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Soil Chemistry

Modeling the Effects of Fertilizer Application Rate on Nitrous Oxide Emissions

^a Dep. of Renewable Resources, Univ. of Alberta, Edmonton, AB, Canada T6G 2E3
^b Agric. and Agri-Food Canada Research Branch, Ottawa, ON, Canada K1A 0C6
^c Conservation and Development, Alberta Agric. and Rural Development, Edmonton, AB, Canada T6H 5T6
^d Crop Diversification Centre North, Alberta Agric. and Rural Development, Edmonton, AB, Canada T6H 5Z2

^* Corresponding author (robert.grant{at}afhe.ualberta.ca)

The attribution of N₂O 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 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 model 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 current 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.

Abbreviations: DOC, dissolved organic carbon • SOC, soil organic C • SON, soil organic N • WFPS, water-filled pore space

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