The effect of biochar-amended biogas digestate on soil nitrous oxide emissions of long-term reduced tillage under varying soil moisture.

The elevated emissions of nitrous oxide (N2O) after fertilization with biogas digestate pose a challenge to sustainable nutrient recycling in agriculture. In recent years, the addition of biochar and reduced tillage were considered important methods for sustainable soil management and have been extensively studied. However, their effect on nitrous oxide (N2O) emissions is still being debated, and the mechanisms are not adequately understood. This study explores how two sustainable farming practices - reduced tillage (RT) legacy and biochar amendment - affect N2O emissions from digestate-fertilized soil. We conducted a 36-day greenhouse pot experiment using soils from a long-term tillage experimental field, treated with no fertilization (N0), biogas digestate-only (BD) or digestate amended with biochar (BDBC). We monitored gaseous fluxes of N2O, ammonia (NH3) and carbon dioxide (CO2), alongside gravimetric water content, soil mineral nitrogen dynamics and nitrogen uptake in Lolium perenne biomass. Our results revealed two distinct peak periods of N2O emissions, likely driven by low water-filled pore space (WFPS) in peak period 1 (PP1, 35-45 %WFPS) and higher WFPS in peak period 2 (PP2, 50-65 % WFPS) and subsequent effects on N2O production pathways. Cumulative N2O emissions were ranged from 9.2 to 29.6 mg N m-2, and RT significantly decreased by 35 % as compared to conventional tillage (CT) soil, although a significant reduction occurred only during PP1. Biochar amendment did not significantly affect cumulative N2O emissions, but significantly decreased N2O emissions of PP2, particularly in RT soil. Our findings show that both RT legacy and biochar amendments can potentially mitigate N2O emissions from biogas digestate under certain soil moisture conditions.