Institut für Bodenkunde und Standortslehre

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The role of crop management practices and adaptation options to minimize the impact of climate change on maize (Zea mays L.) production for Ethiopia
(2023) Feleke, Hirut Getachew; Savage, Michael J.; Fantaye, Kindie Tesfaye; Rettie, Fasil Mequanint
Climate change impact assessment along with adaptation measures are key for reducing the impact of climate change on crop production. The impact of current and future climate change on maize production was investigated, and the adaptation role of shifting planting dates, different levels of nitrogen fertilizer rates, and choice of maize cultivar as possible climate change adaptation strategies were assessed. The study was conducted in three environmentally contrasting sites in Ethiopia, namely: Ambo, Bako, and Melkassa. Future climate data were obtained from seven general circulation models (GCMs), namely: CanESM2, CNRM-CM5, CSIRO-MK3-6-0, EC-EARTH, HadGEM2-ES, IPSL-CM5A-MR, and MIROC5 for the highest representative concentration pathway (RCP 8.5). GCMs were bias-corrected at site level using a quantile-quantile mapping method. APSIM, AquaCrop, and DSSAT crop models were used to simulate the baseline (1995–2017) and 2030s (2021–2050) maize yields. The result indicated that the average monthly maximum air temperature in the 2030s could increase by 0.3–1.7 °C, 0.7–2.2 °C, and 0.8–1.8 °C in Ambo, Bako, and Melkassa, respectively. For the same sites, the projected increase in average monthly minimum air temperature was 0.6–1.7 °C, 0.8–2.3 °C, and 0.6–2.7 °C in that order. While monthly total precipitation for the Kiremt season (June to September) is projected to increase by up to 55% (365 mm) for Ambo and 75% (241 mm) for Bako respectively, whereas a significant decrease in monthly total precipitation is projected for Melkassa by 2030. Climate change would reduce maize yield by an average of 4% and 16% for Ambo and Melkassa respectively, while it would increase by 2% for Bako in 2030 if current maize cultivars were grown with the same crop management practice as the baseline under the future climate. At higher altitudes, early planting of maize cultivars between 15 May and 1 June would result in improved relative yields in the future climate. Fertilizer levels increment between 23 and 150 kg ha−1 would result in progressive improvement of yields for all maize cultivars when combined with early planting for Ambo. For a mid-altitude, planting after 15 May has either no or negative effect on maize yield. Early planting combined with a nitrogen fertilizer level of 23–100 kg ha−1 provided higher relative yields under the future climate. Delayed planting has a negative influence on maize production for Bako under the future climate. For lower altitudes, late planting would have lower relative yields compared to early planting. Higher fertilizer levels (100–150 kg ha−1) would reduce yield reductions under the future climate, but this varied among maize cultivars studied. Generally, the future climate is expected to have a negative impact on maize yield and changes in crop management practices can alleviate the impacts on yield.
Soil water status shapes nutrient cycling in agroecosystems from micrometer to landscape scales
(2022) Bauke, Sara L.; Amelung, Wulf; Bol, Roland; Brandt, Luise; Brüggemann, Nicolas; Kandeler, Ellen; Meyer, Nele; Or, Dani; Schnepf, Andrea; Schloter, Michael; Schulz, Stefanie; Siebers, Nina; von Sperber, Christian; Vereecken, Harry
Soil water status, which refers to the wetness or dryness of soils, is crucial for the productivity of agroecosystems, as it determines nutrient cycling and uptake physically via transport, biologically via the moisture‐dependent activity of soil flora, fauna, and plants, and chemically via specific hydrolyses and redox reactions. Here, we focus on the dynamics of nitrogen (N), phosphorus (P), and sulfur (S) and review how soil water is coupled to the cycling of these elements and related stoichiometric controls across different scales within agroecosystems. These scales span processes at the molecular level, where nutrients and water are consumed, to processes in the soil pore system, within a soil profile and across the landscape. We highlight that with increasing mobility of the nutrients in water, water‐based nutrient flux may alleviate or even exacerbate imbalances in nutrient supply within soils, for example, by transport of mobile nutrients towards previously depleted microsites (alleviating imbalances), or by selective loss of mobile nutrients from microsites (increasing imbalances). These imbalances can be modulated by biological activity, especially by fungal hyphae and roots, which contribute to nutrient redistribution within soils, and which are themselves dependent on specific, optimal water availability. At larger scales, such small‐scale effects converge with nutrient inputs from atmospheric (wet deposition) or nonlocal sources and with nutrient losses from the soil system towards aquifers. Hence, water acts as a major control in nutrient cycling across scales in agroecosystems and may either exacerbate or remove spatial disparities in the availability of the individual nutrients (N, P, S) required for biological activity.
The need to decipher plant drought stress along the soil-plant-atmosphere continuum
(2023) Schweiger, Andreas H.; Zimmermann, Telse; Poll, Christian; Marhan, Sven; Leyrer, Vinzent; Berauer, Bernd J.
Lacking comparability among rainfall manipulation studies is still a major limiting factor for generalizations in ecological climate change impact research. A common framework for studying ecological drought effects is urgently needed to foster advances in ecological understanding the effects of drought. In this study, we argue, that the soil–plant–atmosphere‐continuum (SPAC), describing the flow of water from the soil through the plant to the atmosphere, can serve as a holistic concept of drought in rainfall manipulation experiments which allows for the reconciliation experimental drought ecology. Using experimental data, we show that investigations of leaf water potential in combination with edaphic and atmospheric drought – as the three main components of the SPAC – are key to understand the effect of drought on plants. Based on a systematic literature survey, we show that especially plant and atmospheric based drought quantifications are strongly underrepresented and integrative assessments of all three components are almost absent in current experimental literature. Based on our observations we argue, that studying dynamics of plant water status in the framework of the SPAC can foster comparability of different studies conducted in different ecosystems and with different plant species and can facilitate extrapolation to other systems, species or future climates.
Linking horizontal crosshole GPR variability with root image information for maize crops
(2023) Lärm, Lena; Bauer, Felix Maximilian; van der Kruk, Jan; Vanderborght, Jan; Morandage, Shehan; Vereecken, Harry; Schnepf, Andrea; Klotzsche, Anja
Non‐invasive imaging of processes within the soil–plant continuum, particularly root and soil water distributions, can help optimize agricultural practices such as irrigation and fertilization. In this study, in‐situ time‐lapse horizontal crosshole ground penetrating radar (GPR) measurements and root images were collected over three maize crop growing seasons at two minirhizotron facilities (Selhausen, Germany). Root development and GPR permittivity were monitored at six depths (0.1–1.2 m) for different treatments within two soil types. We processed these data in a new way that gave us the information of the “trend‐corrected spatial permittivity deviation of vegetated field,” allowing us to investigate whether the presence of roots increases the variability of GPR permittivity in the soil. This removed the main non‐root‐related influencing factors: static influences, such as soil heterogeneities and rhizotube deviations, and dynamic effects, such as seasonal moisture changes. This trend‐corrected spatial permittivity deviation showed a clear increase during the growing season, which could be linked with a similar increase in root volume fraction. Additionally, the corresponding probability density functions of the permittivity variability were derived and cross‐correlated with the root volume fraction, resulting in a coefficient of determination (R2) above 0.5 for 23 out of 46 correlation pairs. Although both facilities had different soil types and compaction levels, they had similar numbers of good correlations. A possible explanation for the observed correlation is that the presence of roots causes a redistribution of soil water, and therefore an increase in soil water variability.
Effectiveness of bio-effectors on maize, wheat and tomato performance and phosphorus acquisition from greenhouse to field scales in Europe and Israel: a meta-analysis
(2024) Nkebiwe, Peteh Mehdi; Stevens Lekfeldt, Jonas D.; Symanczik, Sarah; Thonar, Cécile; Mäder, Paul; Bar-Tal, Asher; Halpern, Moshe; Biró, Borbala; Bradáčová, Klára; Caniullan, Pedro C.; Choudhary, Krishna K.; Cozzolino, Vincenza; Di Stasio, Emilio; Dobczinski, Stefan; Geistlinger, Joerg; Lüthi, Angelika; Gómez-Muñoz, Beatriz; Kandeler, Ellen; Kolberg, Flora; Kotroczó, Zsolt; Kulhanek, Martin; Mercl, Filip; Tamir, Guy; Moradtalab, Narges; Piccolo, Alessandro; Maggio, Albino; Nassal, Dinah; Szalai, Magdolna Zita; Juhos, Katalin; Fora, Ciprian G.; Florea, Andreea; Poşta, Gheorghe; Lauer, Karl Fritz; Toth, Brigitta; Tlustoš, Pavel; Mpanga, Isaac K.; Weber, Nino; Weinmann, Markus; Yermiyahu, Uri; Magid, Jakob; Müller, Torsten; Neumann, Günter; Ludewig, Uwe; de Neergaard, Andreas
Biostimulants (Bio-effectors, BEs) comprise plant growth-promoting microorganisms and active natural substances that promote plant nutrient-acquisition, stress resilience, growth, crop quality and yield. Unfortunately, the effectiveness of BEs, particularly under field conditions, appears highly variable and poorly quantified. Using random model meta-analyses tools, we summarize the effects of 107 BE treatments on the performance of major crops, mainly conducted within the EU-funded project BIOFECTOR with a focus on phosphorus (P) nutrition, over five years. Our analyses comprised 94 controlled pot and 47 field experiments under different geoclimatic conditions, with variable stress levels across European countries and Israel. The results show an average growth/yield increase by 9.3% (n=945), with substantial differences between crops (tomato > maize > wheat) and growth conditions (controlled nursery + field (Seed germination and nursery under controlled conditions and young plants transplanted to the field) > controlled > field). Average crop growth responses were independent of BE type, P fertilizer type, soil pH and plant-available soil P (water-P, Olsen-P or Calcium acetate lactate-P). BE effectiveness profited from manure and other organic fertilizers, increasing soil pH and presence of abiotic stresses (cold, drought/heat or salinity). Systematic meta-studies based on published literature commonly face the inherent problem of publication bias where the most suspected form is the selective publication of statistically significant results. In this meta-analysis, however, the results obtained from all experiments within the project are included. Therefore, it is free of publication bias. In contrast to reviews of published literature, our unique study design is based on a common standardized protocol which applies to all experiments conducted within the project to reduce sources of variability. Based on data of crop growth, yield and P acquisition, we conclude that application of BEs can save fertilizer resources in the future, but the efficiency of BE application depends on cropping systems and environments.
Formation of mineral‐associated organic matter in temperate soils is primarily controlled by mineral type and modified by land use and management intensity
(2023) Bramble, De Shorn E.; Ulrich, Susanne; Schöning, Ingo; Mikutta, Robert; Brandt, Luise; Poll, Christian; Kandeler, Ellen; Mikutta, Christian; Konrad, Alexander; Siemens, Jan; Yang, Yang; Polle, Andrea; Schall, Peter; Ammer, Christian; Kaiser, Klaus; Schrumpf, Marion
Formation of mineral-associated organic matter (MAOM) supports the accumulation and stabilization of carbon (C) in soil, and thus, is a key factor in the global C cycle. Little is known about the interplay of mineral type, land use and management intensity in MAOM formation, especially on subdecadal time scales. We exposed mineral containers with goethite or illite, the most abundant iron oxide and phyllosilicate clay in temperate soils, for 5 years in topsoils of 150 forest and 150 grassland sites in three regions across Germany. Results show that irrespective of land use and management intensity, more C accumulated on goethite than illite (on average 0.23 ± 0.10 and 0.06 ± 0.03 mg m−2 mineral surface respectively). Carbon accumulation across regions was consistently higher in coniferous forests than in deciduous forests and grasslands. Structural equation models further showed that thinning and harvesting reduced MAOM formation in forests. Formation of MAOM in grasslands was not affected by grazing. Fertilization had opposite effects on MAOM formation, with the positive effect being mediated by enhanced plant productivity and the negative effect by reduced plant species richness. This highlights the caveat of applying fertilizers as a strategy to increase soil C stocks in temperate grasslands. Overall, we demonstrate that the rate and amount of MAOM formation in soil is primarily driven by mineral type, and can be modulated by land use and management intensity even on subdecadal time scales. Our results suggest that temperate soils dominated by oxides have a higher capacity to accumulate and store C than those dominated by phyllosilicate clays, even under circumneutral pH conditions. Therefore, adopting land use and management practices that increase C inputs into oxide-rich soils that are under their capacity to store C may offer great potential to enhance near-term soil C sequestration.
Release of glucose from dissolved and mineral‐bound organic matter by enzymatic hydrolysis
(2023) Lenhardt, Katharina R.; Brandt, Luise; Poll, Christian; Rennert, Thilo; Kandeler, Ellen
Sorption of dissolved organic matter (DOM) by poorly crystalline minerals during their formation may protect large amounts of carbon in soils from mineralization. We investigated the bioavailability of carbohydrates in DOM and after co-precipitation with short-range ordered aluminosilicates. Carbohydrates originated from soil solutions collected in situ at two depths of a Dystric Cambisol, and from litter extracts. Quantification of substrate-specific degradability was achieved by the addition of β-glucosidase at an optimal concentration and subsequent determination of glucose release. Depending on DOM composition, 0.6–41.4 mg g−1 C−1 of glucose was enzymatically released from dissolved carbohydrates. Co-precipitated carbohydrates were partially accessible, resulting in a glucose release of 0.7–5.2 mg g−1 C−1. Restricted enzymatic depolymerization due to co-precipitation may contribute to accumulation of easily degradable substrates in soils.
Associations of short-range ordered aluminosilicates and organic matter: formation, properties and stabilization of organic matter
(2023) Lenhardt, Katharina Raphaela; Rennert, Thilo
Short-range ordered aluminosilicates (SROAS) typically form during the weathering of volcanic ejecta by polymerization of released aluminium (Al) and silicon (Si). These minerals exhibit variable chemical composition and crystallinity. Tubular imogolite is a SROAS with long-range order; its locally defined Si configuration occurs also in poorly ordered SROAS. Interactions of SROAS and organic matter (OM) promote carbon (C) accrual by protecting OM from microbial degradation in the long term; however, the fundamental processes are poorly understood. Stable mineral-organic associations may form by adsorption of dissolved OM (DOM) on SROAS surfaces and by co-precipitation of DOM with SROAS during mineral formation. The objective of this study was to elucidate the chemical interactions between DOM and SROAS by both processes, and to assess the stability of OM sorbed by SROAS. Therefore, the impact of SROAS composition on DOM adsorption, partitioning of OM moieties by adsorption and co-precipitation, the structure of co-precipitates, and the degradability of co-precipitated OM was investigated. A method to synthesize SROAS at ambient conditions was developed to mimic the surface properties of natural weathering products. Characterization of SROAS structure by solid-state 27Al and 29Si nuclear magnetic resonance (NMR) spectroscopy and Fourier transform infrared (FTIR) spectroscopy evinced a close similarity of synthetic SROAS to their natural analogues. Aluminium-rich SROAS (molar Al:Si>2) resembled proto-imogolite, with Al mainly in octahedral coordination and ≥38% of Si nuclei exhibiting an imogolite-like configuration. Silicon-rich SROAS (molar Al:Si = 1.4) contained tetrahedral Al and Si existed largely in ill-defined environments. Analyses of the specific surface area by nitrogen adsorption revealed marked aggregation of Al-rich SROAS, which was less pronounced in Si-rich SROAS. These results show that the poor crystallinity of Al-rich SROAS permits a very dense spatial arrangement of mass at the submicron scale, while Si incorporation restricts aggregation. Interactions of SROAS and DOM were studied using natural DOM with heterogeneous composition collected in situ from a Dystric Cambisol at two depths, and by water extraction of litter. Solid-state 13C-NMR and FTIR spectroscopy revealed a major contribution of oxidized aromatic moieties to soil DOM, likely originating from lignin degradation, while litter DOM was predominantly composed of carbohydrates. Soil DOM adsorption was driven by surface accessibility and was thus larger for Si-rich SROAS than for Al-rich SROAS, showing the importance of SROAS physical properties for OM retention. Aromatic products of lignin degradation preferentially adsorbed on SROAS, inducing relative enrichment of aliphatic substances, particularly carbohydrates, in the residual DOM. Topsoil DOM adsorption depended more strongly on contact time (1–168 h) than subsoil DOM adsorption, possibly due to qualitative differences of the aromatic fraction. Associations formed by co-precipitation contained more C than adsorption complexes. As Al interacted preferentially with oxidized aromatic compounds, co-precipitation of DOM increased as a function of aromatic C. Nevertheless, marked sorption of carbohydrates from litter DOM evinced possible retention of substances with low affinity for Al by co-precipitation, in contrast to adsorption. Time-dependent (1–72 h) structural evolution of SROAS in the absence of DOM was examined to resolve the mechanisms of SROAS formation. Irrespective of the initial molar Al:Si ratio, amorphous precursors formed by olation during the first hour. After 72 h, up to 50% of Si nuclei exhibited imogolite-like configuration, showing rapid development of short-range order. Dissolved OM interfered in condensation of Al and Si, causing partial exclusion of Si, and slowed crystallisation of the octahedral Al sheet, promoting ill-defined Si species in the co-precipitates. Hence, DOM likely impedes assembly of precursors into structurally ordered particles, in particular, oxidized aromatic DOM in topsoils. The binding strength of DOM to SROAS surfaces may be affected by Si incorporation due to structure modifications. Thus, a mechanistic adsorption study was conducted with oxalic, salicylic and octanoic acid as models of functional moieties in DOM. Adsorption of oxalic and salicylic acid was up to 80–90% lower for Si-rich SROAS than for Al-rich SROAS. Rapid (<1 min) release of hydroxyls, indicating ligand exchange, was observed only for oxalic and salicylic acid, suggesting octanoic acid interacted electrostatically with SROAS surfaces. Chelate complexes of oxalic acid and partial inner-sphere binding of salicylic acid on both SROAS were identified by FTIR spectroscopy. Fast adsorption kinetics were retraced by changes in electrical conductivity using a stopped-flow technique. Ligand exchange by oxalate proceeded at a similar rate as complexation of monomeric Al3+, showing its binding to octahedral Al. Hence, the much lower susceptibility of Si-rich SROAS to ligand exchange with carboxyl groups is caused by tetrahedral Al. Consequently, little OM may be stabilized by chemical bonds with Si-rich SROAS. The degradability of co-precipitated carbohydrates was tested by addition of b-glucosidase, a microbial extracellular enzyme, at optimal concentration and quantification of the reaction product glucose. Glucose release was analysed for initial DOM, co-precipitated OM and for residual DOM to account for compositional changes by co-precipitation. As a result of carbohydrate enrichment in residual DOM, its degradability by b-glucosidase increased. Minor amounts of glucose were released from co-precipitated carbohydrates, showing their restricted accessibility for enzymes due to occlusion. Formation of SROAS in soils likely induces preferential association of lignin degradation products with the mineral matrix and alters the composition of OM introduced to the subsoil. Otherwise easily degradable OM with low affinity for SROAS surfaces can be effectively protected from mineralization by co-precipitation. As the structure of SROAS reflects formation processes and affects their reactivity, future research characterizing natural SROAS will give mechanistic insights into C sequestration and potentially other vital soil functions.
Formation and properties of inorganic Si-contaminant compounds
(2023) Stein, Mathias; Rennert, Thilo
Environmental contamination is the most pressing issue of our global society. Among others, contamination with potentially toxic elements (PTEs) such as cadmium (Cd), copper (Cu), and lead (Pb) threatens organisms, humans, and entire ecosystems. Silicon (Si) is known to benefit the resilience to such abiotic stresses and its application showed to alleviate PTE toxicity. These beneficial effects are predominantly attributed to in planta processes, but PTE immobilization in soil induced by Si addition has also been reported. However, interactions between silicic acid and Cd, Cu, and Pb at undersaturation of their silicates and other mineral phases remains elusive. Silicic acid, which is dissolved Si, may interact with cationic PTEs in soil, altering their environmental fate. At oversaturation, PTEs and silicic acid may precipitate forming metal silicates, whereas at undersaturation PTEs may be incorporated into the network of polymerized silicic acid or inner-spherically complexed on the negatively charged surface of polymeric silicic acid, forming particulate Si-contaminant compounds. Aiming to elucidate the extent and the mechanism of the potential PTE immobilization, long-term formation experiments in aqueous solution, a soil column experiment, and batch adsorption experiments including isothermal titration calorimetry (ITC) experiments were conducted. Long-term formation experiments in aqueous solution were conducted at undersaturation of PTE silicates and other mineral phases. Time-dependent particle size and charge changes were measured in between 211 days using dynamic light scattering and phase analysis light scattering. Solid phases were characterized by Fourier transform infrared (FTIR) spectroscopy and 29Si nuclear magnetic resonance (NMR) spectroscopy. Particle size measurements revealed a positive effect of cationic PTEs on silicic acid polymerization (Cu>Cd>Pb). However, only traces (2.1‰ Cd, 2‰ Cu and 1.4‰ Pb of the initially added PTEs) were bound during the polymerization of silicic acid. Copper was incorporated in the polymeric network of silicic acid during its polymerization as indicated by FTIR spectra and 29Si NMR relaxation experiments. Cadmium was only outer-spherically adsorbed. The long-term formation experiments revealed that particulate compounds form due to silicic acid/PTE interactions at undersaturation of other mineral phases. Soil column experiments were conducted to investigate the formation of Si-contaminant compounds in an acidic soil (pH 4.6). Therefore, a Haplic Phaeozem was preconditioned with Cu and Cd in the absence and presence of additional monomeric silicic acid and subsequently irrigated with artificial rainwater. Interactions of silicic acid and PTEs were investigated by monitoring the elemental composition of the eluates, and the size and charge of the particles eluted. After irrigation, total and exchangeable Si and PTE contents were analysed. Silicic acid application resulted in larger particles in the eluates, indicating silicic acid polymerization. The molar metal:Si ratios of the eluates and the significant correlation between Si and exchangeable metals indicated that particularly Cu formed Si-contaminant compounds in the soil, enhancing its retention. However, translocation of PTEs in particulate form, associated with polymerized silicic acid, was indicated. The negative charge and the very small size of the formed compounds may facilitate translocation from soil into groundwater. Batch adsorption experiments and ITC experiments were conducted to examine mechanism and extent of PTE adsorption to polymeric silicic acid. These experiments did not reveal any adsorption of the metals on polymeric silicic acid at pH 4 to 6 and after 24 h, which was underpinned by the results of the ITC experiments. However, zeta-potential measurements indicated weak electrostatic interactions between the negatively charged silanol groups and the PTEs. These electrostatic interactions may be the initial step of Si-contaminant compound formation. This thesis elucidates extent and mechanisms of silicic acid, either mono- or polymeric, and PTE interactions, showing the formation of particulate compounds from the reaction between silicic acid and cationic PTEs in aqueous solution and in an acidic soil. Particularly Cu formed stable compounds during silicic acid polymerization. However, the interactions showed a low extent and mainly weak electrostatic interactions, concluding that the addition of monomeric silicic acid to acidic soils may not be a quantitatively effective measure to reduce PTE mobility in soils. Aggregation effects, resulting from freezing/thawing or drying/rewetting, however, could alter the mobility of Si-contaminant compounds. These effects may be subject of future research, as well as the spectroscopic detection of Si-contaminant compounds in soils.
Reducing uncertainty in prediction of climate change impacts on crop production in Ethiopia
(2024) Rettie, Fasil Mequanint; Streck, Thilo
Ethiopia, with an economy heavily reliant on agriculture, is among the countries most vulnerable to climate change. It faces recurrent climate extreme events that result in devastating impacts and acute food shortages for millions of people. Studies that focus on their influence on agriculture, especially crop productivity, are of particular importance. However, only a few studies have been conducted in Ethiopia, and existing studies are spatially limited and show considerable spatial invariance in predicted impacts, as well as discrepancies in the sign and direction of impacts. Therefore, a robust, regionally focused, and multi-model assessment of climate change impacts is urgently needed. To guide policymaking and adaptation strategies, it is essential to quantify the impacts of climate change and distinguish the different sources of uncertainty. Against this backdrop, this study consisted of several key components. Using a multi-crop model ensemble, we began with a local climate change impact assessment on maize and wheat growth and yield across three sites in Ethiopia . We quantified the contributions of different sources of uncertainty in crop yield prediction. Our results projected a of 36 to 40% reduction in wheat grain yield by 2050, while the impact on maize was modest. A significant part of the uncertainty in the projected impact was attributed to differences in the crop growth models. Importantly, our study identified crop growth model-associated uncertainty as larger than the rest of the model components. Second, we produced a high-resolution daily projections database for rainfall and temperature to serve the requirement for impact modeling at regional and local levels using a statistical downscaling technique based on state-of-the-art GCMs under a range of emission scenarios called Shared Socioeconomic Pathways (SSPs). The evaluated results suggest that the downscaling strategy significantly reduced the biases between the GCM outputs and the observation data and minimized the errors in the projections. Third, we explored the magnitude and spatial patterns of trends in observed and projected changes in climate extremes indices based on downscaled high-resolution daily climate data to serve as a baseline for future national or regional-level impact assessment. Our results show largely significant and spatially consistent trends in temperature-derived extreme indices, while precipitation-related extreme indices are heterogeneous in terms of spatial distribution, magnitude, and statistical significance coverage. The projected changes in temperature-related indices are dominated by the uncertainties in the GCMs, followed by uncertainties in the SSPs. Unlike the temperature-related indices, the uncertainty from internal climate variability constitutes a considerable proportion of the total uncertainty in the projected trends. Fourth, we examined the regional-scale impact of climate change on maize and wheat yields by crop modeling, in which we calibrated and validated three process-based crop models to guide the design of national-level adaptation strategies in Ethiopia. Our analysis showed that under a high-emissions scenario, the national-level median wheat yield is expected to decrease by 4%, while maize yield is expected to increase by 2.5% by the end of the century. The CO2 fertilization effect on the crop simulations would offset the projected negative impact. Crop model spread followed by GCMs was identified as the largest contributor to overall uncertainty to the estimated yield changes. In summary, our study quantifies the impact of climate change and demonstrates the importance of a multi-model ensemble approach. We highlight the significant impacts of climate change on wheat yield in Ethiopia and the importance of crop model improvements to reduce overall uncertainty in the projected impact.
Regionalising a soil-plant model ensemble to simulate future yields under changing climatic conditions
(2023) Bendel, Daniela Silke; Streck, Thilo
Models are supportive in depicting complex processes and in predicting their effects. Climate models are applied in many areas to assess the possible consequences of climate change. Even though Global Climate Models (GCM) have now been regionalised to the national level, their resolution of down to 5x5 km2 is still rather coarse from the perspective of a plant modeller. Plant models were developed for the field scale and work spatially explicitly. This requires to make adjustments if they are applied at coarser scales. The regionalisation of plant models is reasonable and advantageous against the background of climate change and policy advice, both gaining in importance. The higher the spatial and temporal heterogeneity of a region, the greater the computational need. The (dis)aggregation of data, frequently available in differing resolutions or quality, is often unavoidable and fraught with high uncertainties. In this dissertation, we regionalised a spatially-explicit crop model ensemble to improve yield projections for winter wheat under a changing climate. This involved upscaling a crop model ensemble consisting of three crop models to the Stuttgart region, which has an area of 3,654 km2. After a thorough parameter estimation performed with a varying number of Agricultural Response Units on a high-performance computing cluster, yield projections up to the year 2100 were computed. The representative concentration pathways of the Intergovernmental Panel on Climate Change (IPCC) RCP2.6 (large reduction of CO2 emissions) and RCP8.5 (worst case scenario) served as a framework for this effort. Under both IPCC scenarios, the model ensemble predicts stable winter wheat yields up to 2100, with a moderate decrease of 5 dt/ha for RCP2.6 and a small increase of 1 dt/ha for RCP8.5. The variability within the model ensemble is particularly high for RCP8.5. Results were obtained without accounting for a potential progress in wheat breeding.
Bedeutung der Stickstoffumsetzung und externer Stickstoffquellen für die Entwicklung von FFH-Mähwiesen in Baden-Württemberg
(2023) Kukowski, Sina Louise; Streck, Thilo
1. AIM AND OBJECTIVES OF THE STUDY. The condition of the species-rich lowland hay meadows (habitat type 6510) in Germany is increasingly deteriorating. One cause of the deterioration is the supply of reactive nitrogen (N). To counteract the ongoing deterioration, it is necessary to understand the relationships between external N inputs via the atmosphere and fertilization, internal N turnover in the soil, plant uptake and growth, as well as possible links to the conservation degree of this habitat type. The overall objective of this dissertation is therefore to contribute to a better process-based understanding of the complete N cycle of Fauna-Flora-Habitat (FFH) meadows. 2. MATERIAL & METHODS. The interdisciplinary structure of this thesis includes different approaches to study inputs, turnover and outputs of N. With respect to N input via the airborne pathway, the focus was primarily placed on the hitherto poorly studied relationships between ammonia concentration and specific N-sensitive species groups in FFH lowland hay meadows. These relations were analyzed by means of generalized mixed models (GLM) based on nationwide data. In addition, further site-specific factors with a significant influence on the conservation degree of FFH meadows were identified using GLM. For the quantification of soil-borne N turnover processes, an empirical approach was chosen, including the determination of gross N turnover rates using the 15N isotope dilution method. To record these N dynamics, an intensive monitoring of gross and net N fluxes (mineralization, nitrification, ammonium consumption, nitrate consumption) in soils from different primary substrate and with different meadow conservation degree was carried out in 2016 and 2017. The results were merged using a process-based agroecosystem model (EXPERT-N), which was adjusted for habitat type 6510 to the collected data. The adapted model was applied to other sites of habitat type 6510 distributed throughout the state of Baden-Württemberg, which served to investigate spatial and temporal patterns of relevant nitrogen fluxes over an extended time period (1996 until 2012) and had been characterized in terms of soil and vegetation. 3. RESULTS. The nationwide data show a statistically significant decrease of habitat-typical low-nutrient indicator species and an increase of N indicator species with increasing atmospheric ammonia concentration on lowland hay meadows in Baden-Württemberg. Whether this is an effect of the atmospheric ammonia concentration or whether differences in agricultural land use structure play the decisive role could not be clarified with the available data. The intensive monitoring on selected FFH lowland hay meadows showed that soil-borne gross nitrification rates on soils from calcareous parent substrate (high pH) differed significantly from those from decalcified substrate (low pH). Both gross mineralization and gross nitrification were characterized by high temporal variability at all sites, which could not be explained by measurements of soil temperature and soil water content. Determination of net N turnover rates showed almost no variability and could not be used to draw conclusions about actual gross turnover rates in soil. The N-turnover model adapted for habitat type 6510 was able to represent spatial and temporal patterns over an extensive period of time. Simulation results showed high spatial and temporal variability for most N cycle variables. Soil organic N mineralization has a critical influence on the amount of plant-available N and thus has a direct impact on yield and N removal. On high clay-content soils and sites with high organic matter content, the model overestimated mineralization. External N inputs, such as moderate organic fertilization or atmospheric N deposition, were less crucial for yield. Additional N input is always a driving factor for N turnover in soil in the short term. With already high turnover levels, N turnover continues to increase and thus the risk of nutrient imbalances also increases. In the long term, the decisive factor for the N balance of FFH lowland hay meadows is whether N supply exceeds removal, whether the mineralizable organic N pools are thus increased, or whether a balance between supply and removal can be achieved. If soil internal N turnover is high, as it was the case on most of the simulated sites, a longer depletion phase should be applied before. In summary, this dissertation provides insight into the complexity of N cycling of FFH meadows. Using various approaches (statistical analyses, field trials, process-based modelling), it contributes to a better understanding of site-specific N turnover and the role of external N sources for the development of this ecosystem.
Constraints on microbial pesticide degradation in soils
(2023) Wirsching, Johannes; Kandeler, Ellen
Pesticides are an essential component of intensified agriculture and have contributed significantly to the increase in food production observed in recent decades. Since 1960, pesticide use has increased by a factor of fifteen to twenty, representing a market value of $40 billion in 2016. Soil monitoring campaigns to track pesticide contamination of croplands across Europe are quantifying pesticide residues whose residence times in soils exceed expected values. Diffuse contamination by pesticide residues raises concerns about soil functions, soil biodiversity, and food safety, as well as the transport of contaminants by wind and water to surface waters or to adjacent, organically managed croplands. Data on the frequency of occurrence and concentrations of pesticide residues in soil demonstrate a discrepancy between the determination of persistence and subsequent approval and their actual fate in soil. This raises the question of whether degradability of individual organic compounds has been adequately studied. Microbiological degradation is the most important process for reducing pesticide loads in soils. A reliable estimate of pesticide residence time requires an expanded understanding of the factors limiting microbial degradation. Degradation of anthropogenic organic chemicals in soils occurs much more slowly than would be expected based on their physicochemical properties. While processes that determine the fate of pesticides in soil have often been studied at different spatial and temporal scales, reasons for discrepancies between the observed complete degradation of pesticides under laboratory conditions and their persistence in the field remain unclear. This thesis addresses this challenge by focusing on the central question of why inherently biodegradable compounds in soils display increased persistence under field conditions. Organic contaminants in low but detectable and environmentally significant concentrations could remain in the soil once available concentrations fall below a threshold where bioenergetic growth restrictions come into play. In addition, potential microbial and biophysical limitations and environmental factors such as soil temperature and soil moisture are often examined separately in current degradation studies. Combinations of temperature and soil moisture changes associated with different concentration levels have been less well examined, resulting in an incomplete understanding of the degradation process. Another key factor in the demonstrated discrepancy between predicted and actual persistence in the field could be due to laboratory experiments that cannot account for field-scale processes. Therefore, degradation rates determined in laboratory experiments cannot be confidently extrapolated to the field scale. . This thesis identified further important regulatory mechanisms for microbially mediated pesticide degradation. The previously unknown concentration-dependent degradation dynamics and the concentration-dependent influence of limiting environmental conditions on microbial degradation emphasize the importance of studies using a realistic concentration range. Evidence of deep transport of a highly sorptive pesticide such as glyphosate primarily via preferential flow pathways into the subsoil with lower degradation dynamics underscores the need to include processes that can only be verified in field studies as part of risk assessments. The results of this thesis suggest that the biodegradation rates of pesticides are not homogeneous at field scales and may account in part for the discrepancy between complete degradation of pesticides under laboratory conditions and their persistence in the field. Laboratory studies in which soil samples are pooled and mixed to obtain a single "representative" sample can provide a simplified understanding of the process, but the complexity, particularly that of soil heterogeneity, of pesticide distribution and microbial degradation associated with prevailing climatic conditions, requires calibration of previously used methods in field studies and possibly at landscape, watershed, or regional scales. The scale-dependent degradation aspect will become even more important in the future; as soil properties and processes that control the toxicological aspects of contaminants include temperature and moisture, and changes associated with climate change will lead to an increase in extreme precipitation, longer dry periods, and soil erosion.
Microplastics interactions with soil organisms
(2022) Schöpfer, Lion; Kandeler, Ellen
Microplastics (MP) are plastic particles from 100 nm to 5 mm with different shapes and chemical compositions. In aquatic ecosystems, MP have proven to affect the biological fitness of aquatic organisms, enter the food web, and act as vectors of pollutants. Agricultural soils are sinks for MP due to inputs via sewage sludges, plastic mulches, and organic fertilizers. However, ecological consequences of MP in agricultural soils are unknown. This doctoral thesis aimed to evaluate the risk of conventional and biodegradable MP for soil organisms in agricultural soils. A microcosm study was combined with a field study and a nematode study to investigate background concentrations, the persistence, and the biodegradation of MP in the soil, and effects of MP on soil microorganisms and nematodes. In the microcosm study, the influence of plastic type, particle size, and soil moisture on the biodegradation of MP in the soil and on effects on soil microorganisms were examined under controlled conditions (25 °C, 230 days). The abundance and composition of the main soil microbial groups was analyzed via phospholipid fatty acids (PLFAs) as biomarkers; activities of C cycling enzymes driving the decomposition of differently complex substances were analyzed as proxies for C turnover. To understand better the role of MP as an interface for specific microbial processes in the soil, e.g. the enzymatic hydrolysis of MP, enzyme activities of individual MP particles extracted from the soil were measured. The site of the field study was a conventionally managed agricultural soil (silt-loam Luvisol) of the Heidfeldhof, University of Hohenheim. No practices associated with significant inputs of MP have been conducted at the site in the past (sewage sludge, organic fertilizers, plastic mulch). In a randomized complete block design, the effects of MP, organic fertilizers (digestate and compost), and their interactions on soil microbiological indicators (microbial biomass, soil enzymes) were studied. Before the setup of the field study, MP background concentrations (particle-based) in the soil were analyzed. The persistence of added MP in the soil was evaluated by comparing MP concentrations in the soil after 1 month and 17 months with initial MP concentrations after addition. In the nematode study, the soil-dwelling nematode Caenorhabditis elegans was exposed to MP feed suspensions on agar plates. The uptake of MP through nematodes and the influence of plastic type and concentration on MP effects on nematode reproduction and body length were examined. In all studies, artificially fragmented MP from a conventional polymer (low-density polyethylene, LDPE) and a biodegradable polymer blend (poly(lactic acid) and poly(butylene adipate-co-terephtalate), PLA/PBAT) were used. The occurrence of both LDPE- and PLA/PBAT-MP is likely in agricultural soils because these are used for plastic mulches and compost bags. Results from this thesis suggest that (1) agricultural soils, including those without management practices related to significant MP entry, contain various MP, indicating diffuse MP inputs via atmospheric deposition, littering, and the abrasion of machinery coatings (a possible newly identified pathway), (2) also biodegradable MP persist and are slowly biodegraded in the soil implying a long term exposure risk for soil organisms to MP, (3) MP have no acute negative effects on microorganisms and C turnover, (4) MP form a specific habitat in the soil, the plastisphere, where MP-specific processes take place, e.g. the enzymatic hydrolysis of PLA/PBAT, (5) MP can enter the soil food web via nematodal uptake and affect nematode reproduction, which could destabilize the soil food web.
Soil microorganisms as hidden miners of phosphorus in soils under different cover crop and tillage treatments
(2022) Hallama, Moritz; Kandeler, Ellen
Phosphorus (P) is one of the most limiting plant nutrients for agricultural production. The soil microbial community plays a key role in nutrient cycling, affecting access of roots to P, as well as mobilization and mineralization of organic P (Porg). This thesis aimed to better understand the potential of cover crops to enhance plant-soil-microbe interactions to improve the availability of P. This dissertation consists of a meta-analysis of and two field experiments. The used methods showed that microbial P, the activity of P-cycling enzymes and PLFAs increased under cover crops, indicating an enhanced potential for organic P cycling. Gram- positive and Gram-negative bacteria, and to a lesser extent also arbuscular mycorrhizal fungi, increased their abundance with cover crops. However, saprotrophic fungi could benefit most from the substrate input derived from cover crop roots or litter. Enzyme-stable Porg shifted towards pools of a greater lability in the active soil compartments (rhizosheath and detritusphere). The effects of agricultural management, such as cover crop species choice and tillage, were detectable, but weaker compared to the effect of the presence of cover crops. With the obtained results, the research aims of this thesis could be successfully addressed. We were able to confirm that cover crops have the potential to improve main crops’ access to P. Furthermore, we presented and discussed three pathways of P benefit. In the plant biomass pathway, P is cycled through cover crop biomass and becomes available for the main crop upon litter decomposition. The microbial enhancement pathway describes how the cover crop’s interaction with soil microbes increases their abundance and activity, thereby increasing the availability of Porg. Some cover crop species seem to be capable of utilizing a biochemical modification pathway, where changes in the sorption capacity of the soil result in a greater quantity of plant-available phosphate. However, the latter pathway was apparently not important in the crop rotations used in our field experiments. The data also allowed us to characterize ways in which plant-soil-microbe interactions under cover crops affected the relationship of soil microbial functions to the enzymatic availability of Porg pools. Cover crops increased the abundance and activity of microbes, especially fungi, as well as microbial P. This enhancement in P-cycling potential shifted Porg toward pools of greater availability to added enzymes. However, the relation between enzymes and Porg pools is complex and is possibly affected by soil P composition and other site characteristics, indicating the need for further research in this area. Finally, we elucidated how the choice of cover crop species and agricultural management can shift the relative importance of the pathways for the P benefit of the main crop, while site-specific management allows farmers to adapt to local conditions and to optimize the functions of their agroecosystems. In conclusion, our results indicate that the pathways of cover crop derived P benefit take place simultaneously. We confirmed the potential of cover crop biomass for the cycling of P, and we suggest that our observed increases in the availability of soil Porg are related to microbial abundance and activity. The interactions of cover cropping and tillage indicate also that P benefit can be optimized by management decisions. Finally, these new insights into soil phosphorus cycling in agroecosystems have the potential to support further development of more sustainable agricultural systems.
Modeling microbial regulation of pesticide turnover in soils
(2022) Chavez Rodriguez, Luciana; Streck, Thilo
Pesticides are widely used for pest control in agriculture. Besides their intended use, their long-term fate in real systems is not well understood. They may persist in soils, thereby altering ecosystem functioning and ultimately affecting human health. Pesticide fate is assessed through dissipation experiments in the laboratory or the field. While field experiments provide a close representation of real systems, they are often costly and can be influenced by many unknown or uncontrollable variables. Laboratory experiments, on the other hand, are cheaper and have good control over the governing variables, but due to simplification, extrapolation of the results to real systems can be limited. Mechanistic models are a powerful tool to connect lab and field data and help us to improve our process understanding. Therefore, I used mechanistic, process-based models to assess key microbial regulations of pesticide degradation. I tested my model hypotheses with two pesticide classes: i) chlorophenoxy herbicides (MCPA (2-methyl-4-chlorophenoxyacetic acid) and 2,4-D (2,4-Dichlorophenoxyacetic acid)), and ii) triazines (atrazine (AT)), in an ideal scenario, where bacterial degraders and pesticides are co-localized. This thesis explores some potential controls of pesticide degradation in soils: i) regulated gene expression, ii) mass-transfer process across the bacterial cell membranes, iii) bioenergetic constraints, and iv) environmental factors (soil temperature and moisture). The models presented in this thesis show that including microbial regulations improves predictions of pesticide degradation, compared to conventional models based on Monod kinetics. The gene-centric models achieved a better representation of microbial dynamics and enable us to explore the relationship between functional genes and process rates, and the models that used transition state theory to account for bioenergetic constraints improved the description of degradation at low concentrations. However, the lack of informative data for the validation of model processes hampered model development. Therefore, in the fourth part of this thesis, I used atrazine with its rather complex degradation pathway to apply a prospective optimal design method to find the optimal experimental designs to enable us identifying the degradation pathway present in a given environment. The optimal designs found suggest to prioritize determining metabolites and biomass of specific degraders, which are not typically measured in environmental fate studies. These data will lead to more robust model formulations for risk assessment and decision-making. With this thesis, I revealed important regulations of pesticide degradation in soils that help to improve process understanding and model predictions. I provided simple model formulations, for example the Hill function for gene expression and transition state theory for bioenergetic growth constraints, which can easily be integrated into biogeochemical models. My thesis covers initial but essential steps towards a predictive pesticide degradation model usable for risk assessment and decision-making. I also discuss implication for further research, in particular how mechanistic process-based modeling could be combined with new technologies like omics and machine learning.
Multi-objective and multi-variate global sensitivity analysis of the soil-crop model XN-CERES in Southwest Germany
(2021) Witte, Irene; Streck, Thilo
Soil-crop models enjoy ever-greater popularity as tools to assess the im- pact of environmental changes or management strategies on agricultural production. Soil-crop models are designed to coherently simulate the crop, nitrogen (N) and water dynamics of agricultural fields. However, soil-crop models depend on a vast number of uncertain model inputs, i.e., initial conditions and parameters. To assess the uncertainty in the simulation results (UCSR) and how they can be apportioned among the model inputs of the XN-CERES soil-crop model, an uncertainty and global sensitivity analysis (GSA) was conducted. We applied two different GSA methods, moment-independent and variance-based methods in the sense of the Factor Prioritization and the Factor Fixing setting. The former identifies the key drivers of uncertainty, i.e., which model input, if fixed to its true value, would lead to the greatest reduction of the UCSR. The latter identifies the model inputs that cannot be fixed at any value within their value range without affecting the UCSR. In total we calculated six sensitivity indices (SIs). The overall objective was to assess the cross-sub-model impact of parameters and the overall determinability of the XN-CERES applied on a deep loess soil profile in Southwest Germany. Therefore, we selected 39 parameters and 16 target variables (TGVs) to be included in the GSA. Furthermore, we assessed a weekly time series of the parameter sensitivities. The sub-models were crop, water, nitrogen and flux. In addition, we also compared moment-independent (MI) and variance-based (VB) GSA methods for their suitability for the two settings. The results show that the parameters of the TGVs of the four groups cannot be considered independently. Each group is impacted by the parameters of the other groups. Crop parameters are most important, followed by the Mualem van Genuchten (MvG) parameters. The nitrate (NO3-) content and the matric potential are the two TGVs that are most affected by the inter- action of parameters, especially crop and MvG parameters. However, the model output of these two TGVs is highly skewed and leptokrutic. Therefore, the variance is an unsuitable representation of the UCSR, and the reliability of the variance-based sensitivity indices SIVB is curtailed. Nitrogen group parameters play an overall minor role for the uncertainty of the whole XN-CERES, but nitrification rates can be calibrated on ammonium (NH4+) measurements. Considering the initial conditions shows the high importance of the initial NO3-; content. If it could be fixed, the uncertainty of crop groups’ TGVs, the matric potential and the N content in the soil could be reduced. Hence, multi-year predictions of yield suffer from uncertainty due to the simulated NO3-; content. Temporally resolved parameter show the big dependence between the crop’s development stage and the other 15 TGVs becomes visible. High temporally resolved measurements of the development stage are important to univocally estimate the crop parameters and reduce the uncertainty in the vegetative and generative biomass. Furthermore, potential periods of water and N-limiting situations are assessed, which is helpful for deriving management strategies. In addition, it become clear that measurement campaigns should be conducted at the simulation start and during the vegetation period to have enough information to calibrate the XN-CERES. Regarding the performance of the different GSA methods and the different SIs, we conclude that the sensitivity measure relying on the Kolmogorov-Smirnov metric (betaks) is most stable. It converges quickly and has no issues with highly skewed and leptokrutic model output distributions. The assessments of the first-effect index and the betaks provide information on the additivity of the model and parameters that cannot be fixed without impacting the simulation results. In summary, we could only identify three parameters that have no direct impact on any TGV at any time and are hence not determinable from any measurements of the TGVs considered. Furthermore, we can conclude that the groups’ parameters should not be calibrated independently because they always affect the uncertainty of the selected TGV directly or via interacting. However, no TGV is suitable to calibrate all parameters. Hence, the calibration of the XN-CERES requires measurements of TGVs from each group, even if the modeler is only interested in one specific TGV, e.g., yield. The GSA should be repeated in a drier climate or with restricted rooting depth. The convergence of the values for the Sobol indices remains an issue. Even larger sample sizes, another convergence criteria or graphical inspection cannot alleviate the issue. However, we can conclude that the sub-models of the XN-CERES cannot be considered in- dependently and that the model does what it is designed for: coherently simulating the crop, N and water dynamics with their interactions.
Towards a better understanding of land surface exchange processes over agricultural crop stands
(2020) Bohm, Kristina; Streck, Thilo
Weather and climate models are useful tools for projecting the influence of global climate change on the regional scale. These models are critically dependent on an accurate representation of soil-plant-atmosphere interactions, which are simulated by Land Surface Models (LSMs). The present PhD thesis was designed to improve the representation of land surface exchange processes of croplands in the Noah-MP land surface model. This thesis aims: a) to elucidate the nature of the energy imbalance over a winter wheat stand and to identify the appropriate post-closure method for the study region Kraichgau, southwest Germany; b) to improve the representation of the green vegetation fraction (GVF) dynamics of croplands in the Noah-MP for a more accurate computation of surface energy and water fluxes; and c) to determine the effect of aggregating different crop types with various shares into a single generic cropland class on the simulation of water and energy exchange between land surface and atmosphere.
Biological regulation of subsoil C-cycling
(2019) Preußer, Sebastian; Kandeler, Ellen
Soils are the largest terrestrial reservoir of organic carbon (OC). Substantial proportions of the stored OC are found in stabilized form in deeper soil layers. Beside the quality and quantity of C input from plant biomass, C storage in soil is primarily controlled by the microbial decomposition capacity. Various physical, chemical and biological factors (e.g., substrate availability, temperature, water content, pH, texture) vary within soil profiles and directly or indirectly influence the abundance, composition and activity of microbial communities and thus the microbial C turnover. While soil microbiological research has so far focused mainly on processes in topsoil, the mechanisms of C storage and turnover in subsoil are largely unknown. The objective of the present thesis was therefore to investigate the specific influence of substrate availability and different environmental factors as well as their interactions on microbial communities and their regulatory function in subsoil C-cycling. This objective was addressed in three studies. In the first and second study, one-year field experiments were established in which microbial communities from different soil depths were exposed to altered habitat conditions to identify crucial factors influencing the spatial and temporal development of microbial abundance and substrate utilization within soil profiles. This was achieved by reciprocal translocation of soils between subsoil horizons (first study) and topsoil and subsoil horizons (second study) in combination with addition of 13C-labelled substrates and different sampling dates. In the third study, a flow cascade experiment with soil columns from topsoil and subsoil horizons and soil minerals (goethite) coated with 13C-labelled organic matter (OM) was established. This laboratory experiment investigated the importance of exchange processes of OM with reactive soil minerals for the quality and quantity of dissolved OM and the influence of these soil micro-habitats on microbial abundance and community composition with increasing soil depth. In the first study, the reciprocal translocation of subsoils from different soil depths revealed that due to comparable micro-climatic conditions and soil textures within the subsoil profile, no changes in microbial biomass, community composition and activity occurred. Moreover, increasing microbial substrate utilization in relation to the quantity of added substrate indicated that deep soil layers exhibit high potential for microbial C turnover. However, this potential was constrained by low soil moisture in interplay with the coarse soil texture and the resulting micro-scale fragmentation of the subsoil environment. The bacterial substrate utilization was more affected by this spatial separation between microorganisms and potentially available substrate than that of fungi, which was further confirmed by the translocation experiment with topsoil and subsoil in the second study. While the absolute substrate utilization capacity of bacteria decreased from the more moist topsoil to the drier subsoil, fungi were able to increase their substrate utilization and thus to partially compensate the decrease in C input from other sources. Furthermore, the addition of root litter as a preferential C source of fungal decomposer communities led to a pronounced fungal growth in subsoil. The third study demonstrated the high importance of reactive soil minerals both in topsoil and in subsoil for microbial growth due to extensive exchange processes of OM and the associated high availability of labile C. In particular copiotrophic bacteria such as Betaproteobacteria benefited from the increased C availability under non-limiting water conditions leading to a pronounced increase in bacterial dominance in the microbial communities of these soil micro-habitats. In conclusion, this thesis showed that subsoil exhibits great potential for both bacterial and fungal C turnover, albeit this potential is limited by various factors. This thesis, however, allowed to determine the specific effects of these factors on bacteria and fungi and their function in subsoil C-cycling and thus to identify those factors of critical importance. The micro-climate in subsoil, in particular soil moisture, was the primary factor limiting bacterial growth and activity, whereas fungi were more strongly restricted by substrate limitations.
Phosphorus-acquisition strategies of canola, wheat and barley in soil amended with sewage sludges
(2019) Faucon, M.-P.; Kandeler, Ellen; Lambers, Hans; Firmin, S.; Michel, E.; Houben, D.; Nobile, Cécile
Crops have different strategies to acquire poorly-available soil phosphorus (P) which are dependent on their architectural, morphological, and physiological root traits, but their capacity to enhance P acquisition varies with the type of fertilizer applied. The objective of this study was to examine how P-acquisition strategies of three main crops are affected by the application of sewage sludges, compared with a mineral P fertilizer. We carried out a 3-months greenhouse pot experiment and compared the response of P-acquisition traits among wheat, barley and canola in a soil amended with three sludges or a mineral P fertilizer. Results showed that the P-acquisition strategy differed among crops. Compared with canola, wheat and barley had a higher specific root length and a greater root carboxylate release and they acquired as much P from sludge as from mineral P. By contrast, canola shoot P content was greater with sludge than with mineral P. This was attributed to a higher rootreleased acid phosphatase activity which promoted the mineralization of sludge-derived P-organic. This study showed that contrasted P-acquisition strategies of crops allows increased use of renewable P resources by optimizing combinations of crop and the type of P fertilizer applied within the cropping system.

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