Analysis of soil organic carbon composition characteristics and causes in Wuming region Karst landforms, Nanning, Guangxi Province, China

Karst landforms are special geological structures in southwestern China that are closely related to local ecological environments and human activity. This study focuses on the Wuming District, Nanning City, Guangxi Province, China, to explore the influence of different factors on the organic carbon storage and density characteristics of typical karst landform soils in the Wuming area. A multi-objective geochemical investigation was conducted from five perspectives: geological age, rock type, landform type, soil type, and land-use type. Surface and deep soil organic carbon (SOC) content data were obtained, and the organic carbon density and storage were calculated for the study area. The results showed significant vertical heterogeneity in soil organic carbon content, with carbon storage increasing with depth, indicating distinct carbon sink characteristics (p < 0.01). Overall soil organic carbon density was highest in the Devonian period among the different geologic ages (up to 3.92 kg/m2 in the surface layer), with acidic rocks resulting from ancient volcanic activity showing the highest organic carbon density among the lithological classifications (4.09 kg/m2). Karst landform soils, primarily limestone, had a higher organic carbon density than that of other terrains, particularly in carbonate rocks intermingled with clastic rocks. Despite the limited distribution of Bog soil within the study area, they exhibited the greatest carbon sequestration capacity among all evaluated soil types, with a soil organic carbon Density of 4.81 kg/m2. The predominant land-use type was arable land, encompassing a vast expanse dedicated to dryland farming—72.66