Biochar produced at high temperature mitigates N2O emission and promotes nitrogen retention in subtropical forest soils

Abstract

Biochar is produced by burning biomass under oxygen-limited conditions, and it has been widely used as a soil amendment to improve soil functions such as nutrient retention. However, whether the impact of biochar application on soil nitrogen (N) transformation and N₂O emission varies with the pyrolysis temperature remains unclear, especially in different forest types in subtropical regions. In this study, a 60-day laboratory incubation experiment was conducted to evaluate the impact of biochar with different pyrolysis temperatures (300°C [BC300], 500°C [BC500], and 800°C [BC800]) on net N transformation rates and N₂O emission in soils collected from Castanopsis kawakamii dominated natural forest (NF) and Chinese fir (Cunninghamia lanceolate, CF) plantation in subtropical China. The results showed that the application of biochar significantly increased soil ammonium (NH₄⁺) content (p < 0.001) but reduced nitrate (NO₃⁻) content (p < 0.001) compared with the control. The soil NH₄⁺ content of the BC800 treatment was significantly higher than that of other treatments (p < 0.001). Biochar application significantly reduced soil net N mineralization (NR_min) and nitrification (NR_nit) rate (p < 0.001), but increased net ammonification (NR_amm) rate (p < 0.001). The application of biochar led to a remarkable decrease in cumulative N₂O emission compared to the control (p < 0.001). In particular, soils treated with high-temperature biochar emitted significantly lower N₂O compared to other treatments (p < 0.001). The partial least squares path model demonstrated that biochar influenced N₂O emission through a direct effect in NF soil and an indirect effect in CF soil. This study highlights the distinct role of biochar, particularly that produced under high pyrolysis temperatures as a soil amendment to mitigate N₂O emission and promote N retention in both subtropical natural and planted forests.