Low Soil Organic Matter Stability Exacerbates the Vulnerability of Karst Areas to Rocky Desertification

Karst rocky desertification represents a severe form of land degradation and has traditionally been attributed to geological and hydrological constraints. However, soil organic matter (SOM), a key regulator of erosion resistance, is highly responsive to management practices, yet its contribution to rocky desertification remains poorly understood. This study investigated three representative soil types (limestone soil, yellow soil, and yellowish‐brown soil) in the karst region of southwestern China to elucidate SOM‐related mechanisms underlying the high erosion susceptibility of limestone soils. Compared with other soil types, SOM in limestone soils exhibited lower hydrophobicity, higher polarity, and greater carbohydrate abundance, indicating a potentially more labile SOM pool that is vulnerable to leaching and degradation. In addition, SOM stabilization in limestone soils was more strongly associated with exchangeable and carbonate‐bound Ca than with Fe‐ and Al‐associated mineral phases. Given the high mobility of Ca under humid karst conditions, Ca‐mediated organo‐mineral associations may be inherently unstable. Spectroscopic analyses further revealed that SOM in limestone soils had higher apparent molecular weight, lower aromaticity, and shorter aliphatic chain lengths, which may limit its accessibility with mineral surface. Collectively, these findings challenge the conventional view that greater higher SOM content necessarily indicates enhanced soil stability. The susceptibility of limestone soils to rocky desertification may be closely linked to both the intrinsic instability of SOM and the fragility of Ca‐mediated stabilization mechanisms. These findings provide new insights into the mechanisms driving karst soil degradation and highlight the potential importance of improving SOM quality for mitigating karst rocky desertification.