Advances in understanding Hg dynamics in mercury contaminated paddy soils under straw amendment

Abstract

It is well-known that interactions between Hg and organic matter could greatly affect biogeochemistry of Hg in soils, and its subsequent uptake by plants. Crop straw is mainly composed of organic matter, and its incorporation into soils may have impacts on Hg dynamics in soil-plant systems. Straw return is being widely encouraged, in order to mitigate straw-burning induced air pollution, as well as to promote plant growth by increasing soil fertility and improving the soil. Meanwhile, potential impacts of straw return on the ecological risk of metals in agricultural soils warrant investigation. While much progresses have been made in understanding the reduced metal bioavailability (e.g., for Cu, Cd and Zn) under straw amendment, little is known about the potential effects of straw return on Hg dynamics in soils and the associated ecological risks. Here, we summarize recent advances in understanding Hg dynamics in soil-rice systems under straw amendment. Recent studies revealed evidently enhanced ecological risk of Hg in paddy soils under straw amendment. Especially, straw return was found to have positive effects on methylmercury (MeHg) concentrations in soils, as well as those in crops. The underlying mechanisms may include: (1) Straw amendment has evident effects on the geochemical fractionation of Hg in soils, by facilitating transformation of Hg from the refractory fractions (e.g., residual fractions, including HgS) to more mobile fractions (e.g., organic-complexed fraction), which may increase availability of inorganic Hg to microbial methylators. (2) Transformation of inorganic Hg to MeHg was facilitated under straw amendment, which could be mainly attributed to the increased availability of inorganic Hg as well as enhanced activities of microbial methylators (e.g., sulfate reducing bacteria). (3) Straw return was found to have positive effects on Hg accumulation in grains of crops. The observed effects could be explained by the increased soil MeHg levels, enhanced mobility of MeHg in the presence of dissolved straw-derived organic matter, as well as changes in plant physiology, e.g., increased uptake of MeHg by root and MeHg transformation from other tissues to grains. Information summarized in this manuscript would improve mechanistic understanding about dynamics and risk of Hg in soil-plant systems and ensure appropriate straw management in Hg-contaminated areas. Particularly, more attentions should be paid to the interactions between 'fresh' organic matter (e.g., those from crop straw), which may have great impacts on Hg speciation, methylation, mobility, and bioavailability in soils. Furthermore, results gained in those pioneering studies highlight the importance to re-consider the policy of straw return in farming soils, especially in Hg-contaminated areas.