6. 2. Uncertainties in understanding of mechanisms and drivers

Enhanced nutrient supply from soil freezing has been accepted as one of mechanisms to explain abruptly increased N2O fluxes. However, Hentschel et al. (2009) found that moderate soil freezing did not affect solute losses of N, DOC, and mineral ions from temperate forest soils, and argued that their results did not support the mechanism that N2O peak fluxes are caused by the enhanced nutrient supply from soil freezing (Goldberg et al., 2010b). While it has been argued that N2O peak flux at spring thaw is mostly produced in the surface layer (Müller et al., 2002; Furon et al., 2008; Wagner-Riddle et al., 2008), Goldberg et al. (2010b) found that released N2O in soil thawing is due to a slow release of subsoil N2O and a delayed activation of N2O reductase in the topsoil after soil frost due to low soil temperatures. The relatively importance of source processes responsible for the increased fluxes of CO2 (i.e., autotrophic or heterotrophic activity), NO and N2O (i.e., nitrification, denitrification or nitrifier denitrification) is poorly understood.

In the respect of drivers of the response, we observed conflicting results concerning the magnitude of increased NO flux after rewetting (see §3.4). How different vegetation types respond to rewetting and thawing events (Teepe and Ludwig, 2004; Matzner and Borken, 2008; Kim et al., 2010b; Shi et al., 2011) is unclear. This is important because different vegetation types can have different phenologies and photosynthesis rates (Vargas et al., 2010b), nutrient cycling rates in detritus (Vogt et al., 1986) and soils (Borken and Beese, 2005; Paré et al., 2006). Plant-mediated effects on soil microclimate, such as soil temperature and soil moisture (Raich and Schlesinger, 1992; Aussenac, 2000), and plant mediated effects on root and rhizomorph dynamics (Vargas and Allen, 2008) are also only beginning to be explored. Novel mechanisms and pathways by which plants emit GHGs have been explored recently (Keppler et al., 2006; Aubrey and Teskey, 2009), but how these pathways respond to rewetting or thawing events has been little studied.

Compared to CO2 and N2O fluxes, our understanding of the effect of rewetting and thawing on CH4, NO and NH3 fluxes and mechanisms and drivers of the variation is limited, with large uncertainties. We encourage the scientific community to perform experiments and observations to better understand their magnitudes and mechanisms.