Model evaluation of different mechanisms driving freeze-thaw N2O emissions

N2O emissions from soil contribute significantly to global warming. Pulse emissions of N2O from soils during freeze-thawing were recently recognized as important atmospheric sources. In this modelling study we explore three different hypotheses for explaining freeze-thaw related N2O emissions: (I) soil frost or snow cover may reduce gas diffusion and create anaerobic conditions that stimulate N2O production via denitrification, (2) microbes that die of frost deliver easy decomposable organic carbon and nitrogen to the soil, which Stimulates microbial growth and vigorous N2O production during freeze-thaw, and (3) the enzyme nitrous oxide reductase, which is responsible for the reduction of N2O to N-2 during denitrification, is more sensitive to low temperatures than other enzymes, so that N2O becomes the dominating end-product of denitrification at low temperatures. These hypotheses were tested with a biogeochemical model that combines hydrology and physics calculations with a newly developed, parameter-poor biochemistry module. The model was first calibrated with field datasets on soil-atmosphere fluxes of N2O, NO and CO2 and soil NO3 and NH4 concentrations that were measured in a spruce forest in Southeast Germany in the years 1994-1997. Subsequently, additional model mechanisms were implemented that allow the model to describe the outlined mechanisms potentially driving freeze-thaw N2O fluxes. After each implementation the model was recalibrated. We were able to mimic dimension and timing of high N2O emissions when either one of the first two hypotheses were assumed, but found no confirmation for the third. The best model fit was achieved by combining hypothesis one and two, indicating that freeze-thaw N2O emissions are not mono-causal. (C) 2009 Elsevier B.V. All rights reserved.