Sources of soil leachable sulphate in the Indian region using sulphur isotopic analysis

Abstract

Sulphur is an essential nutrient for the growth of plants and, in recent years, sulphur deficiency in the soils is becoming a serious problem. In order to manage the sulphur resources in the soil, it is necessary to understand various sources of sulphur and their relative contributions. The sulphur in the soil mainly comes from the substrate rock or via atmospheric deposition either as wet or dry deposition. As these sources have distinct sulphur isotopic compositions (δ34S), δ34S study of leachable soil sulphate is a useful tool to study the origin of sulphur in the soil. δ34S studies have been successful in understanding the biogeochemical cycle of sulphur by identifying the different sources of sulphur. Based on the limited number of δ 34S studies, it has been suggested that sulphur in the soils and rivers of peninsular India is mainly derived from atmospheric fallout or from rain, and the contribution from the bedrock is negligible. On the contrary, sulphur in the rivers from the western Himalayas is indicated to be derived from the rocks. However, such characterization is carried out at only a few locations and its spatial variability is not known. Further, because of contributions from deeper rocks or groundwater, δ34S variation in the river or stream water may not reflect the δ34S of the soils. This study analysed the soils collected from various locations within India for their δ 34S. Soil samples were collected employing a community-outsourced sampling scheme. Sulphate was leached out from the soil samples using an established protocol and was converted to barium sulphate whose δ34S was determined at the IRMS facility of IISER Pune. δ34S values of sulphate from the samples from the Indo Gangetic plain were depleted in 34S compared to that in the rest of India. It is postulated that the soil sulphate in the Indo-Gangetic plain and Himalayan region is likely to be derived mainly from weathering of sulphide phases in the rocks while those from the soil in other parts of India are derived from atmospheric deposition. This contrast in the δ34S values in soils between Himalaya-Gangetic plain and the rest of India mimics the spatial variability reported in the δ34S of bird feathers from the Indian region. The δ34S study of the riverine sulphate has implications for CO2 budget estimation. This work will be helpful in assessing sulphate budget in the riverine system.