The soil stable carbon (C) and nitrogen (N) isotopes are widely used to indicate C3/C4 vegetation history, N sources and transformation processes, respectively. However, land use change, particularly converting forest into farm land, alters soil organic matter (SOM) sources and processes in soils, resulting in a hard understanding of soil C and N fate. In the present study, soil organic carbon (SOC) and soil organic nitrogen (SON) contents, and their stable isotope compositions (δ13C and δ15N) were determined in the five soil profiles under land use change (i.e., conversion of native forest land into shrub land, grass land, maize field, and paddy land) in Lobo county, Guizhou province, Southwest China. A coupling of 13C and 15N isotope in SOM under land use change was verified whether it could provide more accurate indications of sources and transformation processes.

The SOC and SON contents of native forest land at the 0∼20 cm depth were significantly larger than these under other transformed lands. The SOC and SON contents decreased exponentially with increasing soil depth under all land use types, and showed opposite trends with soil pH. The C/N ratios of SOM in the soils under undisturbed native forest decreased from 10 to 7 with increasing soil depth, while an irregular fluctuation along soil profile was shown in other transformed lands. Similarly to the most study in the soils under C3 forest, the δ13C and δ15N values of SOM in the soils under native forest at the 0∼50 cm depth increased with increasing soil depth, with the range of −27.7‰∼−25.7‰ and 6.5‰∼10.0‰, respectively. While decreasing trends of them in the soils below 50 cm depth were attributed to the mixing of 13C and 15N-depleted organic matters from bedrocks. However, the δ13C and δ15N values of SOM along the soil profiles under other transformed lands were intensively irregularly fluctuated between −29.1‰ and −19.0‰, 1.2‰ and 7.9‰, respectively. The single δ13C and δ15N signals in the soil profiles of transformed lands indeed revealed the alterations of historical C3/C4 composition and N transformation processes after land use change, but these indications were not specific. The result of the coupling of 13C and 15N isotope under native forest land reveals a positive relationship between them, which associated with full plant-absorption against 15N-depleted inorganic nitrogen derived from SOM mineralization. This study suggests that the coupling of CN isotope fractionation more likely occurs in the C3 forest ecosystem with high N utilization efficiency. However, the replacement of native forest by farm land or grass land will reduce soil N utilization efficiency.