Coupling metabolisms of arsenic and iron with humic substances through microorganisms in paddy soil

Humic acid (HA) and fulvic acid (FA) are dominating humic substances (HS) in soil. In this study, the effects of HA and FA addition (0.2%–1.5%) on arsenic (As) mobility and microbial community composition in paddy soil were investigated. FA significantly increased the concentrations of As (12-fold), iron (Fe; 20-fold), manganese (Mn; 3-fold) and acetic acid (3-fold) in soil porewater, and also caused significant enrichment of Desulfitobacterium (41-fold). Furthermore, the FA addition significantly increased the relative abundance of Bathyarchaeota (4-fold), a microorganism that is suggested to be important for FA degradation. In contrast, HA slightly increased As (1.2-fold) in porewater, had little effect on Fe, Mn and acetic acid, and 1.5% HA addition significantly decreased As in porewater at day 14 (45%). Both HA and FA addition promoted As methylation. HA increased dimethylarsenate concentration and FA increased monomethylarsenate concentration in porewater. These results highlight the contrasting effects of different (HA vs. FA) organic substances on As fate in paddy soil and advance our understanding of the associations among As, Fe and organic substances through microorganisms in paddy soil.