Cadmium-containing straw return obstructs carbon storage and aggravates nitrogen deficiency in paddy soils

Straw return is environment-friendly strategy for enhancing carbon (C) storage and improving soil health in paddy soils. However, in Cd-contaminated regions, this practice contributes to the reintroduction of cadmium (Cd) from rice straw into soil and may pose risks to carbon and nitrogen (N) transformations. To date, the fate of C and N in paddy soils incorporated with Cd-containing straw is poorly understood. To clarify this, we tracked C and N dynamics over a 91-day incubation of paddy soils amended with rice straw with four Cd levels (0, 5, 10 and 20 μg g^-1) and at four addition rates (0, 0.5%, 1% and 3%). Our results showed that Cd-containing straw significantly inhibited soil organic C accumulation. Notably, after 91-day incubation, addition of 3% Cd-containing straw decreased soil organic C by 2.2%-7.3% relative to the Cd-free straw. This was likely driven by enhanced microbial demands of labile C and N for detoxification, lowered metabolic efficiency, stimulated fungal excavation of organic matter and modified microbial community structure. Meanwhile, inhibited Cd-containing straw decomposition and N-fixation gene decreased labile N concentration, exacerbating N deficiency under Cd-free straw return. Notably, incorporation of 3% Cd-containing straw decreased total dissolved N by 6.6%-27.4% but increased DOC/TDN ratio by 4.1%-21.1% relative to the Cd-free straw. These detrimental impacts become more pronounced for straw containing over 5 μg Cd g^-1. During safe utilization of Cd-contaminated paddy soils, safe threshold for straw Cd should be evaluated to consider both Cd accumulation and its impact on soil nutrient cycling.