Modeling the Effects of Fertilizer Application Rate on Nitrous Oxide Emissions

The attribution of N 2 O emission factors to N inputs from chemical fertilizers requires an understanding of how those inputs affect the biological processes from which these emissions are generated. We propose a detailed model of soil N transformations as part of the ecosystem model ecosys for use in attributing N 2 O emission factors to fertilizer use. In this model, the key biological processes—mineralization, immobilization, nitrification, denitrification, root, and mycorrhizal uptake—controlling the generation of N 2 O were coupled with the key physical processes—convection, diffusion, volatilization, dissolution—controlling the transport of the gaseous reactants and products of these biological processes. Physical processes controlling gaseous transport and solubility caused large temporal variation in the generation and emission of N 2 O in the model. This variation limited the suitability of discontinuous surface flux chambers measurements used to test modeled N 2 O emissions. Continuous flux measurements using micrometeorological techniques were better suited to the temporal scales at which variation in N 2 O emission occurred and at which model testing needed to be conducted. In a temperate, humid climate, modeled N 2 O emissions rose nonlinearly with fertilizer application rate once this rate exceeded the crop and soil uptake capacities for added N. These capacities were partly determined by history of fertilizer use, so that the relationship between N 2 O emissions and current N inputs depended on earlier N inputs. A scheme is proposed in which N 2 O emission factors rise nonlinearly with fertilizer N inputs that exceed crop plus soil N uptake capacities.