Karst Epigenetic Geochemistry Of Selenium-enrichment And Cadmium-resistant Of Babao Rice In Yunnan Province

With the development of society,the contamination of potentially harmful elements such as heavy metals and the enrichment of beneficial elements in soil-crop systems have become hot issues.Crop quality is closely related to ecological geochemical conditions,and it is important to identify the migration and enrichment patterns of bedrock-soil-crop elements to optimize the agricultural planting layout.Rice is one of the important carriers for the uptake and enrichment of soil heavy metals.The southwestern karst region is the main area of heavy metal enrichment in China,but in Yunnan Province,there are the best areas for rice cultivation,among which Babao rice is the first of the"six famous rice"in Yunnan.The origin of Babao rice is the town of Babao in Guangnan,Yunnan Province,which is a typical peak cluster depression.The rice grown in this area is rich in selenium and low in cadmium.Therefore,in the study area of Babao Town,rocks,soils,irrigation water and rice are the main objects of study.Through field sampling of rocks,topsoil,deep soil,profile soil,karst water,non-karst water,surface water,groundwater in the whole area and irrigation water pH gradient rice pot experiments,combined with multivariate statistical analysis methods,we systematically studied the geochemistry of rock-soil-rice element migration in Babao Town,Yunnan Province.The following main insights were obtained.(1)Elemental cadmium(Cd)concentration in the surface soil of Babao Town ranged from 0.17 to 26.70 mg/kg,and 50%of Cd content was greater than 4 mg/kg,distributed in the limestone soil formed by P2y and D3g strata in the southwest of Babao Town.The concentration of selenium(Se)in the surface soils ranged from 0.25-2.97 mg/kg,and the soils were medium Se content soils to high Se content soils.The main source of elemental Cd and other heavy metal elements and metalloid Se element in the soil of Babao Town is rock weathering.The karst area is more weathered in situ than the clastic rock area,and most of the elements are enriched in the karst area and deficient in the clastic rock area.Soils in carbonate areas are more enriched in heavy metals and Se elements.(2)The pot experiment simulated the effect of different pH irrigation water on elemental patterns in soil and elemental content in rice.In the pot experiment(experimental soil Cd=6.2mg/kg,Se=0.53mg/kg,pH=7.61),a synergistic effect was found between Cd and Se contents in rice roots,and an antagonistic effect between Cd and Se contents in rice stems,leaves and seeds.pH=5 irrigation water may be the critical point to cause the synergistic effect between Cd and Se elements.The synergistic effect of Cd and Se was reduced when the pH of irrigation water was greater or less than 5.It is mainly the ion-exchange state of Cd in the soil that affects the Cd content of rice seed uptake.The ion-exchange state of Cd was significantly reduced in soils irrigated with irrigation water with pH>5.However,if the experimental soil is expected to be rich in Se at the same time,irrigating rice with irrigation water with pH greater than 7 can not only reduce Cd content in rice seeds but also enrich Se.The main controlling factor for high Se and low Cd in rice seeds is pH.(3)The pH value of the water environment in Babao Town was 7.20-8.86,showing weak alkalinity.Ca²⁺,Mg²⁺and HCO₃^-in water were not conducive to the absorption of Cd by rice,while Mg²⁺promoted the absorption of Se by rice.Karst groundwater has a significant influence on the distribution of soil pH.The distribution area of karst groundwater is consistent with the distribution area of high pH in soil,and the distribution area of high Cd in Babao Town overlaps with the high Se area,while the pH in soil is also high.Together with the influence of Ca²⁺,Mg²⁺,and HCO₃^-in the water,all are very favorable for high Se and low Cd in rice seeds in this area.(3)The pH value of water environment in Babao Town was 7.20-8.86,showing weak alkalinity.Ca²⁺,Mg²⁺and HCO₃^-in water were not conducive to the absorption of Cd by rice,while Mg²⁺promoted the absorption of Se by rice.Karst groundwater has a significant influence on the distribution of soil pH.The distribution area of karst groundwater is consistent with the distribution area of high pH in soil,and the distribution area of high Cd in Babao Town overlaps with the high Se area,while the pH in soil is also high.Together with the influence of Ca²⁺,Mg²⁺,and HCO₃^-in the water,all are very favorable for high Se and low Cd in rice seeds in this area.