The effect of agriculture on topsoil carbon stocks is controlled by land use, climate, and soil properties in the Argentinean Pampas

The conversion of native grasslands into croplands with a high frequency of soybean (Glycine max L.) in crop rotations has diminished soil organic carbon (SOC) stocks in the Argentinean Pampas. The aims of our study were to determine the amount of SOC lost due to cultivation (dSOC) and to assess the main factors (land use, climate, and soil properties) associated with dSOC. We took paired topsoil samples (0 to 20 cm) from arable and pristine soils (n = 465) and used a path analysis approach to evaluate the direct and indirect effect of different variables on dSOC. The selected variables were SOC in pristine soils (SOCPRIS), clay content, carbon input (C-input), relative soybean harvested area (Sb-%), and mean annual precipitation and air temperature (PP and Temp, respectively). The percentage of sites with SOC < 20 g kg(-1) was 29% in pristine soils and 66% in arable soils. The dSOC ranged from 0 to 82 Mg ha(-1). Depending on the area, dSOC represented 25 to 36% of SOCPRIS stocks. The path analysis explained 60% of the dSOC variation, and the main factor controlling dSOC was SOCPRIS (by direct effect and indirect effects through clay, PP, and Temp), followed by Sb-%. The Sb-% depended on the productive potential of the area (soybean yield) which was associated with Temp and PP. As a conclusion, in the analysed temperate and fully humid environments, soils with greater SOCPRIS and high soybean frequency in the crop rotations presented a greater SOC depletion after conversion to agriculture. This information will be valuable when developing models to predict current and future SOC stocks.