Effect of different rotation systems on mercury methylation in paddy fields

Rice (Oryza sativa) paddy is the hotspot of mercury (Hg) methylation. Given distinct influences of rotation systems on the physicochemical properties of paddy soils, we hypothesized different rotation systems in rice paddies inducing a large difference in Hg methylation. Here, we investigated Hg species distribution, dissolved organic matter (DOM) features, and Hg methylation in five rotation systems (Other farmland newly reclaim into paddy field, i.e., NR-R; Drain the water in winter, i.e., DW-R; Flooding in winter, i.e., FW-R; Rape-Rice rotation, i.e., Ra-R; Wheat-Rice rotation, i.e., Wh-R) of paddy fields to identify such hypothesis. Results shown that FW-R had the strongest Hg methylation, followed by Ra-R and Wh-R, then DW-R, and finally NR-R. We further found that much higher soil organic matter (SOM) and organo-chelated Hg (Hg-o) from straw residues and root exudates were the main cause for the greater Hg methylation in FW-R, Ra-R and Wh-R. This was because the protein-like fraction of SOM facilitated the net production of methyl Hg (MeHg), meanwhile the humin-like fraction had a strong affinity to MeHg in paddy soils. Therefore, it can be concluded that paddy soil under DW-R was the optimum pattern in order to reduce the occurrence of Hg methylation. However, paddy soils under Ra-R and Wh-R were the recommendable patterns if the productivity of paddy fields was considered.