Simulation of the irrigation requirements for improving carbon sequestration in a rainfed cropping system under long-term fertilization on the Loess Plateau of China

The nutrients and water supplied to rainfed agroecosystems need to be properly managed to meet food security and global warming challenges in the coming decades. A revised CENTURY model was adopted to evaluate the effects of long-term fertilization and supplemental irrigation (SI) on carbon (C) sequestration in a conventional 3-year rotation on the Loess Plateau of China (CWWM): spring corn in the first year, followed by 2 years of winter wheat, and fallow millet was sown in June of the third year. The simulation showed that grain C and soil organic C (SOC) benefited from long-term synthetic nitrogen (N) application. The contribution of this treatment to SOC accumulation was less than that of manure, but the concomitant increase in grain C was greater than that under manure treatment. Fertilization coupled with SI exerted a positive effect on grain C and SOC accumulation for both corn and wheat. The supply of 142 mm of irrigation water when the soil moisture status fell below 80% of the soil available water content (SAWC) in April for spring corn improved grain C accumulation by 33.7% compared with the control (CK); similarly, supplying 83 mm of irrigation water to winter wheat in October increased its grain C by 16.8%. Simulated SOC at the end of a 30-year period in which SI was applied once per month was higher than that obtained under SI in a single month. The highest SOC of 2256.5 g m⁻², 4% higher than in CK, was obtained when SI was applied during corn growing season when the soil moisture fell below 50% of the SAWC. These findings implied that irrigation should be seriously considered for improving C sequestration in rainfed agroecosystems.