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

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 nutri-ent retention. However, whether the impact of biochar application on soil nitro-gen (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 degrees C [BC300], 500 degrees C [BC500], and 800 degrees 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 ammo-nium (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 (NRmin) and nitrification (NRnit) rate (p< 0.001), but increased net ammonification (NRamm) rate (p< 0.001). The application of biochar led to a remarkable decrease in cumulative N2O emission com-pared 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.