Analysis of the phytoremediation potential, rice safety, and economic benefits of light to moderate Cd-contaminated farmland in oilseed rape-rice rotation with straw removal: A three-year field trial.

Under the dual pressures of food security and soil cadmium (Cd) pollution in China, the use of an oilseed rape-rice rotation system and phytoremediation has been proposed as an effective measure to extract heavy metals from soil, achieve safe rice production, and alleviate soil heavy metal stress. A three-year field rotation experiment by straw removal was conducted in light to moderate Cd-contaminated soil in Hunan, China. The experiment involved rotating two oilseed rape varieties, LSYH and ZYZ, with two rice varieties, the low-accumulation variety XWX and the high-accumulation variety TYHZ. The structural equation model (SEM) indicated that the correlation coefficients of total nitrogen (N) and available phosphorus (P) with CaCl-Cd in the soil under oilseed rape-rice rotation were higher than those in rice monoculture, indicating that changes in soil nutrients have a significant impact on CaCl-Cd during rotation. During continuous rice monoculture, the Cd content in brown rice exceeded safety standards in all three years. Crop rotation effectively reduced the Cd content in the aboveground parts of rice, particularly inhibiting its transfer to brown rice. By the second year of rotation, the minimum Cd content in brown rice from XWX and TYHZ was only 0.10 and 0.11 mg kg⁻, respectively. Cd levels in rice met the safety standard (<0.2 mg kg⁻). Although the Cd content in the third-year rice was higher than in the second year, it remained significantly lower than in the first year. The TPE in the aerial portion for the ZYZ-XWX rotation was 4.88%, while for the rotation of ZYZ-TYHZ rotation, it was 6.37%. Economic benefits were enhanced by this rotation mode, with profit-to-cost ratios exceeded 1. Overall, oilseed rape-rice rotation is an economical and environmentally friendly method for remediating soil Cd contamination, but it requires selecting the appropriate varieties.