Estimating the Tipping Point between N2O Emissions and C Sequestration in Soil using the DNDC v. CAN Model

Increasing the grassland proportion in the crop rotation has been considered as an effective approach to sequester carbon (C) in the soil. However, its climate mitigation benefits may be overestimated because the associated impact of long-term C sequestration on nitrous oxide (N₂O) emissions remains uncertain. Mechanistic models, such as the DeNitrification and DeComposition model (DNDC v. CAN 9.5.0), are used to simulate changes in soil organic carbon (SOC) and N₂O emissions. This provides the opportunity to estimate future emission trends and to enhance our understanding of the interactions between SOC and N₂O emissions under different levels of grass/clover proportion in arable crop rotations. We hypothesize that increases in N₂O emissions will offset the benefits from the increased SOC over time. The objectives of this study are to (1) calibrate and validate the DNDC model, and (2) estimate and predict the potential tipping point at which the negative climate forcing of N₂O emissions offsets the benefits of C sequestration over long-term timescales. For this, we used long-term measurements of biomass, SOC, and N₂O emissions from two crop rotations with either two or four years of grass-clover in a six-year rotation in Denmark. Preliminary results showed that the DNDC model simulated crop biomass production with fair to high accuracy as indicated by an index of agreement (d) of 0.98, a Nash-Sutcliffe efficiency (NSE) of 1, and a normalized root mean square error (nRMSE) of less than 30%. The simulated biomass was slightly underestimated as shown by a negative mean bias error (MBE). Conversely, the simulations for N₂O fluxes and SOC exhibited poorer agreement, with d-values below 0.7 and nRMSE exceeding 30%. These findings suggest that while the DNDC model effectively predicts crop growth, including annual crops and grass/clover ley, its ability to simulate SOC and N₂O fluxes requires substantial improvement. Our future efforts will focus on refining and optimizing model parameters for SOC and N₂O, with an emphasis on calibration to enhance the model performance and the capacity to predict management-induced long-term dynamics under future climate scenarios. Results of these updated model simulations will be shown at the conference.