4. 1. Carbon dioxide flux from thawing
4. 2. Methane flux from thawing
4. 3. Nitrous oxide flux from thawing
4. 4. Nitric oxide flux from thawing
Tuesday, March 29, 2011
4. Effect of thawing of frozen soil on biogenic gas fluxes
Here we discuss, for each biogenic gas: a) how thawing events influence gas fluxes in multiple ecosystems and in experimental settings; and b) the likely mechanisms and environmental controls underlying the observed patterns.
4. 1. Carbon dioxide flux from thawing
General patterns of responses
Increased RS after thawing has been observed in various terrestrial ecosystems including forest (Wu et al., 2010a), alpine tundra (Brooks et al., 1997), and arctic heath (Elberling and Brandt, 2003), and in incubation experiments with soils from cropland (Kurganova et al., 2007), grassland (Wu et al., 2010b), forest (Goldberg et al., 2008), bog (Panikov and Dedysh, 2000), taiga and tundra (Schimel and Clein, 1996), and Antarctica (Zhu et al., 2009). Reported CO2 flux increases after thawing can range up to 5,000% (Table 1, Fig. 2). Such increases in CO2 flux after seasonal thawing were important to the annual budget of CO2 flux in arable soils (Priemé and Christensen, 2001; Kurganova et al., 2007), but did not affect the annual budget in some natural sites (Coxson and Parkinson, 1987; Schimel and Clein, 1996; Neilsen et al., 2001). Similarly to rewetting of soils CO2 diffusion and production could be affected by increase of water in the soil pore space reducing RS and creating anaerobic conditions that lower autotrophic and heterotrophic respiration (section 3.1). In the following sections we focus on the positive impact of thawing on RS.
Mechanisms and drivers
The mechanism responsible for increased RS following thawing has been commonly hypothesized as enhanced microbial metabolism by substrate supply. A large proportion of microorganisms, fine roots and mycorrhizae die during frozen conditions; these dead cells have low C:N ratios and rapidly decompose during thawing (Priemé and Christensen, 2001; Yergeau and Kowalchuk, 2008). Thawing also disrupts soil aggregates, exposing physically protected organic matter and increase the accessibility of substrate that can be rapidly mineralized (Pesaro et al., 2003; Grogan et al., 2004). The magnitude of increased RS following thawing is controlled by substrate availability, soil properties and characteristics of thawing events. Colder frost temperatures have been shown to increase RS (Matzner and Borken, 2008; Goldberg et al., 2008). Another known control factor is freeze-thaw event frequency: the largest RS increase commonly occurs in the first thawing event (among repeated freezing-thawing cycles) with the effects declining in successive cycles (Kurganova and Tipe, 2003; Goldberg et al., 2008).
Increased RS after thawing has been observed in various terrestrial ecosystems including forest (Wu et al., 2010a), alpine tundra (Brooks et al., 1997), and arctic heath (Elberling and Brandt, 2003), and in incubation experiments with soils from cropland (Kurganova et al., 2007), grassland (Wu et al., 2010b), forest (Goldberg et al., 2008), bog (Panikov and Dedysh, 2000), taiga and tundra (Schimel and Clein, 1996), and Antarctica (Zhu et al., 2009). Reported CO2 flux increases after thawing can range up to 5,000% (Table 1, Fig. 2). Such increases in CO2 flux after seasonal thawing were important to the annual budget of CO2 flux in arable soils (Priemé and Christensen, 2001; Kurganova et al., 2007), but did not affect the annual budget in some natural sites (Coxson and Parkinson, 1987; Schimel and Clein, 1996; Neilsen et al., 2001). Similarly to rewetting of soils CO2 diffusion and production could be affected by increase of water in the soil pore space reducing RS and creating anaerobic conditions that lower autotrophic and heterotrophic respiration (section 3.1). In the following sections we focus on the positive impact of thawing on RS.
Mechanisms and drivers
The mechanism responsible for increased RS following thawing has been commonly hypothesized as enhanced microbial metabolism by substrate supply. A large proportion of microorganisms, fine roots and mycorrhizae die during frozen conditions; these dead cells have low C:N ratios and rapidly decompose during thawing (Priemé and Christensen, 2001; Yergeau and Kowalchuk, 2008). Thawing also disrupts soil aggregates, exposing physically protected organic matter and increase the accessibility of substrate that can be rapidly mineralized (Pesaro et al., 2003; Grogan et al., 2004). The magnitude of increased RS following thawing is controlled by substrate availability, soil properties and characteristics of thawing events. Colder frost temperatures have been shown to increase RS (Matzner and Borken, 2008; Goldberg et al., 2008). Another known control factor is freeze-thaw event frequency: the largest RS increase commonly occurs in the first thawing event (among repeated freezing-thawing cycles) with the effects declining in successive cycles (Kurganova and Tipe, 2003; Goldberg et al., 2008).
4. 2. Methane flux from thawing
General patterns of responses
The reported effects of thawing on CH4 fluxes are variable. Seasonal soil thaw increased CH4 flux in peatland (Tokida et al., 2007), forest (Kim and Tanaka, 2003), and wetlands (Friborg et al., 1997; Song et al., 2006; Ding and Cai, 2007; Yu et al., 2007). In a subarctic peatland, CH4 flux increased from 2.6 mg m−2 d−1 to 22.5 mg m−2 d−1 during thawing, with the latter rate equivalent to approximately 25% of the mid-summer flux (Friborg et al., 1997). A few studies also have shown enhanced CH4 consumption during seasonal thawing periods (Ding and Cai, 2007; Wu et al., 2010b). In addition to affecting rates of CH4 production and oxidation, seasonal soil thaw also may affect CH4 transport mechanisms (Friborg et al., 1997; Kim and Tanaka, 2003; Tokida et al., 2007). For example, surface seasonal thawing in a bog appeared to trigger ebullition events, with flux up to 25.3 mg CH4 m−2 h−1 (Tokida et al., 2007). In Alaskan boreal forest soils damaged by fire, CH4 flux increased 7−142% during seasonal thawing (Kim and Tanaka, 2003). While beyond the scope of this paper, we note that similar to seasonal thaw, longer-term increases in active layer depth with permafrost thaw also tend to increase CH4 flux in high latitude wetlands and lakes (Turetsky et al. 2002; Christensen et al., 2004; Walter et al., 2006; Anisimov, 2007).
Mechanisms and drivers
The mechanisms and drivers underlying various changes in net CH4 flux following thawing are probably linked the response of methanogenesis and methanotrophy to changes in availability of substrates, soil physical properties, soil moisture and redox potential in soil. Freezing increases substrate availability (see §4.1) and limits O2 transport into soil, both of which would promote methanogenesis and storage in deeper soil layers (Yu et al., 2007). Also CH4 typically accumulates subsurface in snow or ice covered ecosystems. During thawing periods, the diffusion barriers disappear, and trapped CH4 is released to the atmosphere (Friborg et al., 1997; Yu et al., 2007). Methane emissions were independent of temperature below the freezing point (Friborg et al., 1997; Yu et al., 2007), suggesting that biological activity was not the dominant control on soil CH4 flux during early soil thaw. However, as the soil active layer becomes thicker, soil CH4 fluxes will be driven by soil aeration and redox controls on methanotrophy and methanogenesis as described above for rewetting (Section 3.2). In particular, due to poor drainage of melting snow and seasonal ice, thawing can create saturated surface soils in the active layer, which can favour CH4 production (Thauer, 1988) and suppress methanotrophy. In contrast, Ding and Cai (2007) found that low temperatures reduced microbial activity of some aerobic microbes, and the resulting presence of more O2 in soil increased methanotrophy and reduced methanogenesis. Overall, the mechanisms and drivers responsible for the various response of CH4 to thawing have not been clearly explored to our knowledge. Further research is needed to identify the mechanisms controlling the response after thawing at multiple ecosystems.
The reported effects of thawing on CH4 fluxes are variable. Seasonal soil thaw increased CH4 flux in peatland (Tokida et al., 2007), forest (Kim and Tanaka, 2003), and wetlands (Friborg et al., 1997; Song et al., 2006; Ding and Cai, 2007; Yu et al., 2007). In a subarctic peatland, CH4 flux increased from 2.6 mg m−2 d−1 to 22.5 mg m−2 d−1 during thawing, with the latter rate equivalent to approximately 25% of the mid-summer flux (Friborg et al., 1997). A few studies also have shown enhanced CH4 consumption during seasonal thawing periods (Ding and Cai, 2007; Wu et al., 2010b). In addition to affecting rates of CH4 production and oxidation, seasonal soil thaw also may affect CH4 transport mechanisms (Friborg et al., 1997; Kim and Tanaka, 2003; Tokida et al., 2007). For example, surface seasonal thawing in a bog appeared to trigger ebullition events, with flux up to 25.3 mg CH4 m−2 h−1 (Tokida et al., 2007). In Alaskan boreal forest soils damaged by fire, CH4 flux increased 7−142% during seasonal thawing (Kim and Tanaka, 2003). While beyond the scope of this paper, we note that similar to seasonal thaw, longer-term increases in active layer depth with permafrost thaw also tend to increase CH4 flux in high latitude wetlands and lakes (Turetsky et al. 2002; Christensen et al., 2004; Walter et al., 2006; Anisimov, 2007).
Mechanisms and drivers
The mechanisms and drivers underlying various changes in net CH4 flux following thawing are probably linked the response of methanogenesis and methanotrophy to changes in availability of substrates, soil physical properties, soil moisture and redox potential in soil. Freezing increases substrate availability (see §4.1) and limits O2 transport into soil, both of which would promote methanogenesis and storage in deeper soil layers (Yu et al., 2007). Also CH4 typically accumulates subsurface in snow or ice covered ecosystems. During thawing periods, the diffusion barriers disappear, and trapped CH4 is released to the atmosphere (Friborg et al., 1997; Yu et al., 2007). Methane emissions were independent of temperature below the freezing point (Friborg et al., 1997; Yu et al., 2007), suggesting that biological activity was not the dominant control on soil CH4 flux during early soil thaw. However, as the soil active layer becomes thicker, soil CH4 fluxes will be driven by soil aeration and redox controls on methanotrophy and methanogenesis as described above for rewetting (Section 3.2). In particular, due to poor drainage of melting snow and seasonal ice, thawing can create saturated surface soils in the active layer, which can favour CH4 production (Thauer, 1988) and suppress methanotrophy. In contrast, Ding and Cai (2007) found that low temperatures reduced microbial activity of some aerobic microbes, and the resulting presence of more O2 in soil increased methanotrophy and reduced methanogenesis. Overall, the mechanisms and drivers responsible for the various response of CH4 to thawing have not been clearly explored to our knowledge. Further research is needed to identify the mechanisms controlling the response after thawing at multiple ecosystems.
4. 3. Nitrous oxide flux from thawing
General patterns of responses
Increased soil N2O flux following thawing have been observed in cropland (Rochette et al., 2010), grassland (Virkajärvi et al., 2010), forest (Maljanen et al., 2010), marsh (Yu et al., 2007), alpine meadow (Hu et al., 2010), and alpine tundra (Brooks et al., 1997). Laboratory incubation experiments showing similar results have been performed with agricultural (Kurganova et al., 2004), grassland (Yao et al., 2010), forest (Goldberg et al., 2008), permafrost (Elberling et al., 2010), and coastal Antarctica soils (Zhu et al., 2009). Episodic N2O peak fluxes of up to 750 μg N2O−N m–2 h–1 (background levels of under 50 μg N2O−N m–2 h–1) were measured after freeze-thaw in arable field (Dörsch et al., 2004). Such increases usually occur when soil temperatures are close to 0 oC (Christensen and Tiedje, 1990; Chen et al., 1995; Müller et al., 2003). Studies examining the thawing effect on N2O flux have reported 6 to 35 d response following rewetting (Table 2) and N2O fluxes increase up to 17,000% (Table 2, Fig. 2). Thaw-induced N2O fluxes constituted a major component of annual N2O fluxes from arable field (Regina et al., 2004; Johnson et al., 2010), temperate grassland (Kammann et al. 1998; Müller et al., 2002), steppe (Holst et al., 2008; Wolf et al., 2010), wetland (Yu et al., 2007) and forest ecosystems (Papen and Butterbach-Bahl, 1999; Wu et al., 2010a; Guckland et al., 2010) with contributions exceeding 50% of the annual budget in some years.
Mechanisms and drivers
Enhanced microbial metabolism (see §4.1) and changing physical protection have been hypothesized as responsible for increased N2O fluxes following thawing. Increased N2O flux following thawing has been shown to predominantly originate from denitrification (Mørkved et al., 2006; Sharma et al., 2006; Wagner-Riddle et al., 2008). Denitrification contributed 83% of the produced N2O immediately after thawing and 72% after 70 h incubation in undisturbed soil cores (Ludwig et al., 2004). Physical mechanisms involving reduced diffusivity can also influence N2O. Anaerobic water-saturated topsoil conditions are created during thawing by reduced drainage of melting ice and snow in the frozen subsoil, and this conditions are known to increase N2O fluxes (Li et al., 2000; de Bruijn et al., 2009). Ice layers prevent the escape of soluble N2O into the liquid water film and may result in supersaturated soil solutions. During thawing periods, the diffusion barriers disappear, and the trapped N2O is released into the atmosphere within a few days (Goldberg et al., 2010b; Virkajärvi et al., 2010). Increased N2O fluxes following thawing may be caused by the combination of these two mechanisms (Koponen et al., 2006; de Bruijn et al., 2009).
The magnitude of increased N2O flux following thawing of frozen soils is influenced by soil texture (Christensen and Christensen, 1991; Lemke et al., 1998), crop species (Kaiser et al., 1998; Johnson et al., 2010), forest type (Teepe and Ludwig, 2004), tillage history (Singurindy et al., 2009), soil water content (Koponen and Martikainen, 2004; Wolf et al., 2010), and the length of the freezing period (Papen and Butterbach-Bahl, 1999; Wagner-Riddle et al., 2007). Soils with clay-dominated aggregates are prone to high N2O flux during thawing periods (van Bochove et al., 2000; Müller et al., 2003). There is little information on the effect of soil water content on N2O fluxes (Röver et al., 1998; van Bochove et al., 2000). Röver et al. (1998) measured large fluxes of N2O after freezing in an agricultural soil at 80% water-filled pore space, while van Bochove et al. (2000) reported that flux of N2O from a clay soil were significantly larger at a volumetric water content of 39% than at 28%.
Increased soil N2O flux following thawing have been observed in cropland (Rochette et al., 2010), grassland (Virkajärvi et al., 2010), forest (Maljanen et al., 2010), marsh (Yu et al., 2007), alpine meadow (Hu et al., 2010), and alpine tundra (Brooks et al., 1997). Laboratory incubation experiments showing similar results have been performed with agricultural (Kurganova et al., 2004), grassland (Yao et al., 2010), forest (Goldberg et al., 2008), permafrost (Elberling et al., 2010), and coastal Antarctica soils (Zhu et al., 2009). Episodic N2O peak fluxes of up to 750 μg N2O−N m–2 h–1 (background levels of under 50 μg N2O−N m–2 h–1) were measured after freeze-thaw in arable field (Dörsch et al., 2004). Such increases usually occur when soil temperatures are close to 0 oC (Christensen and Tiedje, 1990; Chen et al., 1995; Müller et al., 2003). Studies examining the thawing effect on N2O flux have reported 6 to 35 d response following rewetting (Table 2) and N2O fluxes increase up to 17,000% (Table 2, Fig. 2). Thaw-induced N2O fluxes constituted a major component of annual N2O fluxes from arable field (Regina et al., 2004; Johnson et al., 2010), temperate grassland (Kammann et al. 1998; Müller et al., 2002), steppe (Holst et al., 2008; Wolf et al., 2010), wetland (Yu et al., 2007) and forest ecosystems (Papen and Butterbach-Bahl, 1999; Wu et al., 2010a; Guckland et al., 2010) with contributions exceeding 50% of the annual budget in some years.
Mechanisms and drivers
Enhanced microbial metabolism (see §4.1) and changing physical protection have been hypothesized as responsible for increased N2O fluxes following thawing. Increased N2O flux following thawing has been shown to predominantly originate from denitrification (Mørkved et al., 2006; Sharma et al., 2006; Wagner-Riddle et al., 2008). Denitrification contributed 83% of the produced N2O immediately after thawing and 72% after 70 h incubation in undisturbed soil cores (Ludwig et al., 2004). Physical mechanisms involving reduced diffusivity can also influence N2O. Anaerobic water-saturated topsoil conditions are created during thawing by reduced drainage of melting ice and snow in the frozen subsoil, and this conditions are known to increase N2O fluxes (Li et al., 2000; de Bruijn et al., 2009). Ice layers prevent the escape of soluble N2O into the liquid water film and may result in supersaturated soil solutions. During thawing periods, the diffusion barriers disappear, and the trapped N2O is released into the atmosphere within a few days (Goldberg et al., 2010b; Virkajärvi et al., 2010). Increased N2O fluxes following thawing may be caused by the combination of these two mechanisms (Koponen et al., 2006; de Bruijn et al., 2009).
The magnitude of increased N2O flux following thawing of frozen soils is influenced by soil texture (Christensen and Christensen, 1991; Lemke et al., 1998), crop species (Kaiser et al., 1998; Johnson et al., 2010), forest type (Teepe and Ludwig, 2004), tillage history (Singurindy et al., 2009), soil water content (Koponen and Martikainen, 2004; Wolf et al., 2010), and the length of the freezing period (Papen and Butterbach-Bahl, 1999; Wagner-Riddle et al., 2007). Soils with clay-dominated aggregates are prone to high N2O flux during thawing periods (van Bochove et al., 2000; Müller et al., 2003). There is little information on the effect of soil water content on N2O fluxes (Röver et al., 1998; van Bochove et al., 2000). Röver et al. (1998) measured large fluxes of N2O after freezing in an agricultural soil at 80% water-filled pore space, while van Bochove et al. (2000) reported that flux of N2O from a clay soil were significantly larger at a volumetric water content of 39% than at 28%.
4. 4. Nitric oxide flux from thawing
General patterns of responses
Increased soil NO fluxes following thawing have been observed only in a field study (Laville et al., 2011) and in a laboratory incubation study (Yao et al., 2010). In a French crop field, NO fluxes following thawing increased up to 10 ng N m−2 s−1 and decreased to pre-event values within 24h while the flux average was 1.7 to 2.3 ng N m−2 s−1 in two years (Laville et al., 2011). Incubation with the soils of steppe, mountain meadow, sand dune and marshland in Inner Mongolia showed that NO fluxes was 0.5−8.0 μg N m–2 h–1 at -10 oC and it increased to around 30 μg N m–2 h–1 following thawing (at 5 oC) (Yao et al., 2010). The relative lack of studies on this subject—the response of NO to both rewetting and thawing—suggests that there is a large research potential to elucidate drivers and constrain the NO flux response.
Increased soil NO fluxes following thawing have been observed only in a field study (Laville et al., 2011) and in a laboratory incubation study (Yao et al., 2010). In a French crop field, NO fluxes following thawing increased up to 10 ng N m−2 s−1 and decreased to pre-event values within 24h while the flux average was 1.7 to 2.3 ng N m−2 s−1 in two years (Laville et al., 2011). Incubation with the soils of steppe, mountain meadow, sand dune and marshland in Inner Mongolia showed that NO fluxes was 0.5−8.0 μg N m–2 h–1 at -10 oC and it increased to around 30 μg N m–2 h–1 following thawing (at 5 oC) (Yao et al., 2010). The relative lack of studies on this subject—the response of NO to both rewetting and thawing—suggests that there is a large research potential to elucidate drivers and constrain the NO flux response.
5. Overall change of gases fluxes following rewetting and thawing
An analysis of published field and laboratory studies showed that CO2, CH4, N2O, NO and NH3 fluxes increase on average 7.6 times (± standard error 1.1) following rewetting and thawing with no significant difference between these events (R2 = 0.896, P < 0.001; Fig. 3). Similarly, laboratory studies showed that CO2 and N2O fluxes increase on average 5.8 times (± standard error 1.5) following rewetting and thawing with no significant difference between these events (R2 = 0.726, P < 0.001; Fig. 3).
6. Knowledge gaps and future directions
6. 1. Uncertainties in understanding of the responses
6. 2. Uncertainties in understanding of mechanisms and drivers
6. 3. Temporal and spatial resolution
6. 4. Experimental settings
6. 5. Model improvement
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