Effects of Iron-Modified Biochar on N2O Emissions and Nitrogen-Cycling Microbial Potential During Laboratory Incubation of an Alkaline Loess Soil

Alkaline agricultural soils pose a significant challenge for N2O mitigation. Iron−modified biochar (FBC) holds potential for greenhouse gas reduction, yet its mechanisms in alkaline soils remain poorly understood. This study focused on alkaline loess in Yan’an, Shaanxi, and established three treatments (CK, BC1%, FBC1%), each with three replicates. Through a 90-day indoor cultivation experiment, combined with Raman spectroscopy and high-throughput sequencing, the study investigated the mechanism by which FBC suppresses N2O emissions. Results showed that the specific surface area of FBC increased by 46.2%, the ID/IG ratio decreased, the arrangement of carbon atoms became more ordered, and the carbon-based microcrystalline structure was stabilized. After 90 days of cultivation, the pH of the FBC-treated sample was 0.17 lower than that of the CK-treated sample, the total nitrogen content decreased by 11.90%, and the cumulative N2O emissions were reduced by 23.20%. High-throughput sequencing revealed microbial mechanisms underlying N2O mitigation: Proteobacteria relative abundance significantly increased by 5.52%, while nitrogen cycling functional genes (amoA, nirK, etc.) showed selective relative abundance responses. These findings demonstrate that under alkaline loess conditions, the core mechanism of FBC emission reduction lies in the synergy of physical retention and biochemical regulation, where optimized pore structures hinder nitrogen oxide diffusion into anaerobic microenvironments. This study elucidates the dual efficacy of iron-modified biochar in nitrogen retention and gaseous emission reduction in alkaline soils, offering a promising material strategy for agricultural N2O reduction.