Microscale zero-valent iron creates a trade-off between arsenic remediation and methane emissions in paddy soil–rice systems
Abstract
Arsenic (As) contamination and methane (CH 4 ) emissions are major environmental concerns in flooded paddy fields. Microscale zero-valent iron is widely used for As immobilization owing to its high affinity for As, yet its effects on CH 4 cycling and the underlying microbial mechanisms remain poorly understood. This study explored coupled responses of As mobility, rice As accumulation and CH 4 dynamics to mZVI via microcosm incubation and rice-planting trials. mZVI substantially lowered As mobility by altering soil As speciation. It also promoted the stabilization of As in Fe-bound fractions. Magnetic separation further verified that aging ZVI-derived Fe minerals contributed to As sequestration, with magnetic fractions containing 3.23-fold higher As concentrations than non-magnetic solids. Accordingly, mZVI markedly decreased As accumulation in rice roots, stems, leaves, and grains by 83.3%, 92.9%, 87.4%, and 34.0%, respectively. It also reduced grain inorganic As(V) and dimethylarsinic acid by 68.2% and 77.3%, respectively. However, the environmental benefits of mZVI for As remediation were accompanied by enhanced CH 4 emissions. Compared with the control, mZVI accelerated CH 4 production during anaerobic incubation. This enhancement was associated with improved methanogenic potential, as indicated by increased mcrA abundance and enrichment of key methanogenic genera, including Methanobacterium, Methanocella, and Methanosarcina. Despite enhanced methane oxidation potential with higher pmoA and enriched type-II methanotroph Methylocystis, such promotion could not counteract intensified methanogenesis. Overall, our findings demonstrate that zero-valent iron amendment creates a trade-off between effective As remediation and increased CH 4 emissions in paddy soils. The results highlight the necessity of integrating greenhouse gas mitigation strategies with iron-based remediation practices to achieve sustainable management of As-contaminated rice ecosystems.