Effects of Long-Term Land Use Patterns on Labile Organic Carbon Fractions and Carbon Pool Management Index of Mollisols Humus Layers

Land use patterns significantly influence the quantity and composition of litter in the soil humus layers, thereby affecting the dynamics of soil organic carbon. However, the differences in labile organic carbon fractions and the carbon sequestration index under different land use patterns, as well as their impact on soil carbon storage in the humus layers of mollisols—without migration loss and soil erosion—remain unclear. Labile organic carbon is classified into fractions such as dissolved organic carbon, easily oxidized carbon, particulate organic carbon, and microbial biomass carbon, which are identified through different chemical extraction methods. This study investigates the impact of long-term land use patterns on organic carbon dynamics, organic carbon pools, KOS, and CPMI in mollisols across five treatments: SC (continuous soybean cultivation), MC (continuous maize cultivation), MSR (maize–soybean rotation), GB (grass belt), and FB (forest belt). It also selects three soil depths (0–20 cm, 20–40 cm, and 40–60 cm) over an 11-year period for analysis. The results indicate that soil organic carbon, labile organic carbon fractions (EOC, POC, DOC, and MBC), and CPMI decrease with soil depth, while KOS increases. Non-tillage treatments enhance SOC accumulation in the humus layers, with FB exhibiting the highest organic carbon content, surpassing GB, MC, SC, and MSR by 22.88%, 52.35%, 60.64%, and 80.12%, respectively. Non-tillage treatments can enhance the accumulation of labile organic carbon fractions, aligning with the observed trends in soil organic carbon, with the FB treatment identified as optimal. Additionally, these treatments can increase labile organic carbon fractions and CPMI, thereby improving soil stability. To minimize SOC loss, land use patterns should encourage the conversion of farmland to grassland and forest, with the FB treatment recommended as the optimal strategy for the protection of mollisols and the sustainable development of these soils over the long term. This approach is significant for understanding the soil carbon cycle, rationally planning land use strategies, and providing a reference for enhancing soil quality and ecosystem carbon sinks.