Mercury methylation and bioaccumulation in purple paddy soil systems with different acid-base properties

Paddy soil is recognized as a hotspot for mercury (Hg) transformation. Soil acid-base property (expressed as pH) plays a crucial role in Hg methylation and accumulation in paddy systems. However, it is challenging to study this process in soils with varying pH values due to the rarity of a single soil type spanning a wide pH range. Purple paddy soil, vital for rice cultivation in southwestern China, occurs naturally in three types based on pH: acidic, neutral, and calcareous. This variation makes purple paddy soil an ideal subject for these studies. Thus, this study investigated Hg transformation and bioaccumulation across these three purple soil types. The results showed that during the rice growing seasons, both methylmercury (MeHg) concentration and methylation potential were higher in acidic purple paddy soil than in neutral and calcareous types. In addition, total mercury (THg) and MeHg concentrations in plant tissues grown in acidic purple paddy soil were significantly higher than in other two purple soil types. These results suggest that acidic purple paddy soil was conducive to Hg methylation and accumulation in rice plants. Given that soil acidification is becoming a serious global issue, the findings could offer fundamental data on the potential risk of Hg exposure due to soil acidification.