Rice straw return is being encouraged in China as an economical and environmentally friendly method to get rid of rice residuals,while the effects of rice residual burial on the biogeochemical behaviors of mercury in contaminated soils are relatively unknown.In the present study,rice residuals,including rice root and straw,were added into a mercury contaminated paddy soil to investigate the effects of rice residual burial on speciation and potential bioavailability of soil-bound mercury.Potential bioavailability of mercury to plants or soil fauna was assessed by CaCl2 or BSA(bovine serum albumin)extraction method.Our results indicated that potential bioavailability of MeHg in soils was less affected by rice residual addition.However,concentrations of MeHg in soils increased significantly(2-8 times)after rice residual addition,possibly due to the elevated microbial activities or complexation of HgS by DOM originated from rice residual decomposition.The enhanced MeHg levels in soils could subsequently increase the exposure of plants or soil fauna to MeHg.In contrast,potential bioavailability of inorganic mercury(IHg)decreased significantly in rice residual added soils,which could be attributed to the immobilization of IHg by particulate organic matter in rice residuals.And thus rice residual burial may decrease the risk of IHg in soils.This study reported for the first time that rice residual return may play a key role on speciation and bioavailability of mercury in contaminated soils,and should be considered in risk assessment of mercury.