Abstract

Soil organic matter (SOM) has substantial influence on geochemical cycle, soil stability and global climate change. Information on SOM source, carbon and nitrogen dynamics in ecologically fragile karst ecosystems is lacking; especially in upland rocky desertification areas. Therefore, four typical soil profile varying degrees of disturbance in Yunnan Karst area -Guizhou Plateau, southwestern China were selected and parameters such as total organic carbon (TOC), nitrogen (TN), δ¹³C and δ¹⁵N were assessed. The small δ¹³C variation (-15.64 to -25.82‰) in SOM reflect the coexistence of C3 and C4 plants with no indication of vegetation change. The increase in δ¹³C and δ¹⁵N in the upper soil layers indicate mineralization and selective preservation, while the decrease at depth indicates mixing of ¹³C-depleted SOM and translocation of dissolved organic carbon. Conversely, the irregular fluctuation in δ¹⁵N is linked to mineralization, nitrification and denitrification due to heterogeneity of karst soils. The coupled negative correlation between δ¹³C and δ¹⁵N indicates low soil N utilization efficiency and mixing of isotopically distinctive carbon and nitrogen source. The results further revealed strong positive correlations in C/N ratios versus TOC (r²= 0.79, 0.84, 0.94, 0.94) and TN (r² = 0.73, 0.81). Variation in TOC, TN, δ¹³C and δ¹⁵N were influenced by vegetation cover, topography, hydrogeological condition and disturbance. This karst rocky desertification area has the characteristic of native forest with limited vegetation cover that had experienced erosion, leaching, inefficiency in N utilization with degree of disturbance. Our findings suggest that urgent attention and control measures are needed to improve soil quality and stability in rocky karst desertification areas.