Nitrous oxide emissions in Fe-modified biochar amended paddy soil are controlled by autotrophic nitrification

We investigated the potential of ferric iron-modified biochar to lessen autotrophic nitrification and lower nitrous oxide (N2O) emissions in paddy soils. A 15N tracing incubation was conducted to investigate the changes in soil gross nitrogen (N) transformations under various biochar amendments (control, unmodified biochar, and Fe-modified biochar). Acetylene and 1-octyne were used to assess the relative contributions of ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) to N2O emission from paddy soil. The Fe-modified biochar increased the rate of NH4+ immobilization by 26 % and 383 % compared to the control and unmodified biochar treatments, respectively. The gross rate of autotrophic nitrification was reduced to 5.43 μg N g−1 d−1 in the Fe-modified biochar treatment, compared to 6.74 μg N g−1 d−1 in the control treatment and 9.38 μg N g−1 d−1 in the unmodified biochar treatment. Soil pH had varying effects on N2O emissions involving AOB and AOA. The N2O yields of AOA were more sensitive to Fe-modified biochar applications. AOB, specifically the Nitrosopira-AOB genus, dominated N2O production in all treatments. Overall, this study suggests that Fe-modified biochar holds greater potential than unmodified biochar in reducing N2O emissions from paddy soils by stimulating NH4+ adsorption, restraining autotrophic nitrification rates, and AOB-dominant N2O production pathways.