DAYCENT model analysis of past and contemporary soil N2O and net greenhouse gas flux for major crops in the USA

The DAYCENT ecosystem model (a daily version of CENTURY) and an emission factor (EF) methodology used by the Intergovernmental Panel on Climate Change were used to estimate direct and indirect N2O emission for major cropping systems in the USA. The EF methodology is currently used for the USA greenhouse gas inventory but process based models, such as DAYCENT, may yield more reliable results because they account for factors such as soil type, climate, and tillage intensity that are ignored by EF. Comparison of mean annual soil N2O flux estimated by DAYCENT and EF with measured data for different cropping systems yielded r2 values of 0.74 and 0.67, and mean deviations of −6 and +13%, respectively. At the national scale, DAYCENT simulation of total N2O emission was ∼25% lower than estimated using EF. For both models, N2O emission was highest in the central USA followed by the northwest, southwest, southeast, and northeast regions. The models simulated roughly equivalent direct N2O emission from fertilized crops, but EF estimated greater direct N2O emission than DAYCENT for N-fixing crops. DAYCENT and EF estimates of the gaseous component of indirect N2O emission (NO + NH3) differed little, but DAYCENT estimated approximately twice the indirect emission from NO3 leaching since it included the contribution of N from N-fixing crops while EF did not. DAYCENT simulations were also performed for no tillage cropping, pre-1940 crop management, and native vegetation. DAYCENT-simulated N2O, CO2, and CH4 fluxes were converted to CO2–C equivalents and combined with fuel use estimates to estimate net global warming potential (GWPnet). GWPnet for recent non-rice (Oryza sativa L.) major cropping was 0.43 Mg C ha−1 yr−1 under conventional tillage and 0.29 Mg C ha−1 yr−1 under no tillage, for pre-industrial cropping was 0.25 Mg C ha−1 yr−1, and for native systems was −0.15 Mg C ha−1 yr−1. Results from DAYCENT suggest that conversion to no tillage at the national scale could mitigate ∼20% of USA agricultural emission or ∼1.5% of total USA emission of greenhouse gases.