Modeling the effects of fertilizer application rate on nitrous oxide emissions

The attribution of N2O emission factors to N inputs from chemical fertilizers requires an understanding of bow 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 N2O emission factors to fertilizer use. In this model, the key biological processes-mineralization, immobilization, nitrification, denitrification, root, and mycorrhizal uptake - controlling the generation of N2O 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 N2O in the model. This variation limited the suitability of discontinuous surface flux chambers measurements used to test modeled N2O emissions. Continuous flux measurements using micrometeorological techniques were better suited to the temporal scales at which variation in N2O emission occurred and at which motel testing needed to be conducted. In a temperate., humid climate, modeled N2O 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 N2O emissions and carrent N inputs depended on earlier N inputs. A scheme is proposed in which N2O emission factors rise nonlinearly with fertilizer N inputs that exceed crop plus soil N uptake capacities.