| High geological background soil is widely distributed in southwest and south China.Different with the artificial sources of Cd pollution,the various accumulation patterns and the complex causes in high geological background soils made the rice Cd content exceed the limits of the food security in a wide range of farmland,which pose serious threat to the rice quality and personal health and safety.Therefore,to explore the mechanism of Cd migration and accumulation in rice under soils with different geological high background can provide a scientific basis for Cd pollution control in paddy fields with geological high background.Water management is a common and effective agronomic measure to control Cd pollution.Different water management measures can significantly affect the accumulation of Cd in rice.At present,there are few studies on the characteristics and mechanism of water management on the migration and accumulation of Cd in rice under the soil with high geological background.There are significant differences in Cd tolerance and accumulation among different rice varieties.The selection of rice varieties with low Cd accumulation is one of the hotspots for rice production in soils with high geological background.Based on this,four typical secondary enrichment type of geological high background soil in guangxi,suxu(SX),luoxu(LX),maling(ML)and yunbiao(YB)were selected.Firstly,the characteristics of soil pollution and soil properties were analyzed.Secondly,guangxi local hybrid rice cultivars xunliangyou were planted on these four soils,two different water management methods(conventional flooding,alternating between dry and wet)were set up to study the effects of water regulation and soil type on Cd absorption and accumulation in rice.Finally,7 different rice were selected to be grown in two soils(SX and LX)to compare the Cd absorption and accumulation characteristics in rice,and possible influencing factors were also explored.The main findings are as follows:1.Abstract:Taking the geological high background farmland soils(MaLing,WuMing,JiangNan,and YunBiao)of different development stages and parent material types in the karst area of Guangxi as research obj ects,the occurrence status of heavy metals,soil physical and chemical properties,and functional diversity of microbial communities were compared.Redundant analysis(RDA)method was used to obtain the key environmental factors affecting the soil microbial community structure in high geological background.The results showed that:(1)The development stage and the type of parent material significantly affected the basic properties of soil and the concentration of heavy metals.The soil heavy metal accumulation was higher in MaLing area at earlier development stage.The content of Cd in the soil of MaLing area developed from carbonate limestone is higher than that of soils from other areas developed from the sediments of Quaternary rivers.(2)There are significant differences in microbial activity and community structure in different soils.The soil microorganisms ’carbon source utilization capacity and microbial diversity index in the Jiangnan region were significantly lower than those in the other three regions.The microbes in YunBiao,Wuming and Maling soils mainly use Amines and Amino acids as carbon sources,While Sugars and polymers in Jiangnan.(3)Redundant analysis(RDA)showed that soil organic matter,pH,total As,Cd,Pb,and Available As were the main environmental factors that caused the differentiation of soil microbial carbon source utilization.Correlation analysis showed that the utilization intensity of Amines,Amino acids,and Phenolic acids was positively correlated with soil organic matter and pH,and negatively correlated with As,Available Cu,and Affective As.2.Using hybrid rice cultivars xunliangyou as the material,pot experiment was carried out to study the characteristics and influencing factors of Cd accumulation in four geological high background paddy soils of SX,LX,ML and YB with different water management modes(alternating wet and dry with conventional flooding).The results showed that:(1)the geological background of soil formation had a significant effect on the growth of rice.The growth of rice in SX,LX and YB soils developed from quaternary river sediments was significantly superior than that in ML soils derived from carboniferous carbonate rocks,and the water management did not change the growth of rice.(2)Cd accumulation in rice grains under alternate wet and dry treatment was significantly higher than that under conventional water management(P<0.05).The Cd content in rice was different under soils with different geological backgrounds.The Rice Cd in SX soil exceeded the standard,rice Cd in LX soil did not exceed the standard,and rice Cd in ML and YB soil exceeded the standard under the alternate treatment of dry and wet.The accumulation sequence of Cd in different parts of rice was as follows:root>stem>leaf>grain>shell.(3)correlation analysis showed that bioavailability of Cd,pH,Eh,soil organic matter and Fe2O3 content were the main influencing factors for Cd accumulation.(4)the structural equation model of Cd migration and accumulation in rice indicated that water treatment would change the transport route of grain Cd.The main transport route of grain Cd in conventional water treatment was leafgrain and stem-grain.(5)the empirical prediction model of grain Cd was obtained by stepwise regression analysis,which had better prediction results under SX,ML soils with different water treatment conditions.3.Conventional rice varieties were selected:Yue Jing si miao(YJ),You Zhan No.8(YZ)and Huang Hua zhan(HH),and hybrid rice varieties such as Tai liangyou(T),Shen liangyou(S),Y liangyou(YL)and Ning liangyou(NL)were selected to grow in SX and LX soils with the same parent material and different Cd content to compare the Cd accumulation characteristics,and to explore possible influencing factors.The results showed that:(1)soil and varieties significantly affected rice growth,yield and Cd accumulation.Specifically,rice of the same variety grew well in LX soil,and hybrid rice of the same soil was superior to conventional rice.The Cd accumulation of rice seeds in SX soil was higher than that in LX soil,and the Cd content of conventional rice was higher than that of hybrid rice under the same soil condition.(2)correlation analysis showed that Cd content in grains was positively correlated with bioavailable Cd in soil,and negatively correlated with pH,soil organic matter,rice plant height,root and stem biomass.(3)redundancy analysis showed that rice varieties and soil types were the main factors affecting Cd accumulation in rice grains.(4)through cluster analysis and screening,YL and S were identified as two rice varieties with low accumulation.YL was more suitable for planting in SX region,while S was more suitable for planting in LX region. |
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