Browsing by Person "Pekrun, Carola"

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Hidden miners – the roles of cover crops and soil microorganisms in phosphorus cycling through agroecosystems
(2019) Hallama, Moritz; Pekrun, Carola; Lambers, Hans; Kandeler, Ellen
Background Phosphorus (P) is a limiting nutrient in many agroecosystems and costly fertilizer inputs can cause negative environmental impacts. Cover crops constitute a promising management option for sustainable intensification of agriculture. However, their interactions with the soil microbial community, which is a key driver of P cycling, and their effects on the following crop, have not yet been systematically assessed. Scope We conducted a meta-analysis of published field studies on cover crops and P cycling, focusing on plant-microbe interactions. Conclusions We describe several distinct, simultaneous mechanisms of P benefits for the main crop. Decomposition dynamics, governed by P concentration, are critical for the transfer of P from cover crop residues to the main crop. Cover crops may enhance the soil microbial community by providing a legacy of increased mycorrhizal abundance, microbial biomass P, and phosphatase activity. Cover crops are generally most effective in systems low in available P, and may access ‘unavailable’ P pools. However, their effects on P availability are difficult to detect by standard soil P tests, except for increases after the use of Lupinus sp. Agricultural management (i.e. cover crop species selection, tillage, fertilization) can improve cover crop effects. In summary, cover cropping has the potential to tighten nutrient cycling in agricultural systems under different conditions, increasing crop P nutrition and yield.
Influence of biogas-digestate processing on composition, N partitioning, and N₂O emissions after soil application
(2023) Petrova, Ioana; Pekrun, Carola
The ever-growing need for agricultural products represents a global issue, particularly with a view to the limited availability of cultivable land. According to the latest estimates, the arable land per capita decreases and, in 2050, is expected to account for about 60% less than in the 1960s. In order to meet the demand, agriculture has evolved into industrial-like structures. This development often goes along with nutrient surpluses (e.g., excess of nitrogen and phosphorus) and increased emissions, caused by mismanagement and inappropriate agricultural practices (e.g., over-fertilization). Biogas plants offer a possibility to valorize organic residues and wastes, but potentially aggravate this problem since additional organic residues (referred to as digestates) with considerable nutrient contents are generated as by-products. A simple approach to adjust nutrient levels in the affected regions is the transfer of manures and digestates. However, to make this feasible, a reduction of water content (and consequently of total mass/volume) of digestates is required. Up to now, various techniques for digestate downstream processing are available. Previous research mainly addressed single processing stages or differences between feedstock mixtures. Only limited information was found about the influence of a completed downstream processing on total mass reduction and nitrogen concentration in digestate. Studies about the (gaseous) N losses that occur after the application of the respective intermediate and final products to soils were equally scarce. Therefore, the aims of the current doctoral thesis were to determine (i) the mass reduction achieved by the gradual removal of water within competing processing chains, (ii) the nitrogen partitioning after every single processing step and its recovery in the end products, and (iii) the amount of greenhouse gases (especially N₂O) released after the application of intermediate and end products to soils in comparison to untreated, raw digestate. For that purpose, two commercial, full-scale biogas plants were examined, which completely processed either the solid or the liquid fraction after mechanical screwpress separation of raw digestate. The separated solid fraction was subsequently dried and pelletized, while the liquid fraction was treated by vacuum evaporation with partial NH₃ scrubbing. As final products, digestate pellets and N-enriched ammonium sulfate solution were generated. Calculation of a mass flow balance served as the basis for determining (total) mass reduction, the partitioning of fresh mass and nitrogen during digestate processing, and the recovery of initial N in the products. Additionally, the environmental impact of utilizing digestate as an organic fertilizer was studied by measuring the N₂O release after application to soil under field and laboratory conditions. A further in-depth analysis was performed to observe the main factors influencing the production and release of climate-relevant N₂O from digestate pellets. It was found that the mass reduction caused by water removal during subsequent processing accounted for 6% (solid chain) and 31% (liquid chain) of the total mass of raw digestate. Liquid processing required 40% less thermal energy per ton of water evaporated than solid processing. At the end of the downstream processing, the recovery of initial nitrogen in pellets was 33% lower than in ammonium sulfate solution. Regarding the environmental impact of digestate application to soil, mechanical solidliquid separation showed the potential to reduce N₂O emissions. Contrary to expectations, pelletizing of dry solid boosted the emissions, which was linked to the properties and composition of the pellet. Here, indigenous microbial activity triggered N₂O production and release from denitrification immediately after wetting. Overall, the present work has shown that the subsequent processing of separated solid or liquid digestate generates different products with individual benefits and challenges. Solid digestates are characterized by a high share of recalcitrant organic compounds and therefore can serve, e.g., as soil improver. After processing to pellets, they can be easily transported, stored, and commercialized. However, it is questionable whether the pelletizing process is advisable, since pellets emitted a considerable amount of GHGs during utilization. Liquid processing produces ammonium sulfate solution, which can be utilized as a valuable inorganic fertilizer rich in plant-available N. Besides the discussed advantages, a final decision for or against digestate processing always depends on individual factors, such as local situation and financial means. Smart decision-making must include fertilizer properties, technological performance, and economic feasibility. With a view to future research, additional aspects were identified, such as returning to a laboratory-scale biogas plant for more accurate digestate sampling and analysis, consideration of digestate storage and transport, and economic evaluation of the entire digestate value chain including the assessment of digestate fertilizer value (expressed as e.g., N use efficiency or N fertilizer replacement value).
A new framework to assess sustainability of soil improving cropping systems in Europe
(2022) Alaoui, Abdallah; Hallama, Moritz; Bär, Roger; Panagea, Ioanna; Bachmann, Felicitas; Pekrun, Carola; Fleskens, Luuk; Kandeler, Ellen; Hessel, Rudi
Assessing agricultural sustainability is one of the most challenging tasks related to expertise and support methodologies because it entails multidisciplinary aspects and builds on cultural and value-based elements. Thus, agricultural sustainability should be considered a social concept, reliable enough to support decision makers and policy development in a broad context. The aim of this manuscript was to develop a methodology for the assessment of the sustainability of soil improving cropping systems (SICS) in Europe. For this purpose, a decision tree based on weights (%) was chosen because it allows more flexibility. The methodology was tested with data from the SoilCare Horizon 2020 study site in Germany for the assessment of the impact of the integration of cover crops into the crop rotation. The effect on the environmental indicators was slightly positive, but most assessed properties did not change over the short course of the experiment. Farmers reported that the increase in workload was outweighed by a reputation gain for using cover crops. The incorporation of cover crops reduced slightly the profitability, due to the costs for seeds and establishment of cover crops. The proposed assessment methodology provides a comprehensive summary to assess the agricultural sustainability of SICS.
Organic matter composition of digestates has a stronger influence on N2O emissions than the supply of ammoniacal nitrogen
(2021) Petrova, Ioana Petrova; Pekrun, Carola; Möller, Kurt
Manures can be treated by solid–liquid separation and more sophisticated, subsequent approaches. These processes generate fertilizers, which may differ in composition and N2O release potential. The aim of the study was to investigate the influence of processing-related changes in digestate composition on soil-derived N2O emissions after application to soil. For that purpose, N2O emissions within the first 7 weeks after fertilization with two raw and eight processed digestates (derived from solid–liquid separation, drying and pelletizing of separated solid, and vacuum evaporation of separated liquid) were measured in the field in 2015 and 2016. Additionally, an incubation experiment was run for 51 days to further investigate the effect of subsequent solid and liquid processing on soil-derived N2O release. The results showed that, only in 2016, the separation of digestate into solid and liquid fractions led to a decrease in N2O emissions in the following order: raw digestate > separated liquid > separated solid. N removal during subsequent processing of separated solid and liquid did not significantly influence the N2O emissions after fertilization. In contrast, the concentrated application of the final products led to contradictory results. Within the solid processing chain, utilization of pellets considerably increased the N2O emissions by factors of 2.7 (field, 2015), 3.5 (field, 2016), and 7.3 (incubation) compared to separated solid. Fertilization with N-rich ammonium sulfate solution led to the lowest emissions within the liquid processing chain. It can be concluded that the input of less recalcitrant organic C into the soil plays a greater role in N2O release after fertilization than the input of ammoniacal N. Digestate processing did not generally reduce emissions but apparently has the potential to mitigate N2O emissions substantially if managed properly.