DAYCENT MODEL ANALYSIS OF PAST AND CONTEMPORARY AGRICULTURAL SOIL N2O AND NET GREENHOUSE GAS EMISSIONS IN THE USA

Authors: DEL GROSSO, S - CSU, FORT COLLINS, CO; MOSIER, ARVIN; PARTON, W - CSU, FORT COLLINS, CO; OJIMA, D - CSU, FORT COLLINS, CO

Submitted to: Soil and Tillage Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/1/2005
Publication Date: 8/1/2005
Citation: Del Grosso, S.J., Mosier, A.R., Parton, W.J., Ojima, D.S. 2005. Daycent model analysis of past and contemporary agricultural soil n2o and net greenhouse gas emissions in the usa. International Journal of Soil and Tillage Research. 83:9-24.

Interpretive Summary: The national inventory of greenhouse gases, e.g. carbon dioxide, methane and nitrous oxide from all U.S. sources is prepared annually by the U.S. Environmental Protection Agency. The agricultural sector contributes the largest part of the U.S. nitrous oxide emissions according to this inventory, primarily from agricultural soil management. The inventory is based upon an emission factor based calculation method that does not allow for accounting for a number of factors which directly influence nitrous oxide production in the soil, e.g. climate, crop, fertilizer source, management or soils. In attempt to improve the numbers in the U.S. inventory for nitrous oxide derived from agriculture a process based model, DAYCENT, is being developed to estimate nitrous emissions for the country. This manuscript describes the use of DAYCENT to perform an estimate of nitrous oxide emissions from the U.S. that accounts for differences in the factors, noted above that impact nitrous oxide emissions from soils.

Technical Abstract: The DAYCENT ecosystem model was used to estimate direct and indirect N2O emissions and net greenhouse gas fluxes for agricultural land in the USA. N2O fluxes and NO3 leaching estimates by DAYCENT and by the IPCC (1997) emission factor methodology (EF) were compared with field data for different cropping systems. To estimate national GHG fluxes, the continental USA was divided into 63 minor agricultural regions and the primary cropping systems in each region were simulated. In addition to recent modern agriculture, pre-1940 crop management and native vegetation were also simulated. The 63 minor regions were aggregated into 5 major regions and decadal mean N2O, CO2, and CH4 fluxes were estimated for the 5 major regions using DAYCENT and EF. For both models, N2O emissions were highest in the central USA followed by the northwest, southwest, southeast, and northeast regions. At the national scale, DAYCENT simulations of total N2O emissions were ~30% lower than estimated using EF. EF simulated slightly higher direct N2O emissions from fertilized crops but substantially higher direct N2O than DAYCENT for N fixing crops. DAYCENT estimated indirect N2O emissions from NH3 and NOx gases and NO3 leaching to be almost twice as high as EF. DAYCENT and EF estimates of the gaseous component of indirect N2O emissions (NO + NH3) differed little but DAYCENT estimated approximately twice the indirect emissions from NO3 leaching because the EF leaching estimate does not include the contribution of N from N-fixing crops. To consider the suite of greenhouse gases, the CO2 costs of N fertilizer production and DAYCENT simulated values for soil CO2, N2O, and CH4 fluxes were converted to a common unit of CO2-C equivalents and summed to calculate net greenhouse gas flux (GHGnet). DAYCENT estimates that recent modern cropping is a GHGnet source of ~30 g CO2-C m-2 equivalents annually, pre-industrial cropping was a source of ~26 g CO2-C m-2 equivalents annually, and native systems were a sink of ~13 g CO2-C m-2 equivalents annually.