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[Objective]Soil aggregate active organic carbon is an important component of soil aggregate organic carbon,which plays an important role in the stability of soil ecosystem.Research on the influence of soil microorganisms on ALOC formation and stability has important theoretical and practical significance for further recognizing change characteristics and change process of meadow soil organic carbon in the alpine environment,and carry out organism directional control based on the microbial living environment.[Methods]Three research areas are randomly selected in alpine meadow of Northern Tibet.Un-degraded grassland,mild degraded grassland and seriously degraded grassland in each research area are selected as the sampling areas;undisturbed soil samples are collected in three sampling points in each sampling area according to surface layer (0-10 cm) and subsurface layer (10-20 cm).Wet screening method is applied for grading soil aggregates.The influence of soil microorganisms on ALOC formation and stability of alpine grassland is analyzed through measuring ALOC content,microbial biomass carbon (MBC) and cellulose decomposing enzyme activity (CEA).[Results](1) MBC has significant (p < 0.05) promotion role to >0.25 mm graded ALOC formation and accumulation in un-degraded grassland surface layer and sub-surface soil.<0.25 mm graded ALOC content is decreased at different degrees with increase of soil MBC content.The >0.25 mm graded ALOC is prominently decreased with increase of MBC content,but sub-surface <0.25 mm graded ALOC content shows an opposite trend in mild degraded grassland surface and severely degraded grassland surface soil.(2) >0.25 mm graded and <0.25 mm graded ALOC contents are decreased at different degrees with the increase of CEA in un-graded grassland surface and sub-surface soil.CEA has different promotion roles to increase of >0.25 mm graded and <0.25 mm graded ALOC contents in mild and severely degraded grassland surface and subsurface soil.[Conclusion]The influence of soil MBC and CEA on ALOC formation and stability are prominently different due to grassland degradation degree,soil depth and soil aggregate size in the plateau cold drought environment.