To what extend that biochar addition promotes organic carbon increase in saline-sodic soils, however, remains poorly understood. Here, we evaluated soil organic carbon (SOC) contents change before and after biochar addition, and deciphered which driving factor or process govern SOC change with biochar application. There was a limited increase in SOC, about by +1.16%~+12.8%, even biochar was applied at the rate of 10% of bulk soil weight. However, soil dissolved organic carbon (DOC) increased significantly by up to 67%. About half SOC was stored in small macroaggregates (250-2000 μm, CPOC), and SOC in silt and clay-sized particles (<53 μm) decreased obviously with biochar addition. Microorganism biomass, represented by phospholipid fatty acid, increased with biochar amendment, of which actinomycetes, fungi, protozoon, and bacteria with straight-chain saturated fatty acids (OB) increased remarkably. DOC was governed by ACT and soil N:P ratio, while SOC mostly depended on CPOC. Biochar addition aggravated nitrogen limitation in saline-sodic soils, and the roles of microorganisms on regulating SOC greatly depended on nitrogen bioavailability. Biochar amendment had greatly changed interactions between environmental factors and SOC in saline-sodic soils. The effects of nutrients on soil carbon changed to strongly negative from strongly positive after and before biochar addition, meanwhile, aggregation was the only factor with positive effects on soil carbon change. How to mitigate nutrient limitation and improve soil aggregation process should be considered in priority when biochar was used to increase SOC in saline-sodic soils.