Abstract: Aims The mechanism of the carbon cycle and the function of the grassland ecosystem as a carbon sink are both continuously impacted by the rising nitrogen deposition. An important measure that can reveal changes in the soil's carbon pool is active organic carbon. For a thorough understanding of the carbon cycle process and the creation of sensible ecosystem management strategies, investigating the impact of nitrogen addition on the active organic carbon fractions of grassland soil is essential. Methods In this paper, five different nitrogen addition treatments were set up in the typical temperate steppe of Nei Mongol. Soil organic carbon fractions, soil physical and chemical properties, aggregate stability, microbial activities and extracellular enzyme activities were measured. Pearson correlation and structural equation model (SEM) were used to examine the results. Important findings Nitrogen addition reduced the contents of dissolved organic carbon (DOC), microbial biomass carbon (MBC) and easily oxidizable organic carbon (EOC). Additionally, the contents of DOC, MBC and EOC all decreased with the increase of soil depth. The 5 g·m–2·a–1 nitrogen addition treatment significantly promoted the decomposition of active organic carbon fractions. The effect of nitrogen application on soil active organic carbon fractions was regulated by biotic (microbial biomass, extracellular enzyme activity, etc.) and abiotic (soil physical and chemical properties, aggregate stability, etc.) factors. Nitrogen addition reduced soil density (SD), increased mean mass diameter (MMD) and the proportion of large aggregates, increased the contact between organic matter and substrate, promoted the decomposition of active organic carbon, and reduced DOC and EOC. Nitrogen addition inhibited the activities of polyphenol oxidase (POX) and peroxidase (PER), reduced the decomposition of difficult-to-decompose organic matter, and reduced the content of EOC and MBC. In addition, the addition of nitrogen increased the activities of β-glucosidase (BG) and cellulose hydrolase (CB), promoted the utilization of DOC by microorganisms, and reduced the content of DOC. The above results indicated that nitrogen addition treatment can affect the decomposition process of active organic carbon by changing soil physicochemical properties and the secretion of extracellular enzymes from microorganisms, promoting the release of carbon in typical temperate grassland soil. This provided a theoretical basis for further exploration of grassland soil carbon dynamics under nutrient addition in the future.

Key words: nitrogen addition, active organic carbon, soil aggregates, biotic factors, physicochemical properties