Combined effects of biochar and biogas slurry on soil nitrogen transformation rates and NO emission in a subtropical poplar plantation.

It has been widely accepted that biochar has a great potential of mitigating soil nitrous oxide (NO) emission. However, the underlying mechanism about how biochar affects nitrogen transformation and the pathways of soil NO production is under discussion. A N-tracer incubation experiment was conducted to investigate the short-term effects of biochar on soil N transformation rates and source partitioning of NO emissions in soils from a poplar plantation system. A two-factor experimental design was adopted using biogas digestate slurry and biochar as soil amendments. In total, there were 12 treatments, including three rates of biochar: B0 (control), B2 (80 t ha), and B3 (120 t ha), and four rates of biogas digestate slurry: C (0 m ha), L (125 m ha), M (250 m ha), and H (375 m ha). We observed significantly lower rates of net nitrification (N) and mineralization (M) in biochar-treated soils. The N tracer analysis revealed a significant decrease in gross autotrophic (O), heterotrophic nitrification (O), and mineralization (M) rates while an increase in gross immobilization (I and I) rates in biochar amended soils. When biogas slurry was applied, biochar only significantly reduced O except in the moderate slurry treatment. Regardless of the slurry application, biochar consistently suppressed NO emission by 58-89 %, and nitrification was the dominant pathway accounting contributing >90 % to cumulative NO emissions. Moreover, soil cumulative NO emissions significantly negatively correlated with soil ammonium contents and positively with M, M, and N, showing that biochar decreased NO emission via a reducing effect on nitrification rates and associated NO emissions. Our results also highlight that application of N fertilizer greatly influence the biochar's impacts on soil N transformation rates and NO emission, calling for further studies on their interactions to develop mitigate options and to improve N use efficiency.