Abstract. The conversion of natural vegetation to managed ecosystems may negatively influence soil organic carbon (SOC) and total nitrogen (TN) stocks, particularly in the fragile ecosystems. The objective of present study was to assess SOC and TN stocks losses by combining deep sampling with mass-based calculations upon land-use changes in a typical karst area of Southwestern China. We quantified the changes from native forest to grassland, secondary shrub, eucalyptus plantation, sugarcane and corn fields (both defined as croplands), on the SOC and TN stocks down to 100 cm depth using fixed-depth (FD) and equivalent soil mass (ESM) approaches. The results showed that converting forest to cropland and other types significantly led to SOC and TN losses, although the effect magnitude partly depended on both sampling depths and soil mass considered. On average, the shifting from native forest to cropland led to SOC losses by 19.1 %, 25.1 %, 30.6 %, 36.8 % and 37.9 % for the soil depths of 0–10, 0–20, 0–40, 0–60 and 0–100 cm, respectively, which highlighted that shallow sampling underestimated SOC losses. Moreover, the FD method underestimated SOC and TN losses for the upper 40 cm layer, but overestimated the losses in the deeper profiles. We suggest that the ESM together with deep sampling should be encouraged to detect the differences in SOC stocks. In conclusion, the conversion of forest to managed systems, in particular croplands significantly decreased in SOC and TN stocks, although the effect magnitude to some extent depended on sampling depth and calculation approach selected.
Combined deep sampling and mass-based approaches to assess soil carbon and nitrogen losses due to land-use changes in karst area of Southwestern China