Objective Recently, the development and utilization of natural selenium−rich land resources have attracted increasing attention. However, the genesis of selenium−rich soil in plain areas far from parent rocks is still unclear. This paper aims to take the Lixiahe area in Hai'an as an example to explore the distribution characteristics and genesis of selenium−rich soil in plain areas, thereby guiding the development and utilization of selenium−rich land resources.

Methods The paper systematically collated geochemical data from 1800 groups of surface soil samples and 1000 groups of soil profile samples in the study area, and combined with previous research data, conducted a comprehensive analysis and study of the data. The grain size characteristics of sediments were used to infer the depositional environment of the parent material.

Results The selenium (Se) content in the surface soil of agricultural land in the study area reaches up to 0.63 mg/kg, and the area of the enrichment zone (≥ 0.3 mg/kg) is 66 km². In the vertical soil profile, the average Se content in plough layer ( 0~20 cm ), plough pan ( 20~40 cm ), subsoil ( 50~80 cm ), subsoil ( 80~100 cm ) and deep soil ( 150~180 cm ) were 0.23 mg/kg, 0.18 mg/kg, 0.13 mg/kg, 0.12 mg/kg, and 0.11 mg/kg, respectively, showing a very obvious surface accumulation; the selenium content in surface soil was highly positively correlated with the parent material layer, with a correlation coefficient between 0.6 and 0.8 (P<0.01). There was a highly significant negative correlation between ground elevation and soil Se content, with a correlation coefficient of −0.6 (P<0.01), indicating that the higher the terrain, the lower the soil Se content; soil organic matter (SOM) was significantly positively correlated with Se, playing an important role in the adsorption and fixation of selenium during soil formation.

Conclusions The parent material layer was mainly controlled by the ancient lagoon−sand dam sedimentary system. The selenium−rich soil in the area was mainly formed by the supergene enrichment of the parent material developed in the ancient lagoon facies. The lagoon facies sedimentary environment controls the selenium enrichment boundary of the soil. Based on multiple linear regression analysis, the controlling factors of soil selenium enrichment were revealed. Among them, SOM played a decisive role in the enrichment of Se in this area, and the contribution rate to soil Se enrichment was 70%.