Soil redox tuning for regulating Ni, As and Cd uptake by rice

Considered an emerging contaminant in rice, nickel (Ni) will be subject to European regulatory limits starting from 2026. Its presence in rice grain raises food safety concerns due to its toxicity and allergenic potential, alongside carcinogenic elements like arsenic (As) and cadmium (Cd). European rice is mainly grown in the Mediterranean area, where geogenically Ni-rich soils increase the risk of grain accumulation, although agronomic strategies capable to limit its mobility are still to be developed. The modulation of soil redox potential (Eh) in paddy fields, achieved through a dry–wet alternating irrigation regime with controlled timing, represents a key strategy for regulating the mobilization of contaminants through redox-sensitive processes. Alternate wetting and drying (AWD), increasingly adopted by rice farmers to reduce water use and methane emissions, has contrasting effects on the accumulation of As and Cd in rice. Water management, when aligned with plant development, can reduce the accumulation of As and Cd by acting differentially on key uptake phases: reproductive phase for As, pre-flowering to maturity for Cd. In contrast, the effect of water regime on Ni behavior in the soil–plant system—particularly in relation to phenological stage and mechanisms of uptake and translocation—remains poorly understood. To explore these dynamics, a mesocosm experiment was conducted by applying eight irrigation treatments, including continuous flooding (CF), AWD, and short drying periods timed to specific growth phases. Drying periods applied in the early growing phases often reduced Ni availability in solution and its uptake during stem elongation and panicle initiation, but had limited impact on final Ni accumulation in plant tissues. In contrast, late-season dry treatments (applied at milky and dough stages) were more effective in reducing Ni content in rice plant, suggesting root–shoot remobilization mechanisms occurring during advanced growth phases. Although drying led to a moderate reduction in Ni concentration in solution, a drop in the Fe/Ni ratio was observed, which may affect the ionic competition at the root level and influence Ni uptake through non-specific transporters shared with Fe. These results provide a basis for optimizing soil redox tuning through appropriate water management. The alternation of soil submersion with short periods of aerobic conditions applied at targeted growing phases could support the simultaneous reduction of Ni, As, and Cd concentrations in rice grains.