Mechanisms underlying the mitigation of both N2O and NO emissions with field-aged biochar in an Anthrosol

The mechanisms associated with N2O and NO processes following biochar application remain unclear in an Anthrosol under intensively vegetable production. An incubation experiment with N-15 tracing technique and quantitative polymerase chain reaction (qPCR) was performed to investigate the responses of pathways and the microbial mechanisms of N2O and NO production to the application of wheat biochar (Bw) and swine manure biochar (Bm) field aged for one year in an Anthrosol under 60% water holding capacity. The application of both types of biochar decreased the cumulative N2O emissions by 12.9-20.0%, with an obvious mitigation effect observed after Bw application. The reduction in N2O emissions derived from autotrophic nitrification and denitrification induced by biochar were coupled with a decrease in ammonia-oxidizing bacteria (AOB) amoA abundance and the ratio of (nirK + nirS)/nosZ, respectively. Biochar increased the relative contribution of heterotrophic nitrification by 54.2-58.3%, with stronger stimulating effects from the Bm amendment than from the Bw amendment. Moreover, cumulative NO emissions were strongly reduced by an average of 35.5% by biochar, with no differences between Bw and Bm. Furthermore, covariation in NO flux and NO2--N content, together with the alternations in the abundance of AOB amoA, indicated that nitrifier denitrification might play a vital role in NO emissions. The present study highlights biochar's promising effects on mitigating N2O and NO emissions by weakening autotrophic nitrification and denitrification processes. Meantime, heterotrophic nitrification should be taken into consideration when comprehensively assessing the mitigation potential of biochar.