| Now,most of Cd-contaminated farmland soil in China is mildly and moderately polluted.Blocking or reducing Cd entering the edible part of rice is the main technical measures for safe production in Cd-polluted paddy soil,according to China’s national conditions and agricultural conditions.Fe,is the most abundant transition element in soil,which is an important factor affecting the migration and transformation of Cd in a soil-rice system.However,as Fe is the redox sensitive element,the variation of pH and Eh in soil environment could result in oxidation(reduction)or dissolution(precipitation)of Fe,and consequently influencing Cd transfer into rice.In this study,the transformation of Fe driven by the simultaneous changes in pH and Eh in paddy soil,and its influence mechanism on Cd transfer from soil into rice were investigated.The main results are as follows:(1)Partial reductive dissolution of crystalline Fe oxides(such as hematite or goethite)occurred when soil pe+pH was below 5.7.In this case,some crystalline Fe oxides transformed into amorphous or low-crystalline(like ferrihydrite)and organically-complexed forms.The phase-transformation of Fe oxides under low pe+pH conditions increased the contents of amorphous,low-crystalline and organically-complexed Fe oxides in larger sized aggregates.The increases in Cd adsorption onto these Fe oxides and ultimately retention in the larger sized aggregates contributed to the reduction of Cd availability in paddy soil.(2)The reduction of pe+pH in rhizosphere soil could significantly inhibit Cd transfer form soil to rice.On one hand,the content of amorphous or low-crystalline Fe oxides increased obviously at low pe+pH level,which resulted in a 22.6%-31.8%increase of Fe/Mn-oxides-bounded Cd content(OX-Cd).The increase in OX-Cd content was the important factor for the 15.3%-33.9%decrease in Cd availability in soil.On the other hand,the low pe+pH condition in rhizosphere facilitated the formation of iron plaque,resulting in a 2.3-3.04-fold increase in Cd accumulation in iron plaque.In contrast,the amount of iron plaque formed under alternately wetting and drying treatments was strikingly higher than that under prolonged flooding treatments.(3)Tillering stage and booting stage were the critical periods for Cd uptake by rice.Flooding in paddy soil could significantly reduce Cd transfer into rice.Because the low pe+pH condition due to soil flooding enhanced FeRB activity and FeRB-mediated Fe(III)reduction.Fe(III)reduction increased Fe2+content in soil,improving the competitive uptake by rice between Fe2+and Cd2+.Meanwhile,Fe(III)reduction promoted the accumulation of amorphous Fe oxides and Cd adsorption onto them,and facilitated the formation of low-crystalline fraction of iron plaque and Cd sequestrated in them.This results revealed the underlying mechanism of FeRB-mediated Fe(III)reduction on Cd transfer in a soil-rice system.In conclusion,the transformation of Fe driven by the change of pe+pH conditions in paddy soil directly affects Cd transfer from soil to rice plants.This study reveals the mechanism of Fe transformation caused by variation in pe+pH on Cd uptake in rice.The results of this study would provide theoretical support for the prevention and control of Cd-polluted soil,as well as safe production of rice. |
