Arsenic (As) contamination is a global problem. Rice is efficient in accumulating As in grains, thus rice consumption has become the main route of dietary As intake for populations consuming rice as the staple food. Organic substances are recommended to be added to the soil for carbon sequestration and soil fertility. However, organic substances application has been demonstrated to remarkably influence As mobility and transformation in soil.

Straw incorporation is widely performed in As contaminated paddy fields. Our results showed that rice straw application significantly increased the abundance of iron-reducing bacterium, and the significant positive correlation between dissolved As with Fe and the abundance of iron-reducing bacterium suggested the coupling of As and Fe reduction mediated by iron-reducing bacterium. In addition, rice straw application significantly increased the abundance of methanogenic bacteria, and the significant positive correlation between DMA in rice grains and the abundance of methanogenic bacteria indicated that methanogenic bacteria could be involved in As methylation after straw application. From this study, it is clealy demonstrated that rice straw application significantly enhanced As release from iron minerals and promoted As(V) reduction to As(III) in soil, accelerated As methylation, enhanced As volatilization and elevated DMA accumulation in polished grains, but decreased As(III) accumulation.

Rice straw can be degraded to humic substances (HS) by microorganisms in the soil. Humic acid (HA) and fulvic acid (FA) are dominating humic substances (HS) in soil. We found that fulvic acid addition caused significant enrichment of Desulfitobacterium (41-fold), and significantly increased the concentrations of As (12-fold), iron (Fe; 20-fold), manganese (Mn; 3-fold) and acetic acid (3-fold) in soil porewater. In contrast, HA slightly increased the concentrations of As (1.2-fold) in porewater, and little effect on Fe, Mn and acetic acid. Under both FA and HA treatments, As was primarily complexed with HS, only a small fraction of free As was available for methylation. Therefore, although there was abundant putative As methylating microorganisms, such as Methanobacterium and Methanosarcinaceae, As methylation was limited.

These results illustrated that different organic substances exhibited different effects of on As mobility and transformation in paddy soil through regulating soil microbial community. These studies would advance our understanding of the associations among As, Fe and organic substances through microorganisms in paddy soil.