Earthworms can mobilize soil arsenic through their casts

The mobilization of arsenic (As) in paddy soil has received much attention because it might accelerate the transfer of As from soil to rice. This study aims to elucidate whether earthworms can mobilize As through their casts. Cast samples were collected from 23 different paddy fields in the Red River delta. We first analysed different forms of As through fractionation and then performed batch experiments under reducing conditions to identify factors that govern the mobility of As in casts. Because the dissociation of casts may induce colloids that carry As, the colloidal properties of cast suspensions were also examined. The median value of As in casts (obtained from aqua regia digestion) was 5.11 mg kg^-1, which was lower than that in the surrounding soil (6.7 mg kg^-1). Compared with the surrounding soil, casts contain less As, possibly because cast As is more labile and more easily lost due to leaching. Various processes, including the reductive dissolution of Fe oxides, decomposition of organic matter, and sorption competition of soluble anionic substances, such as P, Si and DOC, were found to strongly correlate with the release of As from casts. We propose that earthworms, via their casts, may accelerate the As cycle in paddy soils, potentially intensifying As exposure to human health. The dissociation of cast could release colloids containing As; therefore, the cotransport of As with cast-induced colloids should also be considered in future works.