A Study On Cadmium Accumulation By Rice And Greenhouse Gases Emission In Upland-soil System

Rice is most important food crop in the World and the China as well.About 93%of the total rice cultivation(28.6 million ha)area used for paddy rice production in 2001 and 90%of fresh water is used for rice production in south China.Current predictions suggested 12.90 billion m3 reduction in water by 2030 and about 3.65 million ha of paddy rice area already decreased by 2001.Upland rice is one of the best options in the areas under water scarcity and upland rice is mainly distributed in northern part of China at 10000-13200 ha of land.Evidences have shown that most of agricultural paddy soils near mining areas are heavily contaminated with cadmium(Cd)and lead(Pb)causing their accumulation in rice grain.Alkaline amendments have been widely used in polluted water,soil,and paddy rice but very few studies in upland rice.A series of experiment was performed to get the better understanding of soil Cd speciation,bioavailability,accumulation and microbial diversity in upland rice rhizosphere.Cd and mineral nutrients(Fe,Zn,Cu,Mn,Ni,Ca,and Mg)accumulation of upland rice(Huyou2)was performed in an experimental field with hard-ridged plots containing soils from three different sources.Exogenous Cd was added into the soils at rates of 0,0.25,0.5,1,2,4,8,16 mg kg-1 five years prior to commence this experiment,in order to ensure that the Cd in the soils had sufficient time to stabilize.In upland rice rhizosphere,the bacterial community indices results demonstrated no significant effect of long-term soil Cd contamination and bacterial communities were not affected by soil physio-chemical properties except soil Cd.PCoA analysis showed the dominant bacterial communities were divided into three groups at low,medium and high(0.25-2,4-8,16)levels of soil Cd.Sequential extraction of soil Cd results showed that the exchangeable and reducible fractions were dominant in acidic soils.Soils with low pH have high amount of each fractions as compared to the soil with high pH and same trends were observed in DTPA-extractable Cd from three soils.The mean Cd content in the plant parts,from highest to lowest,were as follows:root,stem,leaf,and brown rice.The Cd content in the brown rice of the upland rice was below the limit of Cd in rice(0.2 mg kg-1)from China(GB 2762-2017)when the amount of Cd added was ?1 mg kg-1.This observation can be mainly attributed to lower(TFsoilgrain)transfer of Cd in upland rice.Significant differences were observed between Cd concentrations present in brown rice from the three different soils which can be mainly attributed to the differences in DTPA-extractable soil Cd because of different soil pH.Addition of high concentrations of Cd to soil was found to reduce both the uptake of Fe,Zn,and Mg by the roots and their accumulation in brown rice.In contrast,addition of high concentrations of Cd to soil appeared to enhance both Ni uptake by the roots and accumulation of Ni in brown rice.Altogether,results of this study suggest that it may be possible to cultivate upland rice in slightly Cd-polluted soils.Keeping in view the first experiment results,A pot experiment was performed using two upland rice(Huyou2;Hanyou737)and two paddy rice(Taigeng8;Yixiang2292),with two liming amendments,dolomite(CaMg(CO3)2)and lime powder(CaCO3),in order to study the influence of liming on bioavailability and the accumulation of Cd and Pb in rice and methane(CH4)and nitrous oxide(N2O)emission in upland and paddy conditions.Upland conditions reduced 90%of soil CH4 emission as compared to paddy conditions.Liming amendments reduced the soil CH4 emission by 45%and 39%with dolomite,and it decreased by 35%and 33%with lime treatment both in upland and paddy conditions,respectively.N2O emission decreased by 44%and 52%with dolomite,and with lime application it was reduced by 37%and 44%in both upland and paddy conditions respectively.Reduction in DTPAextractable Cd was between 37-53%and 43-80%with dolomite and 16-37%and 2472%Cd decreased with lime application in upland and paddy conditions respectively.Soil DTPA-extractable Pb reduced by 27-44%and 25-53%with dolomite and 1640%and 11-42%with soil applied lime in upland and paddy conditions respectively.Cd accumulation in rice grain was decreased by 47-88%and 62-79%with dolomite and 31-86%and 45-52%reduction by lime application in upland and paddy rice respectively.Rice grain Pb reduced by 58-91%and 66-78%with dolomite application and 32-71%and 44-71%with lime in upland and paddy rice,respectively.Addition of lime to the soil was found to reduce both the uptake of Fe,Zn,and Cu by the roots and their accumulation in root but no significant reduction in brown rice.Our results showed that dolomite and lime significantly reduced soil N2O and CH4 emission and Cd and Pb accumulation in rice grain,but dolomite is more effective.Altogether,results of this study suggest that it may be possible to cultivate upland rice in Cd-Pb polluted soils to reduce soil CH4 emission and Cd and Pb toxicity and accumulation and N2O emission in upland rice can be minimized by soil liming of 3 g kg-1 and optimizing the macro and micronutrients composition of the soil.