Impact of Plant Residues on Arsenic Bioavailability in Paddy Soil: Exploring the Role of Coupled Iron–Nitrogen Cycling

Soil arsenic (As) contamination poses significant threats to human health, particularly through rice consumption. While the effects of rice straw return on soil As bioavailability have been extensively documented, the regulatory effects of green manure (such as alfalfa) and their differential mechanisms remain unclear. More importantly, how plant residues regulate As mobilization through coupled iron‐nitrogen cycling currently lacks systematic investigation. Composting is considered to stabilize organic matter, yet its influence on coupled iron‐nitrogen cycling processes and As bioavailability requires further mechanistic exploration. This study conducted a pot experiment to compare the effects of direct application versus composted application of rice straw and alfalfa on As speciation, bioavailability, and subsequent accumulation in rice. The results demonstrated that both straw and alfalfa promoted As release through ammonium oxidation and Fe(III) reduction, with alfalfa exhibiting a more pronounced activation effect due to higher dissolved organic carbon (DOC) production. In contrast, composting treatment effectively alleviated As mobilization and reduced As accumulation in rice plant tissues by decreasing DOC content and slowing organic matter decomposition. This study elucidates the mechanisms by which plant residue type and processing method differentially regulate As migration through coupled iron‐nitrogen cycling, providing important theoretical insights and practical guidance for safe utilization of As‐contaminated paddy fields: composting pretreatment can effectively reduce As accumulation risk in rice while promoting plant growth.