Controlling Factors and Predictive Models of Total Mercury and Methylmercury Accumulation in Rice (Oryza sativa L.) from Mercury-Contaminated Paddy Soils.

It is urgent to detect the major controlling factors and establish predictive models of mercury (Hg) accumulation in rice. A pot trial was conducted, exogenous Hg was added to 19 paddy soils at 4 concentration levels in this study. The major controlling factors of total Hg (THg) in brown rice were soil THg, pH and organic matter (OM) content, while those of methylmercury (MeHg) in brown rice were soil MeHg and OM. THg and MeHg in brown rice could be well predicted by soil THg, pH and clay content. The data from previous studies were collected to validate the predictive models of Hg in brown rice. The predicted values of Hg in brown rice were within the twofold prediction intervals of the observations, which demonstrated the predictive models in this study were reliable. The results could provide theoretical foundation for the risk assessment of Hg in paddy soils.

Kinetic characteristics of and critical stages for mercury accumulation in rice (Oryza sativa L.).

By studying the dynamic characteristics of and key growth stages for mercury (Hg) enrichment in rice, the Hg migration and translocation processes in this species can be better understood. In this study, a pot experiment was conducted, wherein two rice cultivars, Tianyouhuazhan (TYHZ, indica) and Zhendao 18 (ZD18, japonica), were selected and planted for analysing the Hg accumulation kinetic characteristics in rice plants. The plants were sampled at each growth stage, and the biomass and total Hg (THg) and methylmercury (MeHg) concentrations of each tissue were measured. The relative Hg contribution rates (CRs) in whole rice plants and rice grains were calculated, and the growth stage with the highest relative contribution was identified as the key growth stage for Hg accumulation. The results indicated that in rice, the MeHg translocation capability was stronger than the THg translocation capability. Significant differences in the kinetic characteristics of Hg accumulation were found between the two rice cultivars, and the TYHZ rice grains had a stronger Hg accumulation ability than the ZD18 rice grains. The key growth stages for THg accumulation in whole rice plants of both cultivars were the tillering and booting stages, while that for MeHg accumulation was the tillering stage. The key period for Hg accumulation in rice grains was the grain filling stage for both cultivars. The insights from this study could provide scientific guidance for the safe production of rice in Hg-contaminated soil.

Kinetic characteristics and predictive models of methylmercury production in paddy soils.

Understanding the mercury (Hg) methylation process is important for the management of paddy soils contaminated by Hg. In this work, samples of eighteen paddy soils with varying soil properties were spiked with inorganic Hg and subjected to a 90 d flooding period. Soil pH and redox potential (Eh) were measured in situ at intervals, and soils were sampled for the analysis of methylmercury (MeHg). The Hg methylation efficiency increased with flooding time and reached a relatively steady state at 30 d of incubation, ranging from 0.08% to 2.52%, and was significantly correlated with the in situ soil pH and Eh. The Elovich equation could adequately describe the kinetic production of MeHg. MeHg production was well predicted by the in situ soil pH and Eh of flooded soils, in addition to the organic matter content of air-dried soil samples and flooding time. The two predictive models explained 78% and 68% of the variability of the Hg methylation efficiency. The results suggested that the methylation of inorganic Hg in paddy soils after flooding can be predicted as a function of routinely measured soil properties and flooding time, a correlation that can be utilized to improve understanding of the extent of Hg methylation and the management of Hg-contaminated paddy soils.