Browsing by Subject "Nutrient management"

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The challenge to achieve a balanced fertilization management in intensive organic vegetable production – approaches for improvement
(2024) Stein, Sophie; Möller, Kurt
The nutrient supply – especially in stockless farms – is a major challenge for organic vegetable farms. Due to the lack of animal husbandry, the nutrient cycle within the farm is limited. At the same time, the nutrient flows in vegetable growing are very high due to the high nitrogen (N) and potassium (K) requirements of the crops. If the nutrient flows are not adapted to the needs of the crops, severe nutrient imbalances, yield or quality losses can occur. Fertilization with basic fertilizers such as manure or compost, based on the N-demand of the crops, can lead to an accumulation of phosphorus (P) in the soil in the long term. Fertilization with N-rich organic commercial fertilizers, on the other hand, is to some extent contradictory to the principles of organic farming due to their origin – mostly from conventional animal husbandry. Another possibility for the exclusive N supply of the farm is the targeted use of legumes with the ability for biological N-fixation (BNF). However, since the cultivation of fodder legumes or grain legumes plays a minor role in vegetable production without livestock, their use for fertilizer management is limited. The aim of this thesis was to optimize fertilizer management in vegetable production based on balanced nutrient budgets of the three main nutrients N, P and K, as well as to increase the supply of N within the farm through the N-fixation of legumes. In a farm survey, the status quo of nutrient management was determined and analyzed using nutrient budgets. In two systemic field trials, the contribution of the strategies legume winter cover crop and living mulch to the on-farm N supply was tested. The status quo of the budgets on organic vegetable farms without livestock shows average budget surpluses of 67.5 kg N ha-1, 2.06 kg P ha-1 and 0.26 kg K ha-1. Another result is that an increase in the N content of base fertilizers such as solid manure or champost leads to an increase in the P and K balance. In the long term, such a strategy carries the risk of nutrient enrichment in the soil. Strategies based on the use of large amounts of keratin fertilizers, on the other hand, have the opposite effect, namely a long-term depletion of P and K. The integration of legumes into the vegetable crop rotation shows that both strategies – leguminous winter cover crops as a preceding crop to white cabbage and leguminous living mulch in combination with white cabbage – lead to similar or even slightly higher cabbage yields than cultivation without legumes or green cover. The direct N-effect of the leguminous winter cover crop on the first crop of white cabbage is about 2/3 of the total N-effect, the N-effect on the second crop of winter wheat is about 1/3 of the total N-effect of the BNF. In the case of the living mulch biomass, which is only incorporated after cabbage cultivation, the direct N effect of BNF on the first main crop is less than 40 %. The N effect on the subsequent winter wheat crop is over 60 %. The results of this thesis show that organic vegetable farms face at least two major challenges in terms of nutrient management: They must implement nutrient management that ensures a balanced nutrient budget in the soil and on the farm while reducing N surpluses through more efficient use of internal and external N sources. This thesis provides important figures for the characterization and balancing of nutrient management in vegetable production. These figures can be used to address the challenge of balanced fertilization management.
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.