Sequential selective extractions based on the method proposed by Bloom NS et al(2003)were used for examining the added Se on the occurrence of mercury(Hg)in paddy soils.The fractions and leaching media were as follows: water soluble(deionized water),“human stomach acid” soluble(0.01 M HCl+0.1 M CH3COOH),fulvic acid chelated(1 M KOH),humic acid chelated(1 M KOH +1M HCl),elemental Hg(12 M HNO3),mercuric sulfide(aqua regia),and residual Hg(HF and HCl,1:1 in volume).The results suggested that Hg fractions occurred in raw paddy soils were different from that obtained by Tessier sequential extractions,which dominated by the humic acid chelated fraction,followed by elemental Hg and mercuric sulfide.If Se(added as Na2SeO3)was added and weathered a period of time(approximately three months),fractions of humic acid chelated,elemental Hg,and mercuric sulfide were significantly varied.In detail,the average abundance of humic acid chelated Hg was decreased from 60.3±3.1%to 30.2±5.9%,while the average abundance of elemental Hg and mercuric sulfide was increased from 16.9±2.1%to 34.6±2.4%and from 17.2±5.1%to 30.7±4.9%,respectively(Figure 1).We hypothesized that the added Se could change the speciation of Hg in paddy soils,resulting in the redistribution of Hg in the different extracted fractions.Paddy soils usually appeared to be acid and reducing conditions(Ding,2008),which facilitate the formation of Se(0)and HSe(Ι)in the rhizophere(Mayland et al.,1989).If this is correct,other forms of Hg(such as Hg(Π)and MeHg(Ι))in paddy soil can be transformed to HgSe(s),a Hg speciation similar to mercuric sulfide,based on the mechanism proposed by Khan and Wang(2009).This could be the main reason for the abundance of mercuric sulfide increased after Se was added.In the meantime,elemental Hg was also obviously increased,with reasons remain unclear.To clarify this,specific chemical speciation of Hg in different fractions should to be figured out.Unfortunately,in the present work,Hg was sequential extracted into fractions as biogeochemical sense,rather than specific compounds.Further efforts are urgently needed to better understanding the transformation of Hg when Se was added.This is also helpful to understand the Hg-Se antagonism in the environment and to find a technique to remediate Hg contaminated paddy soil.