Context: The Loess Plateau region faces severe soil degradation with organic carbon content typically below 1.2 %, substantially lower than the 2 % threshold for optimal soil function. Balancing soil carbon sequestration with agricultural productivity remains a critical challenge for sustainable intensification in these degraded dryland soils. Objective: This study aimed to determine whether optimized organic-inorganic fertilization could simultaneously enhance soil organic carbon sequestration and wheat productivity, and to elucidate the underlying mechanisms of carbon pool dynamics and nutrient cycling. Methods: A randomized complete block experiment with five treatments was established: control (CK), organic fertilizer only (M, 7500 kg/ha sheep manure), chemical fertilizer only (NPK: urea 212 kg/ha, calcium superphosphate 170 kg/ha, potassium sulfate 120 kg/ha), conventional organic-inorganic combination (MNPK, 7500 kg/ha sheep manure plus standard NPK rates), and doubled organic-inorganic combination (2MNPK, 15,000 kg/ha sheep manure plus doubled NPK rates: urea 424 kg/ha, calcium superphosphate 340 kg/ha, potassium sulfate 240 kg/ha). Results and conclusions: Compared to conventional chemical fertilization (NPK), the 2MNPK treatment increased grain yield by 17.2 % (from 3642 to 4268 kg/ha) and total soil organic carbon by 18.1 % (from 11.5 to 13.6 g/kg). Relative to conventional organic-inorganic fertilization (MNPK), 2MNPK enhanced light fraction organic carbon by 25.6 % (from 30.8 to 38.7 g/kg), the most dynamic carbon pool driving carbon sequestration. Compared to control, 2MNPK increased grain yield by 29.8 %, light fraction organic carbon (partially decomposed plant residue carbon) by 69.0 %, and total soil organic carbon by 34.7 %. Microbial biomass carbon and nitrogen increased by 84.2 % and 112.1 % respectively. Available nitrogen, phosphorus, and potassium increased by 106.7 %, 175 %, and 135.3 %. Structural equation modeling revealed fulvic acid carbon as a key mediator promoting light fraction organic carbon formation (beta = 0.70, P < 0.001), which dominated total carbon accumulation. Optimized organic-inorganic fertilization achieved synergistic enhancement of carbon sequestration and crop productivity through improved nutrient cycling, enhanced microbial activity, and hierarchical carbon transformation pathways.