The soil stable carbon (C) and nitrogen (N) isotopes are widely used to indicate C-3/C-4 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 (delta C-13 and delta N-15) 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 C-13 and N-15 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 similar to 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 C-3 forest, the delta C-13 and delta N-15 values of SOM in the soils under native forest at the 0 similar to 50 cm depth increased with increasing soil depth, with the range of -27.7 parts per thousand similar to-25.7 parts per thousand and 6.5 parts per thousand similar to 10.0 parts per thousand, respectively. While decreasing trends of them in the soils below 50 cm depth were attributed to the mixing of( 13)C and N-15-depleted organic matters from bedrocks. However, the delta C-13 and delta N-15 values of SOM along the soil profiles under other transformed lands were intensively irregularly fluctuated between -29.1 parts per thousand and -19.0 parts per thousand, 1.2 parts per thousand and 7.9 parts per thousand, respectively. The single delta C-13 and delta N-15 signals in the soil profiles of transformed lands indeed revealed the alterations of historical C-3/C-4 composition and N transformation processes after land use change, but these indications were not specific. The result of the coupling of C-13 and N-15 isotope under native forest land reveals a positive relationship between them, which associated with full plant-absorption against N-15-depleted inorganic nitrogen derived from SOM mineralization. This study suggests that the coupling of C-N isotope fractionation more likely occurs in the C-3 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.

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