Differential impacts of maize straw return on cadmium and arsenic bioavailability in alkaline-contaminated wheat-maize rotation systems: Insights from a field study

Managing cadmium (Cd) and arsenic (As) contamination in alkaline agricultural soils remains a significant challenge. This field-scale study evaluated the impact of maize straw (MS) return on Cd and As dynamics, bioavailability, and microbial responses in Cd- and As-contaminated soils under a wheat-maize rotation system in Xinxiang, China. Two MS return rates (X-MS-1% and X-MS-2%) were applied and monitored across wheat growth stages. The X-MS-2% return increased dissolved organic carbon (DOC) by 35.2% and slightly elevated the surface soil pH (by ≤ 0.4 units) at ripening. Bioavailable Cd decreased by 38.3% at ripening, whereas bioavailable As increased by 21.6%, indicating contrasting mobilities of these heavy metals (HMs). Sequential extraction revealed that X-MS-2% facilitated the stabilization of Cd into less available fractions but enhanced the mobility of As, particularly in surface layers. Consequently, Cd uptake in wheat grains decreased by 58.3%, while As increased by 23-25.9% from flowering to ripening. High-throughput sequencing revealed that X-MS-2% return enhanced Actinobacteria (0.25-0.29%), Firmicutes (0.05-0.14%), and Bacillus (0.10-0.28%), taxa linked with DOC fluxes, Cd, and metal dynamics. These findings demonstrate that MS return effectively mitigates Cd bioavailability and uptake in wheat tissues but may elevate As mobility, underscoring the need for balanced management to minimize secondary contamination risks.