The Mercury–Selenium Antagonistic Interactions In Soil Plant System And Possible Mechanisms

Mercury?Hg?is a toxic metal element,which low concentrations also can result in high toxicity to living organisms.Existing techniques for the Hg pollution remediation in soil exhibit disadvantages such as high cost and negative impact on the soil.Selenium?Se?is a trace element essential to human health.It is reported that Se can reduce the bioavailability of heavy metals such as lead,arsenic and cadmium in the soil,thereby the application of Se can achieve the purpose of detoxification.However,the Se application on the detoxification of Hg in soil and its mechanism is unclear.In this study,the application of selenate and selenite on the detoxification of Hg-contaminated soil were studied by pot experiments.The transformation of Hg/Se fractions in soil before planting and after harvest,the growth of plants and physiological metabolism responds within plants were determined to reveal the detoxification mechanism.The main results are as follows:?1?The Se application transformed the Hg fractions in soil into more stable forms,lead to reduce its bioavailability,thus limiting the absorption and accumulation of Hg in pak choi.However,the selenite application on the inhibition of Hg bioavailability in soil were more than that of the selenate application.?2?The selenite application reduced the Hg concentration in the roots and shoots of pak choi,whereas the selenate application only reduced the Hg concentration in the roots but increased the Hg concentration in shoots of pak choi.It is worth noting that this inhibition occurs significantly at the appropriate level of selenite application(2.5 mg kg^-1).The proportion of mobile Hg fractions?water-soluble and exchangeable fractions?and the I_R values in soil before planting and after harvest have the good correlation with the concentration of Hg in the plants,indicating that both can be used to predict the bioavailability of Hg in soil.?3?The Se application at the highest concentration(2.5 mg kg^-1)can significantly alleviate the Hg uptake by plant,whereas the Se application at low concentrations(?1.0 mg kg^-1)elevated the activities of SOD,POD,CAT and GSH-Px enzymes and the SPAD value,as well as suppressed MDA and proline content,thus significantly improved the growth of pak choi.In addition,the application of selenite can promote the growth of pak choi more than that of selenite under the same Se concentrations.When the concentration of selenite was less than 2.5 mg kg^-1,the activities of antioxidant enzymes and chlorophyll content was increased,thus promoting the growth of pak choi,whereas the selenate treatment showed similar promotion only at the concentration was 0.5 mg kg^-1.?4?The presence of Hg in the soil promoted the transform of Se fractions from mobile forms to more stable forms,thereby reducing the bioavailability of Se in soil,lead to limit the absorption and accumulation of Se in pak choi.At low Se concentrations(?1.0 mg kg^-1),along with the increase of Se concentrations,the effect of selenite exogenous on the absorption and accumulation of Se in pak choi was higher than that of selenate under Hg-contaminated soil condition.There were the significant correlation between the proportions of Se fractions in soil to the Se concentrations in pak choi for co-exposed Hg-and selenite-treated soils,but no correlation was observed with the selenite-treated soils,indicating that the toxic effect of Hg to selenate-treated plants is greater than that of selenite.?5?In Hg-contaminated soil,the application of low-concentration selenite(?1.0 mg kg^-1)can enhance the absorption and accumulation of Se in pak choi,however,the toxicity of Hg can result in negative effect to the growth of pak choi in selenite-treated or low concentrations(?1.0 mg kg^-1)selenate-treated soils.In contrast,with the highest concentration(2.5 mg kg^-1)of selenate treatment,the exogenous of Hg can increase the SOD,POD,CAT,GSH-Px enzyme activity and SPAD value,as well as inhibit MDA and proline content in plants,and significantly promote the growth of pak choi because of reducing the toxicity of selenate.In summary,the application of Se fertilizer to treating Hg-contaminated soil is a potential method for reducing the Hg accumulation of Hg and improving the content of Se and in pak choi,lead to food safety.In addition,this study also clarified that the application of selenite is Se fertilizer forms that can reduce the toxicity of Hg-contaminated soil.