Tracing Fe cycle isotopically in soils based on different land uses: Insight from a typical karst catchment, Southwest China

Iron (Fe) isotopes can effectively unveil the Fe cycle mechanisms under redox and biological conditions during the weathering and pedogenic processes. Fe contents and Fe isotope compositions (defined as δ56Fe) in the soil profiles under secondary forest land, abandoned cropland and shrubland were investigated in a typical karst area in Southwest China. The results showed that the Fe content ranged from 23.92 to 38.56 g/kg, 21.92 to 33.02 g/kg and 12.98 to 27.93 g/kg, and the δ56Fe levels varied from -0.48 ‰ to 0.21 ‰, -0.24 ‰ to 0.11 ‰ and - 0.11 ‰ to 0.16 ‰ from the secondary forest land, abandoned cropland and shrubland, respectively. The correlation analysis results showed that Fe transportation and isotopic fractionation were regulated by the redox processes through soil pH and soil organic matter (SOM) in the abandoned cropland and shrubland. Heavier Fe isotope may be accumulated in the deeper soil of secondary forest land due to Fe-oxide precipitation. The Fe isotope fractionations were greatly altered by soil organic carbon (SOC) in surface soils due to negative surface charges. Soil pH also plays a key role in enriching lighter Fe in a medium-acidic environment (shrubland) by ligand-controlled dissolution and reductive dissolution. Long-term cultivation in abandoned cropland and grazing in shrubland reshaped the Fe cycle in soil profiles by changing soil pH and SOC contents. However, the similar values of δ56Fe in different land use soils indicated that the agricultural activities have no significant impact on the Fe transformation in karst soil ecosystems. The land utilization is reasonable in the Yinjiang County. This study provided effective data and insightful analysis to understand the Fe cycle processes in the karst soils under varied land uses.