Contribution of natural and agricultural activities on Fe dynamics: Insights from Fe isotope in soils under different land-use types

Abstract

Evaluating the contribution of natural and agricultural factors on Fe dynamics in soils is crucial to the study of material cycling in critical zone observation. Five soil profiles under cultivated land, abandoned cultivated land, grassland, shrub land, and secondary forest from Puding karst catchment in Guizhou Province, China, were analyzed to identify natural and agricultural contributions to Fe processes. Vertical distribution changes of Fe content and Fe isotopic compositions (δ56Fe) were investigated. The results presented that natural chemical weathering accompanied by soil erosion occurs primarily in the upper layer, leading to the loss of Fe from the surface to the bottom during the eluviation-illuviation process. Fe isotopic fractionation occurred in soils rich in natural organic matter (i.e., secondary forest and shrub land), resulting in the accumulation of lighter Fe isotopes. Soil impacted by anthropogenic activity have higher Fe content (e.g., cultivated land or abandoned cultivated land). The reduction of Fe is the principal cause of light Fe isotopes in the upper layer, which is caused by periodic artificial irrigation on cultivated land. Compared to worldwide soils, Puding soils are impacted by natural organic matter, eluviation-illuviation, and agricultural activities, which tend to modify Fe isotopic signatures on a local scale within soil profiles. Fe isotopic fractionations in bulk soils among different land-use soils are very limited. This study provides insightful analysis based on Fe isotopic to recognize natural and anthropogenic activities on Fe dynamic, and offers information on material cycling in the Puding karst catchment.