Water management impacts on the accumulation of potentially toxic elements in rice grain: A meta-analysis

Rice is the primary staple for over half of the world’s population and can be a source of potentially toxic elements (PTE), including heavy metals and metalloids. Non-continuous flooding practices (NCFP) reduce methane emissions and water use. However, they can increase or decrease the bioavailability of PTE through their effect on soil redox potential, pH, and other geochemical processes. This meta-analysis aims to evaluate the effect of NCFP on PTE accumulation in rice grain, explain the mechanisms of their observed increase/decrease, and highlight research gaps. Fifty-five studies comparing PTE grain concentration under continuous flooding and NCFP were selected, with 130, 67, 100, 36, 30, 19, 11, and 11 observations for As, inorganic arsenic (iAs), Cd, Pb, Cr, Hg, MeHg, and Ni, respectively. Data on PTE other than As and Cd were limited; thus, for those both pot and field studies were included. On average, NCFP decreased As, iAs, Pb, Hg, and MeHg grain concentrations by 39%, 24%, 21%, 63%, and 81%, but increased Cd and Ni concentrations by 115% and 187%, respectively. The effect on Cr was not clear due to discrepancies between pot and field experiments. Severe drying resulted in greater decreases in As and iAs but showed no significant difference from mild drying for Cd. Further field studies are needed to clarify NCFP effects, especially for Ni and Cr, where data are limited, results were highly variable, and mechanisms governing availability less understood.