Final Report Improving DNDC Modeling Capability to Quantify Mitigation Potential of Nitrous Oxide from California Agricultural Soils

This project refined a biogeochemical model, DeNitrification-DeComposition (DNDC), so that it can be used to quantify nitrous oxide (N2O) emissions as well as N2O mitigation potentials under California conditions. Specifically, we (1) compiled N2O measurements from California fields across a wide range of crops and farming management practices (FMPs); (2) refined DNDC and tested the model against the collected data to verify the model's ability of representing diverse crops and management practices in California; (3) updated a California-specific GIS databases for statewide greenhouse gases (GHG) modeling; (4) estimated N2O emissions from California croplands; and (5) assessed efficiency of alternative FMPs in mitigating N2O emissions. We improved DNDC in the following aspects: we (1) offered two irrigation simulation options for the DNDC regional mode: an irrigation index and irrigation event; (2) compiled irrigation management data into the California-specific database and added the capacity of varying irrigation water depth with county air temperatures; (3) expanded the database by including more cropping systems and multi-years information of climate and crop area; and (4) improved manure parameterization to reflect application of slurry manure in California croplands. Besides these improvements, we also updated the fertilization and crop residue returns in the database. We verified DNDC against the N2O measurements collected across a range of California cropping systems. The DNDC simulations matched observed N2O peaks, which primarily responded to fertilization and/or irrigation. Altogether, we tested DNDC against 52 cases to verify its applicability for quantifying N2O emissions from different cropping systems in California. The correlation between the DNDC-modeled and measured annual/seasonal N2O emissions is significant with an Rof 0.87, a slope of 1.00, and a P of < 0.001, indicating that DNDC reliably predicted the N2O emissions from the cropping systems. We used the validated DNDC and the updated database to simulate historical N2O emissions from California croplands. The simulated annual N2O emissions from California croplands ranged between 3.49 × 10 and 6.37 × 10 metric ton (MT) N yr with a mean annual emissions of 5.23 × 10 MT N yr from 2000 to 2015. The estimates further indicated that (1) the direct N2O emissions from California croplands showed a significant decreasing trend since 2002 primarily due to significant reductions of croplands area and N inputs as well as N2O emission rate; (2) major contributing crops of N2O were corn, non-legume hay, alfalfa, and cotton; and (3) the counties with substantial N2O emissions are San Joaquin, Fresno, and Tulare. Additional DNDC simulations were carried out to assess uncertainties of the N2O estimates. Uncertainty analyses were performed for (1) soil properties, (2) irrigation water depth, and (3) management event scheduling, using the 2012 activity database. The largest uncertainties of the DNDC N2O estimates would arise from soil properties. The range of the DNDC N2O estimate would vary between 3965 Mg N and 7799 Mg N. The efficiency of different FMPs on mitigating N2O emissions was assessed by conducting a series of simulations with varying FMPs and comparing the simulations under alternative FMP scenarios against the baseline simulation. We set eight alternative scenarios by exclusively modifying a FMP applied under the baseline scenario, including nitrogen application, application of nitrification inhibitor, tillage, planting of cover crop, or irrigation management, to investigate impacts of each management practice on N2O emission. The practices identified with relative high mitigation efficiency (> 20% reduction) included combing N rate reduction by 15% and application of nitrification inhibitors, planting non-leguminous cover crops, surface drip irrigation, and subsurface drip irrigation.