6. 1. Uncertainties in understanding of the responses

In general, rewetting or thawing is associated with increases in CO2, N2O, NO and NH3 fluxes, substantially affecting seasonal and annual flux budgets. However, some studies showed no response or small increased fluxes following rewetting or thawing events that did not substantially affect annual flux rates (Garcia-Montiel et al., 2003; Neill et al., 2005; Muhr and Borken, 2009; Muhr et al., 2010). Some studies showed reduced CO2 or N2O fluxes during drying periods, but the abruptly increased fluxes following rewetting did not compensate for the reduced or uptake rates during the dry period at the seasonal scale (Borken and Matzner, 2009; Goldberg and Gebauer, 2009; Joos et al., 2010).

Overall, there are lack of knowledge in current understanding of the responses of soil gases following rewetting or thawing and their impact on annual budgets. Many studies report the magnitude of peak flux or increase rate of flux following rewetting or thawing but studies often do not identify 1) whether peak fluxes are significantly different from fluxes of pre-drought or pre-frozen periods, 2) change of soil moisture or soil temperature, 3) time lag between rewetting or thawing events and peak fluxes, 4) peak flux durations, 5) cumulative emissions in peak fluxes, and 6) their contributions to annual budget. Efforts to collect such information will contribute to improving our understanding of the response.

In addition, while change of these soil gas fluxes have been studied, how the relative proportion of CO2, CH4, N2O, NO and NH3 emitted following rewetting and thawing changes relative to “background” fluxes is poorly understood. Also the effect of rewetting and thawing on dissolved biogenic soil gas has been only rarely studied (Matzner and Borken, 2008). To our knowledge, there is only one study showing indirect evidence of this effect, which found that rainfall after thawing in spring increased concentration of dissolved N2O in soil solutions in forest (Xu et al., 2009). Their result suggests that the increased N2O following rewetting can be dissolved in the soil solution as well as be emitted (Xu et al., 2009); such N2O in the soil solution can drain to surface or groundwater, and be a source of indirect N2O flux (IPCC, 2006). Therefore, it is important to understand and quantify the effect of rewetting and thawing on dissolved soil gases.