Factors Regulating Aggregate Stability and Soil Organic Carbon Accumulation Capacity in Abandoned Grasslands Under Combined Fertilization

Abandoned grassland constituted a critical transitional stage during natural grassland restoration, and soil organic carbon (SOC) dynamics dominated the reconstruction of regional carbon-sink functions. However, the mechanisms underlying aggregate-associated SOC formation and stabilization under fertilization intervention in abandoned grasslands remained poorly understood. A one-year in situ fertilization control experiment was conducted using a space-for-time substitution approach in 12-year (12a) and 31-year (31a) abandoned grasslands in the Bayanbulak region. Combined inorganic–organic fertilization (NOF) used organic fertilization (OF; sheep manure) and inorganic nitrogen fertilization (NF). Aggregate stability, glomalin-related soil protein (GRSP), and soil SOC accumulation under different fertilization regimes were examined. Relative to no fertilization (CK), NOF significantly raised mechanically- and water-stable aggregate mean weight diameter (MWD) and geometric mean diameter (GMD) by 8.45–15.91% and 27.41–41.96% at 12a and 31a. Compared with CK, NOF significantly decreased the percentage of aggregate destruction (PAD) by 52.09% (12a) and 45.92% (31a). SOC mainly accumulated in LAs and SAs, and aggregate-bound SOC fell with shrinking particle size. Compared with CK, OF markedly raised bulk SOC by 12.53% (12a) and 13.97% (31a) relative to CK; OF significantly increased macroaggregate T-GRSP and EE-GRSP by 17.94–20.91% and 30.11–38.96% across 12a and 31a abandonment. Macroaggregates served as the primary storage pool for SOC and T-GRSP. Fertilization, particularly the inorganic–organic combination, substantially promoted SOC and GRSP accumulation and improved the SOC contribution of macroaggregates. GRSP was extremely positively correlated with SOC and aggregate stability. Grassland abandonment significantly strengthened the coupling between GRSP and SOC. Combined organic–inorganic fertilization improves soil structure and aggregate stability, whereas sole organic fertilization yields rapid increases in soil SOC concentration over the short term. Collectively, all fertilization treatments with organic inputs promote SOC accumulation in abandoned grasslands, which greatly benefited the restoration of degraded grasslands.