Nitrous oxide emissions and biogeochemical responses to soil freezing-thawing and drying-wetting

Soil freeze-thaw (FT) and dry-wet (DW) cycles are brief transitory biophysical changes, but these events have important implications in determining the timing and magnitude of N2O emissions and may represent a significant proportion of annual N2O emissions from agricultural systems. It is often assumed that FT and DW cycles influence the processes of N2O production and emission in a similar manner, however, research has yet to systematically identify the similarities and differences in the mechanisms which lead to potentially higher N2O fluxes during FT compared to DW cycles. Herein, we present the first review to do so; in addition, we identify strategic research areas required for improving the understanding of FT and DW processes leading to N2O emissions. There are key differences between the mechanisms that contribute to N2O fluxes during FT and DW cycles, centered on the duration and spatial extent of anaerobiosis, temperature sensitivity of microbial activity, relative gas diffusivity, and soil water dynamics. These differences might increase the risk of N2O emissions during FT cycles relative to soil DW cycles. Current research gaps include (i) the identification of organic substrates made available due to FT and DW cycles, and their contribution to ensuing N2O fluxes, (ii) an understanding of how cryosuction dynamics potentially influence N2O production and emission, (iii) understanding and predicting the air-entry potential of soil as it relates to N2O fluxes, (iv) identifying the relative significance of dissolved N2O in soil water and its solubility changes during FT and DW phases, and (v) determining microbial community and functional changes across soil spatial and temporal scales. Advances in these areas are recommended for improving process descriptions in biogeochemical models in order to more accurately predict N2O emissions from soils prone to FT and DW cycles.