Modelling freeze-thaw related N2O emissions: recent advances & future perspectives

Soil freeze-thaw (FT) cycles induce high nitrous oxide (N₂O) emissions across all ecosystems, whereby flux rates are highest for agricultural systems, where more than half of the annual N₂O emissions may result from FT related fluxes. Globally, neglecting FT related N₂O emissions may lead to an underestimation of the annual N₂O budget by almost a quarter. However, FT related N₂O emissions are hardly implemented in and simulated by state-of-the-art ecosystem models yet, because of a lack of knowledge about the actual mechanisms explaining timing and magnitude of the observed N₂O emission peaks.

Here we review recent advances in process understanding, which can be summarized into three approaches: (i) a frozen (top)soil (or snow) layer that acts as physical barrier for gas diffusion, (ii) the production of additional decomposable substrate during freezing-thawing, and (iii) temperature-depending changes in the biochemical balances within the denitrification process. We implemented the different mechanisms in the LandscapeDNDC ecosystem model, which provides an advanced representation of soil nitrogen processes, and validate their effects on site scale, before we evaluate their importance on regional scale and as part of the annual N₂O budget. This will enable us to improve national to global estimates of annual N₂O emissions and lower the current uncertainty due to the neglect of FT related N₂O fluxes.