Biochar stimulates NH₄⁺ turnover while decreasing NO₃^- production and N₂O emissions in soils under long-term vegetable cultivation

A ¹⁵N-tracer incubation experiment was conducted to investigate the short-term effects of biochar on gross N transformation rates and nitrous oxide (N₂O) emissions in soils under 1-year and 10-year vegetable cultivations. Biochar was applied at three rates: 0 (control), 10, and 30 t ha^-1. Gross N transformation rates in the two vegetable soils varied in response to biochar application. Specifically, organic N oxidation into NO₃^- (O_Norg) was almost negligible in the biochar-amended soils, and biochar application at 10 t ha^-1 did not change either the rate of mineralization of organic N into NH₄⁺ (M_Norg) nor the inorganic N supply capacity (INS, O_Norg + M_Norg) in both soils, when compared to the control. However, 30 t ha^-1 biochar decreased INS significantly, by inhibiting M_Norg, in the 1-year vegetable soil but increased INS, by stimulating M_Norg, in the 10-year vegetable soil. The rates of NH₄⁺ oxidation into NO₃^- (O_NH4), NO₃^- immobilization into organic N, and dissimilatory NO₃^- reduction into NH₄⁺ were not influenced significantly by biochar application in the 1-year vegetable soil, resulting in no significant differences in NO₃^- production potential. Conversely, biochar decreased NO₃^- production potential significantly in the 10-year vegetable soil, by inhibiting O_NH4 and increasing NH₄⁺ immobilization into organic N (I_NH4), with more obvious effects under higher biochar application rates. Overall, the results demonstrate the capacity of biochar to stimulate NH₄⁺ turnover and to decrease NO₃^- production potential in soil under long-term vegetable cultivation; however, the effect is limited under short-term vegetable cultivation. In addition, N₂O emissions decreased significantly in biochar-amended vegetable soils.