Biochar stimulates nitrogen loss in anoxic soil through ammonium oxidation coupled with iron reduction

Anaerobic ammonium oxidation coupled with iron (Fe) reduction, known as Feammox, is an important nitrogen (N)-cycling pathway in anoxic soils. Biochar, widely employed as a soil amendment, has been reported to influence N dynamics through its redox-active moieties, but the effects on Feammox of biochar produced with different pyrolysis temperatures remain poorly understood. We conducted a slurry incubation in rice-paddy soil with ¹⁵N-isotope tracing to examine the impact of biochar pyrolyzed at 300 °C and 600 °C on Feammox under different Fe availability conditions (addition of ferrihydrite, goethite, or no Fe). Biochar pyrolysis temperature and Fe availability impacted rates of Feammox. Specifically, adding biochar pyrolyzed at 600 °C to soil at a 2 % mass ratio stimulated N loss through Feammox by 0.03 to 0.07 μg N g⁻¹ soil d^-1, likely by facilitating electron transfer via its surface redox-active moieties. Feammox was highly correlated with Fe reduction despite contributing only a small fraction to overall Fe reduction. Feammox rates were greatest with ferrihydrite addition and lowest with no Fe addition, and correlated with the abundance of a known Feammox bacterium (Acidimicrobiaceae bacterium A6). In contrast, biochar pyrolyzed at 300 °C did not stimulate Feammox, likely due to a lack of electron shuttling capacity. Overall, our findings highlight a stimulation effect of biochar pyrolyzed at high temperature on promoting N loss as dinitrogen, underscoring its potential utility for environmentally-friendly removal of excess N from anoxic soil.