Dissolved organic carbon and its carbon isotope compositions in hill slope soils of the karst area of southwest China: Implications for carbon dynamics in limestone soil

Abstract

Dissolved organic carbon (DOC) can be considered fine indicators of soil quality that influence soil function in specific ways and are very sensitive to changes in soil management practices. In this study, the contents and isotopic compositions of dissolved organic carbon (δ¹³C_DOC) in soils were analyzed at different topographic positions on two typical slopes in a karst area to evaluate the source and fate of DOC in the soil from these regions. The results show that DOC contents of the two slopes were mainly affected by soil organic matter (SOM) input, topography, and soil quality. DOC contents at each topographic location on the two slopes decrease with increasing soil depth. The δ¹³C_DOC of the shrubby slope surface soils varied from -15.1‰ to -22.1‰, and are consistent with SOM δ¹³C and local vegetation. The δ¹³C_DOC of the grassy slope surface soils ranged from -20.0‰ to -20.9‰, similar to isotopic compositions of soil organic carbon (δ¹³C_SOC), but are inconsistent with present-day vegetation. The δ¹³C_DOC varied significantly with depth at each topographic location. Thus, the dependence of δ¹³C_DOC in soil profiles on depth reflects the migration and transformation of DOC in soils.