Biochar's contrasting effects on N2O emissions in acidic upland and flooded paddy soils

Although it is well documented that biochar (BC) application reduces nitrous oxide (N2O) emissions in certain soils, a deeper understanding of the inconsistent results, and the underlying microbial mechanisms across different land-use types remains a significant challenge. In this study, biochar (BC-1%, 3%, and 5%), and quicklime (CaO-0.02%, 0.05%, and 0.08%) were applied to acidic upland and flooded paddy soils. N2O emissions were then determined and partitioned into nitrification, nitrifier denitrification, bacterial and fungal denitrification sources using natural abundance isotopic analyses (δ15Nbulk, δ15NSP, and δ18O), alongside complementary microbial molecular assessments. The results revealed contrasting effects of BC on N2O emissions in different soils. In acidic upland soil, BC achieved greater mitigation of N2O emissions than CaO. This mitigation was probably driven by suppressed bacterial and fungal denitrification, linked to lower abundance of Chaetomium—a high N2O-producing fungal genus—and enhanced expression of the nosZII gene, which encodes N2O reductase. Together, these changes curtailed N2O production from both microbial pathways. In flooded paddy soil, however, BC stimulated N2O emissions compared to the control or CaO treatment. Multiple pathways equally contributed to N2O emissions, with each showing a significant enhancement after BC addition to paddy soils. The present study demonstrated that comprehensive insight into the N2O pathway and microbial mechanisms is crucial, given the contrasting responses in upland and flooded paddy soils. Such mechanistic insight, through precise pathway attribution, highlights a critical need for and should precede future studies under more complex and realistic conditions before large-scale implications can be drawn.