Simulation of soil organic carbon and wheat yield by long-term multiple croppingand rolling green manure based on DNDC model

In response to the unique spring wheat leguminous green manure cultivation model employed in the farmland of the Qinghai Plateau, this study investigates the long-term effects of green manure return on spring wheat yield and soil organic carbon (SOC). A combination of plot experiments and models was used to explore a suitable green manure return method for Qinghai Plateau. A split plot experimental design was adopted, with the main plot being the autumn fallow period, when multiple green manures are used for turning over and returning to the field (G) and no green manure is used for returning to the field (CK). The sub plot is the amount of fertilizer used for subsequent spring wheat, with 100% conventional fertilization (F₁₀₀, 225 kg·ha⁻¹ N and 112.5 kg·ha⁻¹ P₂O₅), 70% conventional fertilization (F₇₀, 157.5 kg·ha⁻¹ N and 78.75 kg·ha⁻¹ P₂O₅), and no fertilizer application (F₀). A total of six treatments were used to calibrate and validate the denitrification decomposition (DNDC) model based on field experimental data from 2019 to 2022. The validated model was used to simulate the long-term changes in SOC and wheat yield under different treatments. The results showed that the DNDC model effectively simulated SOC and wheat yield. The NRMSE values of simulated and measured SOC during the calibration and validation process ranged from 0.017 to 0.295, with R² values between 0.763 and 0.924. The NRSME values of simulated and measured wheat yields ranged from 0.160 to 0.280, with R² > 0.75 for both, indicating consistency between the simulated and measured values. A calibrated and validated model can be used to simulate soil SOC and wheat yield in 0－50 cm soil. The simulation results for 30 years showed that returning green manure to the field increased soil SOC, specifically manifested as F₇₀ + G > F₁₀₀ + G > F₀ + G > F₁₀₀ > F₇₀ > F₀. The 30-year variation of soil SOC storage in the 0－50 cm soil layer was 172.82 kg·ha⁻¹, as follows: 136.91, 65.21, ‒35.61, ‒46.70, and ‒143.95 kg·ha⁻¹. By contrast, the yield appeared to follow a trend of F₇₀ + G > F₁₀₀ + G > F₁₀₀ > F₇₀ > F₀ + G > F₀. The combination of green manure and returning to the field with chemical fertilizer showed a better yield increase effect. Compared with treatment without green manure, the wheat yield under F₀ + G, F₇₀ + G, and F₁₀₀ + G treatment increased by 85.7%, 87.5%, and 20.0%, respectively. The simulated SOC content showed a correlation with wheat yield. Overall, long-term multiple cropping of green manure with 70% chemical fertilizer was found to have the best effect on carbon sequestration and yield increase.