Biochar amendment mitigated N2O 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 (N2O) emissions from agricultural ecosystems remains unclear. Based on a 6-year long-term biochar amendment experiment, we applied a dual isotope (N-15-O-18) labeling technique with tracing transcriptional genes to differentiate the contribution of nitrifler nitrification (NN), nitrifler denitrification (ND), nitrification-coupled denitrification (NCD) and heterotrophic denitrification (HD) pathway to N2O production. Then the field experiment provided quantitative data on dynamic N2O emissions, soil mineral N and key functional marker gene abundances during the wheat growing season. By using N-15-O-18 isotope, biochar decreased N2O 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 N2O production as evidenced by the increase in ammonia-oxidizing bacteria, but did not influence the cumulative NN-derived N2O. The field experiment found that the majority of the N2O emissions peaked following fertilization, in parallel with soil NH4+ and nitrite dynamics. Soil N2O 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 N2O consumption were mainly responsible for net N2O mitigation, as evidenced by the decrease in the ratios of nitrite reductase genes/transcripts (nirS, nirK and fungal nirK) and N2O reductase gene/transcripts (nosZI and nosZII). Based on the extrapolation from microcosm to field, biochar significantly mitigated N2O emissions by weakening the ND processes, since NCD and HD contributed little during the N2O emission peaks following urea fertilization. Therefore, emphasis should be put on the ND process and nitrite accumulation during N2O emission peaks and extrapolated to all agroecosystems. (C) 2021 Elsevier Ltd. All rights reserved.

Automated summary

The study found that biochar can reduce nitrous oxide emissions by decreasing nitrification-coupled denitrification, nitrification-coupled denitrification, and heterotrophic denitrification pathways, while increasing the relative contribution of nitrifier nitrification to total N2O production. Effective mitigation of N2O emissions was achieved by reducing nitrification and increasing N2O consumption.