The within-field spatial variation in rice grain Cd concentration is determined by soil redox status and pH during grain filling.

Rice is a major dietary source of the toxic trace metal Cd. Large variation in Cd concentration in rice grain was documented by global and regional surveys, with this variation difficult to predict from soil tests. Even within individual fields, a large spatial variation is often observed but the factors controlling this within-field spatial variation are still poorly understood. In the present study, we used field- and laboratory-based experiments to investigate the effects of a gentle slope gradient within paddy fields (a common farmers' practice to facilitate water flow from the inlet to the outlet) on Cd availability and grain Cd concentrations in unlimed and limed soils. In our field experiments, a gentle slope resulted in large spatial variations in soil redox potential (Eh) and pH upon soil drainage during rice grain filling. As a result of these variations in Eh and pH, there was a 6- to 7-fold spatial within-field variation in grain Cd concentrations, which were the highest in the irrigation inlet area associated with higher Eh values and the lowest in the outlet area with lower Eh values. Our results highlight that soil Eh, and more importantly, field-moist soil pH during grain filling determine grain Cd concentration and accordingly, incorporating measurements of soil redox status (or water content) and pH of field moist soils (rather than air-dried soils) during grain filling may improve the prediction of grain Cd concentrations. Delaying drainage during grain filling or increasing soil pH by liming is effective in reducing grain Cd accumulation.