Potential Hazardous Element Geochemistry In Agro-ecosystem Of Areas Around Beibu Gulf

The accumulation of potential hazardous elements in farmlands,as well as crops,has become an important issue related to food security and environmental risk.Guangxi coastal economic zone and Hainan Island around Beibu gulf were chosen to be the study area.In order to better understand the geochemical behavior of main potential hazardous elements on epigeosphere,especially in soils,present doctoral dissertation has carried out the following investigations:(1)the input and output fluxes and characteristics of potential hazardous elements in agro-ecosystem;(2)the migration,enrichment,adsorption and leaching of potential hazardous elements,especially As and Mo,in the surface layer of agricultural soils.Likewise,the pollution levels of potential hazardous elements,as well as the food safety,were also estimated for local.Collectively,the main study contents and achievements can be summarized as follows:(1)The quality of farmlands is good in the whole and meets the soil quality standards,except some certain elements.The levels of soil background values of most potential hazardous elements contribute greatly to their levels in sampling soils.The potential hazardous elements in soil samples mainly belong to “No-pollution” level or “low” level,and a small mount reach “moderate” level.According to the food standards,there are a small amount of rice samples out of the limits of Cd and Hg.(2)The main factors influencing the uptake by rice grain included soil pH,TOC,available P and available Fe.What calls for special attention is that soil pH affect the As uptake by rice via the influence to available P and available Fe.The competition for transporters from soil to roots between As and P in rhizosphere soil has been a dominant feature in study area.The results indicate that the effect of As in soil cannot be ignored when the As content was at a relatively low level,but the competition for transporter from soil to root between As and P in rhizosphere soil play a dominant role when there is a relatively high content of As in soils.Among the soil properties investigated,soil pH was the most important factor restricting the supply of soil available Mo.In this study,the dominant factors that directly affect Mo availability were the levels of available P and SOC.However,the absolute levels of soil available Mo had a direct effect on rice.Such levels of soil available Mo are affected by soil pH and SOC.(3)In Guangxi coastal economic zone,the main input approach of Cd,Pb and Cu is atmospheric deposition,and the main input approach of As and Hg is irrigation.In Hainan Island,As,Hg and Cr import to the agricultural soils mainly through irrigation,while Pb and Zn import to the agricultural soils mainly through atmospheric deposition.Moreover,the main input approach of Cd and Cu in Hainan Island is fertilization.(4)Results indicate that Cd is the element of the prior concern for agricultural soils,followed by Pb and Zn,and the rest of heavy metal elements represent little threat to the environment in Guangxi coastal economic zone.The Hg is the element of the prior concern for agricultural soil,followed by Cd and As,in Hainan Island.The most urgent threat occurs in NEB and SB for the high risk of Hg mainly due to the irrigation.