Short-term effects of rice straw biochar on sorption, emission, and transformation of soil NH₄⁺-N.

Although previous work has explored and reported the influence of biochar on the fate and transformation of soil nitrogen (N), the governing mechanisms are still unclear. In this study, an incubation experiment was first conducted to investigate the overall fate of NH4 (+)-N in two soils: Gleyi-Stagnic Anthrosols (pH = 6.31) and Argi-Udic Ferrosols (pH = 5.05) amended with rice straw biochar. In addition, batch sorption experiments were designed to explore the potential mechanisms of NH4 (+)-N transformation in biochar-amended soils. Results showed that the KCl extractable NH4 (+)-N concentrations in the amended Anthrosols and Ferrosols decreased by 9-35 and 5-22 %, respectively, compared to the unamended soils, but limited nitrification of NH4 (+)-N into NO3 (-)-N was observed in both soils. In Anthrosols, biochar increased NH4 (+)-N sorption, but it decreased N biotransformation (mineralization, nitrification, and assimilation) into NO3 (-)-N. It implies that the chemical sorption is a dominant process in the biochar-amended soil. As for Ferrosols, biochar seemed to have less effect on either NH4 (+) sorption or biotransformation. Biochar addition promoted NH3 emission in both soils due to the elevated pH, but the overall amount of the N emission losses were negligible.