Soil copper sources and biogeochemical processes under different land uses: insights from stable copper isotopes in the Cambisols of Southwest China.

Copper (Cu), a toxic trace element, has extensively accumulated in soils due to intensive human activities. Yet current knowledge on Cu sources and biogeochemical processes in Cambisols under varying land uses is scant. In this study, Cu contents, stable isotope compositions (δCu) and their correlations with soil physicochemical properties in soil profiles across different land uses were analyzed, including cropland, abandoned cropland and orchard, shrub-grass land, and secondary forest in a karst watershed of Southwest China. Soil Cu contents in cropland (mean 44.9 mg kg ) were significantly higher than those in abandoned cropland and orchard (mean 37.7 mg kg ), and much higher than those in natural lands (mean 26.3 mg kg ). In agricultural lands, cropland soils (mean -0.216‰) were significantly Cu-depleted compared to abandoned cropland and orchard (0.020‰), resulting from applying Cu-depleted fungicides. Soil δCu values in shrub-grass land and secondary forest land exhibited within a wide range, from -0.627‰ to 0.338‰, attributed to Cu isotope fractionations during pedogenic processes. Soil δCu values decreased with increasing soil depth and were positively correlated with soil organic carbon (SOC) contents, but negatively correlated with the chemical index of alteration (CIA), indicating the influences of clay mineral sorption, organic complexation, and leaching processes on soil δCu patterns. In different regions, δCu values in Cambisols increased in sub-humid climate, but decreased in humid climate with increasing annual precipitation. This study underscores the key roles of mineral sorption, organic complexation, and leaching processes in affecting δCu patterns in Cambisols.