Effect of biochar as a support on mitigation of N2O emissions by zero valent iron from paddy soils: A chemical and microbial mechanistic investigation

Abstract

Biochar (BC) and zero valent iron (ZVI) have great potentials for mitigating soil nitrous oxide (N₂O) emissions. However, effects and the involved chemical and microbial mechanisms of the interaction between them on N₂O emissions remained unclear. Herein, BC, ZVI, and the composite ZVI/BC were used to investigate their effects on nitrate-induced N₂O emissions from paddy soils. Abiotic aqueous and soil microcosm incubations were conducted to explore the response of N₂O emissions to these materials. Results showed that additions of BC, ZVI, and ZVI/BC lowered N₂O emissions from nitrate-fertilized paddy soils, with mitigation efficiencies of 67.0 %, 36.3 %, and 54.0 %, respectively. Specifically, BC increased soil pH and dissolved organic carbon, which stimulated the growth of N₂O-reducing microorganisms (e.g., Bryobacter and Candidatus_Solibacter) and inhibited the growth of N₂O-producing microorganisms (e.g., Terrabacter, Conexibacter, and Pseudeurotium), thereby increasing the nosZ/(nirK + nirS) ratio and decreasing N₂O emissions. Oppositely, abiotic aqueous experiment indicated that ZVI resulted in the accumulation of nitrite (NO₂⁻-N) and N₂O. However, ZVI converted nitrate nitrogen (NO₃⁻-N) to ammonium nitrogen (NH₄⁺-N) via abiotic reduction, decreasing denitrification substrate and N₂O emissions. Further, BC as a support inhibited the chemical reaction of ZVI with NO₃⁻-N to produce NO₂⁻-N and N₂O and improved the mitigation of N₂O emissions by ZVI via inhibiting NO₃⁻-N reduction. Overall, BC has great potential for decreasing N₂O emissions and can further enhance the performance of ZVI in mitigating N₂O emissions from paddy soils. The present study provided a valuable reference for mitigating soil N₂O emissions.