N 2 O Emissions Mitigation in Acidic Soil Following Biochar Application Under Different Moisture Regimes

Managing soil pH has been recognized as one of the promising options for N2O emission mitigation in acidic soils. Rice-straw biochar (BC) application to acidic soils can not only ameliorate soil acidity but also influence N2O emissions. We investigated the impact of various levels of rice-straw-derived biochar, a control (no biochar), 2% biochar, and 4% biochar under 50% and 90% water-filled pore space (WFPS) values. In comparison with the application of biochar at 2%, the application of biochar at 4% more pronouncedly altered soil properties (pH, ammonium (NH4+-N), nitrate (NO3-N), microbial biomass carbon (MBC), and abundance of nosZ and nirK genes). Similarly, more noticeable changes in soil properties were noted under 90% WFPS than under 50% WFPS. The soil pH increased from 5.67 to 7.29 with the 4% biochar application. In comparison with those following the 2% biochar application and the control, soil mineral N and the abundance of nosZ and nirK genes following the 4% biochar application were more augmented, thereby leading to a remarkable reduction in soil N2O emissions. The MBC content in the soil also increased with the BC applications, and the maximum MBC contents of 655 and 428 mg kg−1 dry soil were recorded with the 4% biochar application under 50% and 90% WFPS, respectively. Moreover, in comparison with the control, 4% BC mitigated soil N2O emissions by 83%, whereas cumulative N2O emissions were mitigated by 49%. In comparison with 90% WFPS, 50% WFPS produced 35% more N2O emissions. However, the biochar applications significantly (p < 0.05) reduced N2O emissions under both WFPS values owing to an increase in soil pH, which activated mineral N (NH4+-N and NO3−-N) and enhanced the abundance of nosZ and nirK genes. These results suggest that biochar applications can substantially diminish soil N2O emissions by triggering soil pH, soil C and N pools, and the abundance of nosZ and nirK genes.