Biochar amendment mitigated NO emissions from paddy field during the wheat growing season.

Biochar may variably impact nitrogen (N) transformation and N-cycle-related microbial activities. Yet the mechanism of biochar amendment on nitrous oxide (NO) emissions from agricultural ecosystems remains unclear. Based on a 6-year long-term biochar amendment experiment, we applied a dual isotope (N-O) labeling technique with tracing transcriptional genes to differentiate the contribution of nitrifier nitrification (NN), nitrifier denitrification (ND), nitrification-coupled denitrification (NCD) and heterotrophic denitrification (HD) pathway to NO production. Then the field experiment provided quantitative data on dynamic NO emissions, soil mineral N and key functional marker gene abundances during the wheat growing season. By using N-O isotope, biochar decreased NO emission derived from ND (by 45-94%), HD (by 35-46%) and NCD (by 30-64%) compared to the values under N application. Biochar increased the relative contribution of NN to total NO production as evidenced by the increase in ammonia-oxidizing bacteria, but did not influence the cumulative NN-derived NO. The field experiment found that the majority of the NO emissions peaked following fertilization, in parallel with soil NH and nitrite dynamics. Soil NO emissions during the wheat growing stage were effectively decreased (by 38-48%) by biochar amendment. Based on the correlation analyses and random forest analysis in both microcosm and field experiments, the decrease in nitrite concentration (by 62-65%) and increase in NO consumption were mainly responsible for net NO mitigation, as evidenced by the decrease in the ratios of nitrite reductase genes/transcripts (nirS, nirK and fungal nirK) and NO reductase gene/transcripts (nosZI and nosZII). Based on the extrapolation from microcosm to field, biochar significantly mitigated NO emissions by weakening the ND processes, since NCD and HD contributed little during the NO emission "peaks" following urea fertilization. Therefore, emphasis should be put on the ND process and nitrite accumulation during NO emission peaks and extrapolated to all agroecosystems.