1. Introduction

The rewetting of dry soils, or thawing of frozen soils, represents an abrupt step change in soil biophysical conditions, with implications for biogeochemical cycling. From an organismal perspective, soil rewetting and thawing are analogous because both processes increase the availability of soil water, rehydrate cells, and mobilize nutrients. Both processes are also relatively transient, with the duration of individual rewetting and thawing events varying depending on local climatic conditions, topography, drainage, vegetation type, and soil thermal properties (Balser and Firestone, 2005; Vargas et al., 2010b). The sudden flush of water and nutrients that occurs after rewetting and thawing precipitates major changes in plant and microbial activity, with organisms shifting rapidly from dormant or senescent states to active ones (Kieft et al., 1987; Schimel and Clein, 1996).

It is important to understand the change in fluxes of biogenic gases (i.e., CO2, CH4, N2O, NO and NH3) following rewetting and thawing events, as these biogenic gases are either by-products or end-products of soil-related microbial processes involved in C and N dynamics in soils. These gases also play crucial roles in atmospheric chemistry and radiative forcing as greenhouse gases (GHG). Furthermore, global climate models predict that future climatic change is likely to alter the frequency and intensity of drying-rewetting events and thawing of frozen soils (Meehl et al., 2006; Sheffield and Wood, 2008; Sinha and Cherkauer, 2010). The frequency and intensity of soil frost (i.e., annual soil freezing days and freeze-thaw cycles) is also likely to be modified since warming could lead to a reduction in the thickness of the insulating snowpack and thus colder winter soil temperatures (Henry, 2008; Gu et al., 2008). Thus, it is important to understand how soil rewetting and thawing influences biogenic gas fluxes, given the potential for GHGs such as CO2, CH4 and N2O to serve as either positive or negative feedbacks to future climate change.

While abrupt increases in soil CO2, N2O, NH3 and NO fluxes following rewetting are commonly observed in various agricultural lands and natural lands (Priemé and Christensen, 2001; Saetre and Stark, 2005), rewetting can either increase (Moore, 1998; Knorr et al., 2008) or inhibit (Kessavalou et al., 1998; Teh et al., 2005) CH4 oxidation. Similarly, increases in CO2, CH4 and N2O fluxes following soil thawing have been shown to affect total annual gas budgets (Röver et al., 1998; Papen and Butterbach-Bahl, 1999). Despite this growing number of studies, uncertainties in our understanding of the mechanisms and impacts on annual gas budgets continue to exist. These uncertainties are exacerbated by the coarse temporal sampling resolution in most flux measurements that do not capture the dynamic of the pulse (Groffman et al., 2006; Muhr et al., 2009), and unrealistic simulation of dry-wet and freeze-thaw events (Henry, 2007; Jentsch et al., 2007). These limitations are important for our understanding of soil GHG fluxes because even single pulse events have shown to substantially contribute to annual fluxes (Lee et al., 2004; Xu et al., 2004).

The growing number of studies on the individual importance of rewetting or thawing specifically for CO2 and N2O fluxes have been the focus of previous reviews (Henry, 2007; Matzner and Borken, 2008; Borken and Matzner, 2009; Groffman et al., 2009). This review differs by taking a comprehensive approach dealing with the effect of both rewetting and thawing on multiple biogenic gas fluxes (CO2, CH4, N2O, NO and NH3) and indentifying knowledge gaps and potential future research. The objectives of this manuscript are: 1) to summarize the effects of rewetting and thawing on multiple biogenic gas fluxes (CO2, CH4, N2O, NO and NH3) and highlight common patterns across studies; 2) discuss the underlying mechanisms and drivers of responses; 3) identify knowledge gaps and propose future research questions.