Variable effects of biochar application to soils on nitrification-mediated N₂O emissions

Although a meta-analysis on biochar's effects on N₂O emission reported an overall reduction in N₂O emission by adding biochar to the soils, there are still variations in the changes in N₂O emission, especially from field results. The objectives of this study are 1) to compare the effects of biochar addition on N₂O emission between three agricultural upland field experiments, where soil water status was dry favoring nitrification and 2) to identify main factors explaining biochar's variable effects on N₂O emission. Three field experiments were conducted: Exp A in the cultivated grassland treated with rice husk biochar at 2 ton ha^-1 + urea (CHAR) and with urea only (CON); Exp B in the cabbage field with CHAR and CON treatments; and Exp C in the pepper field with CHAR, CON, and CHAR + DCD (dicyandiamide, nitrification inhibitor) treatments. In Exp A and C, cumulative N₂O emissions significantly increased by 82.5% and 55.8% in the CHAR than CON treatments, respectively, while in Exp B, there was no difference in cumulative N₂O emission between the CHAR and CON. Based on results from using nitrification inhibitor and soil % water filled pore space (WFPS), we assumed that the main N₂O production mechanism was nitrification. Our results suggest that soil water status right after urea application is the primary determinant of different effects of biochar on N₂O emission in addition to soil C status and biochar's adsorption. Principal component analysis using the 25 compiled data also supported our results. This study identified the specific field conditions under which biochar could have stimulating effects on N₂O emission. Mitigation potential of biochar application should be reconsidered if biochar and urea were amended to dry soils with low C contents.