Carbon Nitrogen Isotope Coupling of Soils and Seasonal Variation Characteristics in a Small Karst Watershed in Southern China

Carbon and nitrogen are among the most important biogenic elements in terrestrial ecosystems, and carbon and nitrogen stable isotopes (delta C-13 and delta N-15) are often used to indicate the sources of carbon and nitrogen elements and turnover processes, and the study of C and N isotopes coupling can provide more precise indications. To this end, this study was conducted to investigate the effects of different land use types on soil organic carbon (SOC), soil organic nitrogen (SON) and the coupling relationship of C-N isotopes, as well as to reveal the seasonal variation characteristics of soil C and N. The results showed that SOC and SON contents of forest land were significantly higher than those of agricultural land and grassland. The soil C/N was significantly higher in the dry season than in the rainy season (p < 0.01), indicating that the decomposition rate of soil organic matter (SOM) was faster in the rainy season, which was not conducive to the accumulation of soil C. Soil delta C-13 and delta N-15 coupling showed seasonal characteristics: soil delta C-13 and delta N-15 did not have a good linear relationship in the rainy season, but showed a significant positive correlation in the dry season (r(2) = 0.75, p < 0.05), indicating that there are differences in the soil C-N isotope fractionation coupling under the influence of climatic factors. This study provides a reference for regional land resource management as well as carbon and nitrogen cycle studies in karst areas.