Optimized organic-inorganic fertilization enhances soil carbon sequestration and wheat productivity: Evidence from hierarchical carbon pool analysis

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. 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. 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). 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 (β = 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. These findings demonstrate that intensified fertilization with balanced organic-inorganic inputs can reconcile the trade-off between soil health and agricultural productivity, providing a sustainable intensification strategy for degraded dryland soils. The identified carbon transformation pathways offer mechanistic insights for optimizing soil carbon management in semi-arid agricultural systems.