Optimization strategies for organic fertilizer use in a seven-year field trial: Part II – Greenhouse gas emissions

Summary

Introduction
Biogas production from organic side-streams enables to unleash the energetical potential of biomass while contributing to closing nutrient cycles and reducing methane emissions from farmyard manure. To mitigate the higher risk of mineral nitrogen losses from anaerobically digested liquid organic fertilizers and optimize their nitrogen use efficiency, further treatment is desirable. Promising optimization approaches are biochar amendments, in-field acidification and ammonium-stripping. In order to comprehensively evaluate those optimization approaches and avoid the risk of pollution swapping, the assessment of their climate impact is crucial. The aim of this study was to assess greenhouse gas emissions from optimized liquid organic fertilizers over multiple growing seasons under field conditions.
Methodology
Greenhouse gas emissions were measured in a Swiss field trial over seven years featuring a split-plot design with four blocks and an arable crop rotation without legumes. The trial included two control treatments (zero N fertilization, mineral fertilization), cattle slurry, digested slurry with and without biochar (2 t ha-1 a-1) and liquid digestate. Additionally, in the last two trial years in-field acidification (sulfuric acid, pH 6-6.5) and ammonium-stripping were applied to cattle slurry and digested slurry. Nitrous oxide and methane emissions were measured weekly throughout the growing season over a total of 4.5 years using closed static chambers and gas chromatography. Data were analysed using a machine learning approach combining extreme gradient boosting and Shapley additive explanations (SHAP).
Results and discussion
Over a 4.5-year observation period, farming operations were the main drivers of nitrous oxide and methane fluxes. In particular, fertilization, tillage and harvest operations lead to elevated nitrous oxide emissions. Cumulatively over the growing season, digestates often showed lower greenhouse gas emissions compared to cattle slurry as a result of decreased nitrous oxide emissions following farming operations and lower methane emissions at application. Biochar did not significantly affect nitrous oxide emissions from agricultural digestate, but significantly increased short-lived methane emissions at application. However, results from experiments carried out under controlled conditions suggest that biochar does not increase overall methane emissions from agricultural digestate, but rather temporally shifts them from the storage period to the moment of application. Over a two-year observation period, applying in-field acidification and ammonium-stripping to cattle slurry and digested slurry did not significantly alter their cumulative nitrous oxide and methane emissions. However, in-field acidification decreased nitrous oxide emissions following tillage and harvesting operations and increased methane emissions at fertilization, in particular for digested slurry. Ammonium-stripping did not affect nitrous oxide emissions, but increased methane oxidation over the growing season.
Conclusion
Our greenhouse gas emission monitoring over multiple growing seasons showed that anaerobic digestion has the potential to reduce the climate impact of liquid organic fertilizers. Furthermore, it indicates that treating liquid organic fertilizers with biochar amendments, in-field acidification or ammonium-stripping does not increase overall greenhouse gas emissions following their application, but can significantly alter short-lived emission peaks resulting from critical farming operations.