Migration and transformation of Cd in four crop rotation systems and their potential for remediation of Cd-contaminated farmland in southern China

A crop rotation system combining agricultural production with phytoremediation is an economical and sustainable method of remediation of cadmium (Cd)-contaminated farmland. This study focuses on migration and transformation of Cd in rotation systems and the influencing factors. In a two-year field experiment, four rotation systems were eval-uated: traditional rice and oilseed rape (TRO), low-Cd rice and oilseed rape (LRO), maize and oilseed rape (MO), and soybean and oilseed rape (SO). Oilseed rape is a remediation plant in rotation systems. Compared to 2020, the grain Cd concentrations of traditional rice, low-Cd rice, and maize in 2021 decreased by 73.8%, 65.7%, and 24.0% (below the safety limits), respectively. However, soybean increased by 71.4%. The LRO system featured the highest oil content of rapeseed (about 50%) and economic output/input ratio (1.34). Removal efficiency of total Cd in soil was 10.03% (TRO) > 8.3% (LRO) > 5.32% (SO) > 3.21% (MO). Crop uptake of Cd was influenced by bioavailability of soil Cd, and soil environmental factors regulated the bioavailable Cd. Redundancy analysis (RDA) indicated that soil nitrate-nitrogen (NO3--N) had a dominant impact on bioavailable Cd in soil, with variance contributions of 56.7% for paddy-upland (TRO and LRO) and 53.5% for dryland (MO and SO) rotation systems. The difference reflected that ammonium N (NH4+-N) was a secondary factor in paddy-upland rotations, while it was the available phosphorus (P) in dryland rotations, with variance contributions of 10.4% and 24.3%, respectively. The comprehensive evaluation of crop safety, production, economic benefits, and remediation efficiency revealed that the LRO system was efficient and more acceptable to local farmers, providing a new direction for the utilization and remediation of Cd-contaminated farmland.