Profile characteristics of soil organic carbon in shallow karst fissures and its influencing factors.

Shallow karst fissures are important reservoirs and habitats for soils in karst regions and play a crucial role in controlling rocky desertification. Previous studies have often intentionally or unintentionally avoided karst fissures and examined only bulk soil (non-fissure soil), which may lead to biased research results. Taking seven fissure plots and one bulk soil plot within a Dodonaea viscosa shrub community in a karst faulted basin as research objects, we investigated the profile characteristics of soil organic carbon (SOC) in fissure soils and the controlling factors by using correlation analysis and structural equation modeling. The results showed that: 1) SOC, readily oxidizable organic carbon (ROC), and dissolved organic carbon (DOC) in both fissure soil and bulk soil decreased with increasing soil depth. The average SOC in the 0-75 cm fissure soil profile (26.48 g·kg-1) was 72.2% higher than that in bulk soil (15.38 g·kg-1). The mean ROC and DOC contents were 30.7% and 44.6% higher than those in the bulk soils, respectively. 2) Total nitrogen (TN), alkaline hydrolyzable nitrogen (AN), ROC, DOC, and β-glucosidase (βG) activity were all significantly positively correlated with SOC in fissure soils, whereas soil water content (SWC) showed a highly significant negative correlation with SOC in fissure soil. 3) Soil nitrogen content and carbon fractions were the direct drivers of SOC. SWC in fissure soil had no direct effect on SOC, but indirectly suppressed SOC accumulation mainly by inhibiting soil nitrogen content (TN and AN). This study revealed the distribution patterns, accumulation characteristics and controlling factors of SOC in karst fissures. We recommended that an appropriate proportion of fissure soil should be included to guarantee the accuracy and reliability of results in karst soil research.