【Objective】This study investigated the dynamics and interrelationships between soil labile carbon fractions and carbon-hydrolyzing enzymes under long-term fertilization practices, aiming to provide a theoretical basis for enhancing black soil fertility and optimizing fertilization strategies in Northeast China.【Method】A long-term fertilization experiment established in 1989 in Gongzhuling, Jilin Province, was utilized. Soil samples were collected from five treatments: no fertilization (CK), nitrogen and potassium fertilization (NK), nitrogen-phosphorus-potassium fertilization (NPK), NPK plus straw return (NPKS), and NPK plus manure (NPKM). Measurements included soil nutrients, activities of amylase (AMY), sucrase (SUC), cellobiohydrolase (CBH), β-1,4-glucosidase (βG), and content of particulate organic carbon (POC), easily oxidizable organic carbon (EOC), dissolved organic carbon (DOC), and microbial biomass carbon (MBC). Data were analyzed using ANOVA, correlation analysis, Mantel tests, and partial least squares path modeling to elucidate changes and underlying linkages.【Result】Compared with CK, the NPKM treatment demonstrated the most comprehensive improvement in soil organic carbon and its active components. Soil organic carbon (SOC) content increased by 6.7 g·kg-1, while POC, EOC, MBC, and DOC content was significantly increased by 2.5 g·kg-1, 3.0 g·kg-1, 97.5 mg·kg-1, and 14.1 mg·kg-1, respectively, with the geometric mean of active carbon also rising substantially by 50.1%. Under the NPKS treatment, SOC, POC, and MBC content was also increased significantly by 2.3 g·kg-1, 1.5 g·kg-1, and 93.0 mg·kg-1, respectively. In contrast, both the NPK and NK treatments reduced POC and MBC content, with the geometric mean of active carbon decreasing by 24.7% and 26.8%, respectively. The carbon pool management index increased significantly only under the NPKM treatment by 55.9% compared with CK, while the carbon pool activity decreased significantly by 33.6% under the NK treatment. Regarding enzyme activity, all fertilization treatments enhanced AMY activity compared with CK. Additionally, the NK and NPK treatments significantly increased CBH and βG activities, with the NK treatment showing the greatest increase: CBH rose by 167.3 μmol·g-1·h-1 and βG by 457.7 μmol·g-1·h-1. In terms of soil nutrients, the NPKM treatment significantly increased total nitrogen (TN), total phosphorus (TP), available phosphorus (AP), and available potassium (AK) content, with increases of 0.72 g·kg-1, 1.85 g·kg-1, 148.9 mg·kg-1, and 402.1 mg·kg-1, respectively, compared with CK. Conversely, the NK treatment resulted in a significant decrease in pH by 2 units and a reduction in TP content by 0.2 g·kg-1. Furthermore, the NK treatment increased soil C:P and N:P, while other fertilization treatments generally decreased C:N, C:P, and N:P. Correlation and path analysis indicated that soil TN, TP, AP, and AK directly promoted the accumulation of POC, EOC, and MBC. In contrast, higher C:P and N:P stimulated CBH and βG activities, thereby accelerating carbon decomposition and mineralization, which inhibited the accumulation of POC, EOC, and MBC.【Conclusion】Long-term combined application of chemical fertilizers with organic amendments not only improved soil nutrient content but also increased labile organic carbon by optimizing nutrient balance and modulating carbon-hydrolyzing enzyme activities. Thus, integrating organic manure with fertilizers represents the optimal strategy for enhancing black soil fertility in Northeast China.
