Overwinter greenhouse gas fluxes in two contrasting agricultural habitats

Mid-day field fluxes of nitrous oxide (N₂O), carbon dioxide (CO₂) and methane (CH₄) were measured during late winter/early spring in an arable field and an adjacent fallow in southern Germany. On the arable field, 2 dm high ridges, drawn as seed-beds for potato, were exposed to mild, partly diurnal freezing–thawing. Substantially elevated N₂O emission rates (6–750 µg N₂O-N m^–2 h^–1) were observed throughout the investigation period which coincided with freezing–thawing events in the surface soil (0–5 cm). Soil temperatures in the densely vegetated fallow were more isothermal due to an insulating snow/ice cover, resulting in much lower N₂O emission rates (0–57 µg N₂O-N m^–2 h^–1). CH₄ uptake rates were low in both habitats during soil frost (+2 to –7.5 µg CH₄-C m^–2 h^–1) but increased markedly in the fallow after spring thaw. Our data suggest that N₂O emission peaks may occur recurrently throughout the winter when soils are subjected to diurnal surface thawing. We concluded that microclimatic conditions strongly control N₂O winter loss, thus overriding ecosystem-level differences in off-season nutrient cycling. To further characterize winter-time nutrient cycling and habitat functioning in our sites, we determined NO₃^– and NH₄⁺ contents, fumigation-extractable carbon (C_mic) and nitrogen (N_mic) and enumerated protozoa and nematoda throughout the investigation period. C_mic and microbial C:N ratios in the fallow were higher in winter than during the rest of the year as indicated by a 2-year study, reflecting favorable conditions for microbial C assimilation at low temperatures in the absence of freeze–thaw perturbation. In the arable soil, C_mic contents were significantly reduced during soil freezing but recovered quickly upon warming of the soil. Dynamics of C_mic in the arable soil were paralleled by protozoan biomass and transient shifts in functional composition of the nematode community, indicating that microfaunal predation played an important role in nutrient cycling after freeze–thaw perturbation. Only minor microfaunal dynamics were observed in the climatically more stable fallow, essentially confirming the absence of perturbation at this site. Our findings provide strong evidence that overwinter N₂O formation is regulated by both the physical freeze–thaw susceptibility of the soil and the ecological functioning of the habitat.