Decreased N₂O and NO emissions associated with stimulated denitrification following biochar amendment in subtropical tea plantations

Acidic soils are hotspots of nitrous oxide (N2O) and nitric oxide (NO) and biochar is documented to have the potential for mitigating N2O and NO. The N2O and NO emissions associated with soil functional genes and physicochemical properties under biochar amendment remains unclear in acidic soils. Here, we carried out a two-year field study to examine the responses of soil N2O and NO emissions to biochar amendment in a subtropical tea plantation in China. Measurements of N2O and NO fluxes were taken from inter-row soils using the static chamber method. We also measured the seasonal changes in soil key nitrogen (N)-cycling functional genes and physicochemical properties. Annual N2O and NO emissions averaged 27.31 kg N2O-N ha(-1) yr(-1) and 8.75 kg NO-N ha(-1) yr(-1) for the N fertilizer applied plots, which were decreased by 24% and 16% due to biochar application, respectively. In addition, both potential nitrification (PNR) and denitrification (PDR) rates were stimulated by biochar amendment, which significantly increased the abundances of bacterial amoA (AOB), nirK and nosZ genes. Changes in the composition of the N2O-related microbial functional community were closely associated with soil PNR, pH, DOC, and NO3--N contents. The ratios of NO/N2O were mainly lower than 1, suggesting that N2O was produced mostly through denitrification rather than nitrification. There were negative correlations between soil N2O and NO emissions and soil PDR and pH, and soil N2O emissions were negatively correlated with nosZ gene abundances. Together, the decrease in N2O and NO emissions following biochar application could be largely attributed to the enhanced denitrification process, in which biochar enriched the nirK and nosZ genes abundance, resulting from the enhancement of soil DOC and pH in acidic soils.