Field-aged biochar decreased N2O emissions by reducing autotrophic nitrification in a sandy loam soil

A 15N tracing incubation study was carried out using a sandy loam soil without (control) and with field-aged biochar (biochar) to investigate the mechanisms underlying the effects of field-aged biochar on nitrous oxide (N2O) emissions. During the incubation, carried out at 40%, 60%, and 80% water-filled pore space (WFPS), cumulative N2O emission decreased from 24.13–26.40 μg N kg−1 in the control soil to 18.27–23.94 μg N kg−1 in the biochar soil, with a reduction of 9.3–24.3%. The contribution of autotrophic nitrification to total N2O production was 81.8–87.6% in the control soil under 40–80% WFPS, which was significantly reduced by field-aged biochar to 67.1–78.6%. Under 60% WFPS, the gross rates of autotrophic nitrification and gross mineralization were reduced from 11.95 and 4.43 μg N g−1 d−1, respectively, in the control soil to 7.32 and 0.60 μg N g−1 d−1, respectively, in the biochar soil. The field-aged biochar increased the NH4+ immobilization rate by 440%, primarily by immobilizing NH4+ into the recalcitrant organic N pool. Both the turnover rate of NH4+ mineralization-immobilization and the ratio of nitrification to NH4+ immobilization were reduced under biochar amendment, consequently lowering the supply of NH4+ for nitrifiers. In addition, compared with the control soil, the gross rate of NH4+ adsorption was significantly higher in the biochar soil. Taken together, our results suggest that field-aged biochar contributes to mitigating N2O emissions, primarily by decreasing the autotrophic nitrification rate through a reduced NH4+ supply due to increased mineral N immobilization and adsorption and lowered organic N mineralization.