Desilification of phytolith exacerbates the release of arsenic from rice straw.

Arsenic (As) turnover in rice paddy agro-ecosystems has received much attention because As can enter the food chain through its accumulation in rice, thereby affecting human health. Returning straw to soil is a common practice to retain nutrients for soil and crops, but it also cycles As within the rice paddy field ecosystems. However, there is still a lack of detailed understanding of the fate of As in rice straw, and how or to what extent it is recycled back into the soil environment. This study aims to elucidate the relationship between the microstructure of rice straw and the release of As during rice straw decomposition. The microstructure of rice straw was found to comprise both organic and silica (phytolith) components. These two constituents are inter-embedded to form a composite-like structure that contains up to 6.48 mg As Kg-1. The 30-day batch experiments revealed that the biochemical release of As simultaneously depends upon the decomposition of the organic component and the desilicification of the silica component. Accompanying the release of As was the release of other elements such as Fe, Al, P and S. These elements can further interact with As to form less mobile compounds. The introduction of either Trichoderma harzianum or Bacillus velezensis was expected to accelerate the decomposition of rice straw, and enhance the silica dissolution, hence contributing to an increase in the As release. Despite these expectations, our observations showed the opposite effects. Microorganisms presumably have facilitated the change in solution chemistry or the inclusion of As into the newly-formed precipitates. The biochemical decomposition process can reduce straw particle size, while the negatively-charge surface will involve microsized straw particles in the electrostatic interaction, thereby favoring the dispersibility state. Therefore, the co-transport of micro-sized straw particles with As under field conditions should not be neglected.

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.