Citric acid modified biochar application at a low dosage can synchronically mitigate the nitrogenous gas pollutants emission from rice paddy soils.

Raw biochar with high pH possibly stimulated ammonia (NH) volatilization in the agricultural soil. We hypothesized that the modified biochar (MBC) with low pH can synchronically decrease the NH and nitrous oxide (NO) losses. We performed a two-year experiment to clarify how citric acid MBC influence the NH volatilization and NO emission as well as the underlying mechanisms. Two typical paddy soils, i.e., Hydragric Anthrosol and Haplic Acrisol, receiving equal urea N with 240 kg ha but varied rates of MBC with 0, 5, 10, and 20 t ha (named Urea, Urea + MBC5, Urea + MBC10, and Urea + MBC20, respectively) were studied. The results showed that MBC-amended treatments effectively mitigated the NH volatilization from Hydragric Anthrosol and Haplic Acrisol by 29.6%-57.9% and 30.5%-62.4% in 2017, and by 16.5%-21.0% and 24.5%-35.0% in 2018, respectively, compared to Urea treatment. In addition, significantly lower NO emissions with averaged 38.3% and 43.1% in 2017, and 51.7% and 26.7% were recorded under Hydragric Anthrosol and Haplic Acrisol, respectively, following the MBC application (P < 0.05). Increased MBC addition performed higher efficacy on mitigating NH volatilization, particularly in the first rice season, while this "dosage effect" was not found for NO reduction. Lowered pH in overlying water, enhanced adsorption of NH-N and its nitrification rate likely contributed to the lower NH volatilization as result of MBC addition. The nirS and nosZ gene copies were not changed by MBC, while the nirK gene copies were decreased as result of MBC amendment by 8.3%-25.2% under Hydragric Anthrosol and by 21.8%-24.9% under Haplic Acrisol. Consequent lower ratio of nirK/(nirS + nosZ) explained the mitigation effect of MBC on NO emission. In conclusion, the present two-year study recommends that MBC applied at a low dosage can perform positive effect on controlling the nitrogenous gas pollutants from paddy soil.