Winter and spring thaw nitrous oxide emissions linked to nitrous oxide production in the soil profile

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Hu, Quancai

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University of Guelph

Abstract

The soil processes leading to N2O emissions during winter freezing and spring thawing are not well understood. The principal objectives of this thesis were (i) to determine the crucial factor in triggering a burst in N2O emissions from soils under conventional and best management practices; (ii) to investigate the processes of N2O production in the frozen soil profile; (iii) to elucidate the mechanism of N2O production in the frozen soil profile. The increase of soil water content in the frozen soil layer was the main characteristic of the frozen soils during winter. The crucial factor in triggering N2O fluxes was found to be the high water content in the thawed layer originated from the frozen layer when two agricultural practices were compared in eastern Canada. Total annual and spring thaw N2O emissions measured by a micrometeorological technique from conventional practice soil were higher than that from best management practice soil. Snow depth influenced by agricultural management practices, which affected the depth of soil freezing, was a key factor in influencing N2O fluxes. N2O concentrations at different soil depths were associated with two wet soil layers, i.e., analogous capillary fringes, at the top and bottom of frozen layer. The high water content in the analogous capillary fringes triggered a burst in N2O and CO2 production by enhancing microorganism activity. The correlation between N2O and CO2 concentrations confirmed that biological processes were responsible for N2O production. The occurrence of N2O fluxes measured by chamber method was similar to that by a micrometeorological technique during the periods of soil thawing. 15N2O measurements confirmed the source of N2O fluxes was the 'new' N2O produced in thawed layers. The high 15N enrichment of fluxed N2O confirmed that denitrification was the mechanism responsible for the burst N2O fluxes. A new method for simulating soil freezing and thawing cycles was developed to study nitrous oxide fluxes in the laboratory. The N2O fluxes during these cycles were affected by the degree of soil freezing and water content in soil columns.

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N2O emissions, soils, conventional management practices, best management practices, N2O production, frozen soil profile, soil freezing

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