A regional study was conducted to investigate historical variations in soil organic carbon (SOC) storage in agricultural lands along a climate gradient in the Loess Plateau. A process-based model, Denitrification–Decomposition (DNDC) was used to simulate SOC dynamics. A database containing spatially differentiated climate, soil and farming management information was linked to DNDC to support the regional simulations. DNDC was run for all croplands in the Loess Plateau for 20 years. Simulation results demonstrated that SOC stocks in the region amounted to 1.15 Pg C and the SOC content of 65% farmland was below China's national level. Crop management could effectively improve SOC storage and available water played a key role in controlling cropland SOC dynamics. Analyses of SOC change and climate variability indicate that the regional SOC increased along the moisture gradient. We found that (1) higher annual precipitation increased crop productivity, particularly in rain-fed cropping systems, adding more crop residue (i.e., litter) production to the soils that favors SOC accumulation; and (2) irrigation played a crucial role in maintaining the SOC content in areas with low precipitation, where the enhanced irrigation capacity could substantially elevate the SOC storage in the semi-arid domain. The soil in the Loess Plateau is likely to accumulate SOC and to produce a carbon equilibrium rapidly when available water and water use efficiency increase.
