Availability and uptake of potentially toxic elements by rice as influenced by cover crop techniques

Italian rice farming, accounting for over 50% of European production, faces challenges in aligning environmental sustainability with food safety as demand for high-value organic products increases. Cover crops (CCs) with green manuring and mulching enhances soil fertility and weed control, reducing chemical inputs. However, respect other cereals, rice accumulate toxic elements, such as arsenic (As), cadmium (Cd), and nickel (Ni), in grain. Consequently, it’s urgent to understand how CCs influences their bioavailability in paddy fields and their absorption by rice. The mesocosm experiment presented analyzed the impact of CCs (vetch and ryegrass) and termination techniques (rolling and chopping) on the biogeochemical cycle of PTEs and nutrient. The study compared the effects of different residue management strategies—mulching, manuring, and soil without CCs. The results indicated that the CCs significantly released N-NH4+, P-PO43-, and DOC into the standwater. Chopped treatments accelerated the release more than rolled ones, with vetch increasing the levels of N-NH4+. This enhanced microbial degradation, improving nutrient cycling. The localization of crop residues (mulching vs. green manuring) influenced As, Cd, and Ni concentrations in the soil more than the CC species. In mulching, the diffusion of electron acceptors (DOC) into porewater promoted the reductive dissolution of iron, increasing the mobility of toxic elements adsorbed on it. In green manuring, compared to mulching, the highly reducing environment further amplified the release of As and Ni. Although the CCs enriched Cd and Ni in the standwater after flooding, it was the variation in soil solution contaminants they induced that regulated rice uptake, reflected in plant concentrations over the cropping cycle. While As increased during the vegetative phases following Fe(II), Ni in solution, after peaking, progressively decreased with the reduction in redox potential (Eh), showing higher concentrations in tillered plants. Additionally, the CCs reduced Cd bioavailability compared to the control, which had higher Cd levels in the plant. Overall, the CCs increased As in the grain, especially under green manuring, while reducing Ni concentrations in the grain. Thus, CCs regulate paddy contaminant dynamics through redox-driven mobility and organic matter complexation, leading to distinct accumulation patterns in plant and grain.