequestration. By promoting MB for longest time spans, medium-quality tamarind could best facilitateaggregate formation, and increase silt and clay C.
Measuring and modelling carbon stocks in rubber (Hevea brasiliensis) dominated landscapes in Subtropical China
(2019) Yang, Xueqing; Cadisch, Georg
Rubber plantation has been rapidly expanded in Montane Mainland South East Asia in past decades. Limited by long-term monitoring data availability, the impacts of environmental change on rubber trees carbon stock development still not fully understood. Against global warming background, in order to better facilitate regional forest management, we applied synergetic approach combining field survey and modelling tools to improve predictions of dynamic carbon stock changes. The trade-off analysis regarding to rubber carbon stock and latex production optimization was further discussed in view of sustainable rubber cultivation. The first study explored the impact of regional land-use changes on landscape carbon balances. The Naban River Watershed National Nature Reserve (NRWNNR), Xishuangbanna, China, was selected as a case study location. Carbon stocks were evaluated using the Rapid Carbon Stock Appraisal (RaCSA) method based on tree, plot, land use and landscape level assessments of carbon stocks, integrating field sampling with remote sensing and GIS technology. The results showed that rubber plantations had larger time-averaged carbon stocks than non-forest land use types (agricultural crops, bush and grassland) but much lower than natural forest. During 23 years (1989-2012), the whole landscape of the nature reserve (26574 ha) gained 0.644 Tg C. Despite rubber expansion, the reforestation activities conducted in NRWNNR were able to enhance the carbon stocks. Regional evaluation of the carbon sequestration potential of rubber trees depends largely on the selection of suitable allometric equations and the biomass-to-carbon conversion factor. The second study developed generic allometric equations for rubber trees, covering rotation lengths of 4-35 years, within elevation gradient of 621-1,127 m, and locally used rubber tree clones (GT1, PRIM600, Yunyan77-4) in mountainous South Western China. Allometric equations for aboveground biomass (AGB) estimations considering diameter at breast height (DBH), tree height, and wood density were superior to other equations. We also tested goodness of fit for the recently proposed pan-tropical forest model. The results displayed that prediction of AGB by the model calibrated with the harvested rubber tree biomass and wood density was more accurate than the results produced by the pan-tropical forest model adjusted to local conditions. The relationships between DBH and height and between DBH and biomass were influenced by tapping, therefore biomass and C stock calculations for rubber have to be done using species-specific allometric equations. Based on the analysis of environmental factors acting at the landscape level, we noticed that above- and belowground carbon stocks were mostly affected by stand age, soil clay content, aspect, and planting density. The results of this study provide reference for reliable carbon accounting in other rubber-cultivated regions. In the last study, we explored how rubber trees growth and production response to climate change and regional management strategies (cultivation elevation, planting density). We applied the process-based Land Use Change Impact Assessment tool (LUCIA) calibrated with detailed ground survey data to model tree biomass development and latex yield in rubber plantations at the tree, plot and landscape level. Model simulation showed that during a 40-year rotation, lowland rubber plantations (< 900m) grew quicker and had larger latex yield than highland rubber (≧900m). High planting density rubber plantations showed 5% higher above ground biomass than those at low- and medium-planting density. The mean total biomass and cumulative latex yield per tree over 40 years increased by 28% and 48%, respectively, when climate change scenarios were modelled from baseline to highest CO2 emission scenario (RCP 8.5). The same trend of biomass and latex yield increase with climate change was observed at plot level. Denser plantations had larger biomass, but the cumulative latex production decreased dramatically. The spatially explicit output maps produced during modelling could help maximize carbon stock and latex production of regional rubber plantations. Overall, rubber-based system required for appropriate monitoring scale in both temporal aspect (daily-, monthly-, and yearly-level) and in spatial aspect (pixel-, land use-, watershed-, and landscape- level). The findings from present study highlighted the important application of ecological modelling tools in nature resources management. The lessons learned here could be applicable for other rubber-cultivated regions, by updating with site-specific environmental variables. The significant role of rubber tree not limited in its nature latex production, it also lies in its great carbon sequestration potential. Our results here provided entry point for future developing comprehensive climate change adaption and mitigation strategies in South East Asia. By making use of interdisplinary cooperation, the sustainable rubber cultivation in Great Mekong Regions could be well realized.
Measuring and modelling resource use competition at the crop-soil-hedge interface on a hillside in Western Thailand
(2015) Hussain, Khalid; Cadisch, Georg
Thailand’s western uplands are facing severe soil loss and runoff problems due to intensive cultivation of cash crops for high food, feed, fiber, and fuel demand by an increasing population. Thus the Land Development Department and the International Board for Soil Research and Management in Thailand are promoting the use of soil conservation measures such as contour hedgerows, grass barriers and agroforestry systems based on fruit trees and annual crops. Although such measures have been shown to be effective in controlling soil erosion, farmers often are reluctant to adopt such practices as inclusion of trees reduces the cropped area and yields competition for available resources with crops. Hence, a better understanding of the underlying processes at the crop-soil-hedge interface is needed to design soil conservation systems that are more attractive for farmers. It was hypothesized that soil conservation systems with hedgerows and intercropping will induce spatial patterns of resource use which can be linked to competition while planting patterns affect canopy characteristics and light distributions. This study focused on the following objectives; (i) to improve understanding of competition at the crop soil hedge interface by combining stable isotope discrimination, electrical resistivity tomography and time domain reflectometry, (ii) to identify the effects of intercropping and hedgerows on maize biomass accumulation, nitrogen concentration and light use efficiency, (iii) to evaluate the competition between maize hedges at crop-soil-hedge interface, (iv) to explore possible mitigating options to cope with competition between hedge and maize by using a modelling approach. A field trial was laid out in randomized complete block design with three replicates at Queen Sirikit research farm, Ban Bo Wi village (13°28´ N and 99°15´ E), Suan Phueng District, Ratchaburi province in western Thailand with 20-25% slope magnitude. The experiment was established in 2009 while the research presented here was carried out during the 2010 and 2011 maize growing seasons. Six cropping treatments with following management practices were investigated: (T1, control) maize [Zea mays L.) monocrop, tillage, with fertilizer application (farmer’s practice; (T2) maize intercropped with chili (Capsicum annuum L. cv. Super Hot), tillage and fertilizer application; (T3) maize intercropped with chili, minimum tillage, fertilizer application, and Jack bean (Canavalia ensiformis (DC) L.) relay cropping; (T4) maize intercropped with chili, minimum tillage, fertilizer application, Jack bean relay cropped, and leucaena (Leucaena leucocephala (Lam.) de Wit) hedgerows; (T5) and (T6) as (T3) and (T4), respectively, but both without fertilizer application. Tillage was carried out manually by hoe to around 0-20 cm depth. Plots were 13 x 4 m. Fertilizer was applied to maize at a rate of 62, 22, 36 kg ha-1 of N, P, and K, respectively. Urea (N) application to maize was done in two splits as 31 kg ha-1 of N one month after sowing maize and another split of 31 kg ha-1 of N two months after sowing maize Chili received 92 kg N ha-1 at the time of transplanting and 92 kg ha-1 N as top dressing one month after transplanting. The impact of competition at the crop-soil-hedge interface was studied in 2011, two years after establishment of soil conservation measures, to exclude the establishment period of leucaena with a potentially weak impact on maize. At this time, highest above ground biomass (AGB) production of maize of 1364 g m-2 was witnessed in T2 being statistically different from all other treatments, except T4 and T3; while lowest above ground biomass production of 1034 g m-2 was observed in T5. In hedgerow treatments, maize rows planted distant to hedges produced 46% and 73% higher AGB than maize growing in rows close to the hedgerow (p≤0.0001) in T4 and T6, respectively. Similar effects were observed for plant height, grain nitrogen concentration and grain yield. Mean grain δ13C was significantly higher in T6 (-9.32‰) than in T4 (-10.49‰) and T1 (-10.55‰). Generally, higher δ13C signals mean higher water availability; however the higher δ13C signals in unfertilized T6 treatment imply that lack of nutrients may have led to increased δ13C values. Similarly in T4, δ13C signals were significantly higher in maize grains originating form rows planted close to hedges (10.33‰, p≤0.0001) than samples obtained from maize rows distant to hedges (-10.64‰). These results point out that competition at the crop-soil-hedge interface was driven by nutrient rather than water limitation. The electrical resistivity tomography (ERT) imaging further supported this finding showing that maize monocrop showed higher soil moisture depletion patterns than hedge intercrop with fertilizer (T4) treatment, while hedge intercrop without fertilization (T6) depleted soil moisture least. Light use efficiency (LUE) for maize above ground biomass production was statistically higher LUEAGB (1.56 g DM MJ-1) in maize and chilli intercrop (T2) than in maize sole cropping LUEAGB (1.23 g DM MJ-1). In T4 and T6 maize rows planted close to hedgerows had lower LUEAGB than rows distant to hedgerows. The land equivalent ratio showed that inclusion of hedgerows with fertilizer application in the intercropping treatment enhanced land utilization by 21%. The Water, Nutrient and Light Capture in Agroforestry (WaNuLCAS) model simulated AGB with R2= 0.83, RMSE=6.3, EF=0.82 and CD=1.4 during calibration while model validation also showed promising results with R2= 0. 76, P<0.001, RMSE=4.6 and EF=0.69. Simulations also pointed to major nutrient limitation between maize rows planted close to hedgerows. Simulations showed that negative impacts of hedgerows on crops can be reduced by managing fertilizer application in hedge adjacent maize rows leading to a successful application of agroforestry systems on a long-term basis not only for soil conservation but also for sustainable crop production in tropical uplands. The study figured out the scope of stable isotopic discrimination, ERT, light use efficiency and modelling approaches for evaluating resource use competition at crop-soil-hedge interface on hillside agriculture. The combination of isotopic discrimination and ERT measurements provided useful information for identification of cause-impact relationships. Spatial LUE patterns provided insights for canopy light harvest under various soil conservation options. Furthermore, light use data was also helpful in validation of WaNuLCAS model which did not only provide multiple options to figure out resource use competition at crop-soil-hedge interface but also allowed to test mitigation options for sustainable crop production in tropical uplands. Model scenarios showed that negative impacts of hedgerows on crops growing close to hedges can be reduced by applying minute additional doses of fertilizer only to the crop rows planted close to hedgerows, leading to a sustainable crop production along with soil conservation. Productivity evaluation of investigated cropping systems showed that inclusion of hedgerows and intercropping in tropical hillside agriculture is promising in enhancing crop production and thus can be adopted by farmers with yield advantage.
Mid-infrared spectroscopy and enzyme activity temperature sensitivities as experimental proxies to reduce parameter uncertainty of soil carbon models
(2021) Laub, Moritz; Cadisch, Georg
Models that simulate the dynamics of soil organic carbon (SOC) are crucial to understand the global carbon cycle, but current generation models are subject to major uncertainties due to two model shortcomings. Firstly, their different carbon pools are not connected to measurable SOC fractions. Secondly, there is uncertainty about the response of the different carbon pools to an increasing temperature. The aim of this thesis was thus to link the SOC model pools of the Daisy model to measurable proxies for SOC quality and pool specific temperature sensitivity. In the first study, the drying temperature for soil samples assessed by diffuse reflectance mid infrared Fourier transform spectroscopy (DRIFTS) was optimized to assure optimal representativeness of aliphatic and aromatic-carboxylate absorption bands as proxies for fast- and slow-cycling SOC pools. Their ratio was termed the DRIFTS stability index (DSI). In the second study, the DSI was used to distinguish fast- and slow-cycling SOC model pools at model initialization. In the third study, model initialization using DSI was performed to infer pool specific temperature sensitivities for the different Daisy carbon pools. Furthermore, it was tested whether the measured temperature sensitivities of different extracellular soil enzymes could be used as proxies for pool specific temperature sensitivity. Using a global collection of soil samples revealed that the absorption of all studied DRIFTS absorption bands increased significantly (p < 0.0001) with increasing drying temperature from 32°C to 105°C. This effect was disproportionally strong for the aliphatic absorption band. Due to the strong interference of the residual soil sample moisture content with the aliphatic absorption band, drying at 105°C and storage in a desiccator prior to measurement would be necessary for representative spectra for model pool initialization. In the following, a combination of medium to long-term bare fallow experiments was used, to test the utility of the DSI for SOC pool initialization. Pool partitioning by the DSI was superior to using a fixed pool partitioning under the assumption that SOC was at steady state. The DSI contained robust information on SOC quality across sites. Therefore, in the majority of cases, the application of the DSI led to significantly lower model errors than the steady state assumption. Furthermore, the application of the DSI in Bayesian calibration led to a reduced parameter uncertainty for the turnover of the slow-cycling SOC pool and the humification efficiency. The 95% credibility interval of the slow-cycling SOM pools’ half-life between 278 and 1095 years suggested faster SOC turnover than earlier studies. The DSI used for SOC model pool initialization was then combined with the lignin-to-nitrogen ratio for litter pool initialization to infer pool specific temperature sensitivities. The simulations of five field studies and laboratory incubations with fallow soil and crop-litter inputs were combined. Based on a clear pool definition, pool specific temperature sensitivities could be inferred by Bayesian calibration. However, differences in temperature sensitivities of the same pools between experiments suggested that carbon stability was not the main driver of temperature sensitivities. Instead, the main difference was found between the laboratory incubations (higher Q10 values up to 3) and the field (lower Q10 values centered around 2). In a second approach, the measured Q10 value of phenoloxidase (1.35) was used as Q10 value of the temperature function of both SOM pools and the slow crop-litter pool while ß glucosidase (1.82) was used for the fast crop litter pool. This improved field simulations by 3 to 10% compared to assuming a standard Q10 of 2 for all pools. Thus, site specific Q10 of different soil enzymes showed potential as proxy for site and pool specific temperature sensitivities. Important state variables that explain the observed Q10 value differences between experiments were identified as physical protection of SOC, substrate availability and environmental stress for microorganisms due to fluctuating state variables in the field. In conclusion, the usefulness of the DSI as an indicator of SOC stability and proxy for pool initialization was demonstrated for several soils in central Europe. In addition, it was shown that pool partitioning proxies can help to infer pool specific temperature sensitivity by Bayesian calibration. However, temperature sensitivity was not mainly a function of carbon stability.
Modeling the influence of coastal vegetation on the 2004 tsunami wave impact
(2014) Laso Bayas, Juan Carlos; Cadisch, Georg
A tsunami causes several effects once it reaches inland. Infrastructure damage and casualties are two of its most severe consequences being mostly determined by seaquake intensity and offshore properties. Nevertheless, once on land, the energy of the wave is attenuated by gravity (elevation) and friction (land cover). Despite being promoted as ‘bio-shields’ against wave impact, proposed tree-belt effects lacked quantitative evidence of their performance in such extreme events, and have been criticized for creating a false sense of security. The current study analyzed some of the land uses in sites affected by the 2004 tsunami event, especially in coastal areas close to the coast of Indonesia, more specifically on the west coast of Aceh, Sumatra as well as on the Seychelles. Using transects perpendicular to the coast, the influence of coastal vegetation on the impact of the 2004 tsunami, particularly cultivated trees, was modeled. A spatial statistical model using a land cover roughness coefficient to account for the resistance offered by different land uses to the wave advance was developed. The coefficient was built using land cover maps, land use characteristics (stem diameter, height, and planting density), as well as a literature review. The spatial generalized linear mixed models used showed that while distance to coast was the dominant determinant of impact (casualties and infrastructure damage), the existing coastal vegetation in front of settlements also significantly reduced casualties, in the case of Aceh, by an average of 5%. Despite this positive effect of coastal vegetation in front of a settlement, it was also found that dense vegetation behind villages endangered human lives and increased structural damage in the same case, most likely due to debris carried by the backwash. The models initially developed in Aceh were adapted and tested for the effects that the same tsunami event caused in the Seychelles, where the intensity of the event was a tenth of that in Aceh. These new models suggested no direct effect of coastal vegetation, but they indicated that vegetation maintained dunes decreased the probability of structural damage. Additionally, using satellite imagery with higher resolution than that of the first study and/or from different years before the tsunami, corresponding land roughness coefficients were developed and tested with the existing models. The new models showed no signs of further increase of goodness of fit (AIC). Nevertheless, weather conditions at the acquisition dates as well as coverage and lack of image availability diminished the predictive power of these models. Overall, more than advocating for or against tree belts, a sustainable and effective coastal risk management should be promoted. This planning should acknowledge the location (relative to the sea) of settlements as the most important factor for future coastal arrangements. Nevertheless, it should also consider the possible direct and indirect roles of coastal vegetation, determined by its spatial arrangement as shown in the study models. Sustainability of these measures would only occur when coastal vegetation is regarded as a livelihood provider rather than just as a bio-shield. Practical examples could include, e.g. rubber plantations or home gardens in front of settlements, while leaving escape routes or grasslands and coconut plantations behind these. Therefore, the enforcement of educational programs, the setup and maintenance of effective warning systems and the adequate spatial allocation of coastal vegetation bringing tangible short and mid term benefits for local communities, as well as its adaption to local customs should be considered.