Impacts of In Situ Wheat Straw Incorporation Methods on Cadmium Behavior in Soil–Rice Systems

Cadmium (Cd) contamination in paddy soils poses a severe threat to food safety. Although straw incorporation is a key approach to sustainable agriculture, the mechanisms underlying its regulatory effects on safe rice production in Cd-contaminated fields remain unclear. This field study, conducted at two Cd-contaminated sites with two rice cultivars differing in Cd accumulation (low-Cd ZLY8612 and high-Cd YXY2115), evaluated five wheat straw management practices: straw removal (CK), straw mulching (SM), straw incorporation (SI), straw incorporation with organic fertilizer (SOI), and straw-derived biochar incorporation (SBI). The primary findings revealed that SOI and SBI could effectively reduce Cd availability and promoted Cd transformation to residual fractions, with SBI showing superior immobilization effects. SBI also enriched beneficial taxa (Bacillus, Sphingomonas, and Flavisolibacter), increased Proteobacteria, and reduced Chloroflexi and Acidobacteriota. All treatments enhanced rice yield; however, only SBI reduced grain Cd in the high-Cd cultivar to <0.2 mg/kg with high-Cd soil. Collectively, the combined application of straw-derived biochar incorporation and low-Cd-accumulating rice cultivar is a reliable and recommendable agronomic strategy for safe grain production and sustainable straw recycling in Cd-contaminated rice–wheat rotation fields.