Geological and geochemical background of Se-rich rice production in Yili area, Jiangsu Province

There exist abundant Se-rich soil resources over 400 km² in the Yili area, Jiangsu Province, and a part of natural Se-rich rice has been produced in this area. By systematically studying and analyzing newly-obtained elemental geochemical data from the typical samples about rice seeds, soil, rock and soil solution related to Se-rich rice origin, this paper discusses the main controlling factors and geological environment or geochemical background for producing the natural Se-rich rice in the Yili area, and summarizes distribution and migration characteristics of some trace elements such as Se within rice as well as soil and rock. Some conclusions have been reached:(1) Se concentration in rice is significantly correlated to Se and TOC in soil, with the correlation coefficient (R) being 0.81. Detection rate of Se-rich rice is 100% when Se concentration is more than 0.5 mg/kg in soil; (2) Permian coal-bearing strata seem to be main primitive sources to form Se-rich soil, and the variegated argillaceous siltstone and shale within the strata are important Se-rich rocks, and the average Se concentration is 17.22 mg/kg in these rocks, which is more than 5 times higher than the average Se concentration in the local soil; and there are some obvious positive correlation between rock thickness and Se concentration in its overlying soil:where the sedimentary thickness of the Se-rich rock increases, the Se concentration is higher in the overlying soil layer; (3) The above-mentioned Se-rich rock formation includes a layer of iron and manganese nodule sediments lenticular in form, its thickness is about 5-30 cm, and there are abundant Fe, Mn, Cd, S, P and some other elements in the nodule sediments; the weathering and transferring process to form soil may contain a part of Cd, but the Cd that exceeds the value of the standard sample has not been yet detected in those relevant Se-rich rice seeds; (4) the available Se concentration is lower than 0.02 mg/kg in the cultivated soil, and the ratio of the available Se concentration to Se concentration is from 0.16% to 2% or so in the cultivated soil; and the most cultivated soils that enrich selenium are acidic, their pH values are between 4.6 and 6.5; the original Se-rich rocks are more acidic, with their pH values being generally between 4.2 and 5.5; (5) Se migration process is a series of continuous dilutions or homogeneous movements. Se-rich farmland distribution is influenced by the terrain, rainfall, fault belt and some other factors. The natural Se-rich farmland is formed in the small basin in hills, valley and piedmont plain with fault belts becoming the natural boundary of Se-rich farmland.