Dynamics of Vertical Distribution of Soil Organic Carbon in Black Soil Profile of Northeast China in Response to Changes in Land Cover and Land Use

Anthropogenic land-use change influences soil organic carbon (SOC) dynamics by altering both biotic and abiotic soil factors. The carbon stable isotope ratio of SOC (δ13C) indicates the vegetation sources of organic carbon and legacy effects of historical land use, providing important information for carbon dynamics. However, the mechanisms driving SOC dynamics in deep soils (>100 cm) under different land cover and land-use types remain poorly understood. Here, we analyzed the SOC content and δ13C in thick soil profiles (a thickness of 160 cm or 200 cm) under different land cover/land-use types in the typical black soil region of the Songnen Plain, Northeast China. The results showed that the average SOC content at 0–30 cm depth in natural forest land (38.87 g kg−1) was higher than that in the forest land converted to cultivated land (31.66 g kg−1), artificial forest land (22.63 g kg−1), and perennial cultivated land (18.16 g kg−1). Similarly, the average SOC content below 100 cm depth was higher in natural forest land (7.99 g kg−1) than in artificial forest land (6.90 g kg−1), the conversion of natural forest to cropland (6.59 g kg−1), and perennial cultivated land (4.39 g kg−1). Notably, significant positive correlations between δ13C and SOC were observed in both natural forest land and perennial cultivated land, presenting the synergistic effects on SOC probably influenced by carbon input, microbial communities, and environmental conditions. Further investigation revealed that soil moisture content and pH significantly influenced SOC content, probably by regulating organic matter decomposition rates. The natural forest land with high moisture content and low pH conditions created favorable environments for carbon preservation, whereas long-term cultivated cropland with low moisture content and high pH conditions accelerated carbon mineralization processes. These results indicate that land cover and land-use change not only significantly alter surface SOC content but also drive deep soil carbon cycling dynamics by regulating soil moisture content, pH and δ13C values. This study elucidates the intrinsic relationships between SOC content, δ13C, pH, and moisture content under land-use change, providing scientific support for land use-aware carbon management strategies in black soil regions.