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.
Mackelprang, R., Waldrop, M.P., DeAngelis, K.M., David, M.M., Chavarria, K.L., Blazewicz, S.J., Rubin, E.M., Jansson, J.K., 2011. Metagenomic analysis of a permafrost microbial community reveals a rapid response to thaw. Nature, doi:10.1038/nature10576
ReplyDeletePermafrost contains an estimated 1672 Pg carbon (C), an amount roughly equivalent to the total currently contained within land plants and the atmosphere1, 2, 3. This reservoir of C is vulnerable to decomposition as rising global temperatures cause the permafrost to thaw2. During thaw, trapped organic matter may become more accessible for microbial degradation and result in greenhouse gas emissions4, 5. Despite recent advances in the use of molecular tools to study permafrost microbial communities6, 7, 8, 9, their response to thaw remains unclear. Here we use deep metagenomic sequencing to determine the impact of thaw on microbial phylogenetic and functional genes, and relate these data to measurements of methane emissions. Metagenomics, the direct sequencing of DNA from the environment, allows the examination of whole biochemical pathways and associated processes, as opposed to individual pieces of the metabolic puzzle. Our metagenome analyses reveal that during transition from a frozen to a thawed state there are rapid shifts in many microbial, phylogenetic and functional gene abundances and pathways. After one week of incubation at 5 °C, permafrost metagenomes converge to be more similar to each other than while they are frozen. We find that multiple genes involved in cycling of C and nitrogen shift rapidly during thaw. We also construct the first draft genome from a complex soil metagenome, which corresponds to a novel methanogen. Methane previously accumulated in permafrost is released during thaw and subsequently consumed by methanotrophic bacteria. Together these data point towards the importance of rapid cycling of methane and nitrogen in thawing permafrost.
Tagesson, T., Mölder, M., Mastepanov, M., Sigsgaard, C., Tamstorf, M.P., Lund, M., Falk, J.M., Lindroth, A., Christensen, T.R., Ström, L., 2012. Land-atmosphere exchange of methane from soil thawing to soil freezing in a high-Arctic wet tundra ecosystem. Global Change Biology, DOI: 10.1111/j.1365-2486.2012.02647.x
ReplyDeleteAbstract
The land-atmosphere exchange of methane (CH4) and carbon dioxide (CO2) in a high-Arctic wet tundra ecosystem (Rylekærene) in Zackenberg, north-eastern Greenland, was studied over the full growing season and until early winter in 2008 and from before snowmelt until early winter in 2009. The eddy covariance technique was used to estimate CO2 fluxes and a combination of the gradient and eddy covariance methods was used to estimate CH4 fluxes. Small CH4 bursts were observed during spring thawing 2009, but these existed during short periods and would not have any significant effect on the annual budget. Growing season CH4 fluxes were well correlated with soil temperature, gross primary production, and active layer thickness. The CH4 fluxes remained low during the entire autumn, and until early winter. No increase in CH4 fluxes were seen as the soil started to freeze. However, in autumn 2008 there were two CH4 burst events that were highly correlated with atmospheric turbulence. They were likely associated with the release of stored CH4 from soil and vegetation cavities. Over the measurement period, 7.6 g C m−2 and 6.5 g C m−2 was emitted as CH4 in 2008 and in 2009, respectively. Rylekærene acted as a C source during the warmer and wetter measurement period 2008, whereas it was a C sink for the colder and drier period of 2009. Wet tundra ecosystems, such as Rylekærene may thus play a more significant role for the climate in the future, as temperature and precipitation are predicted to increase in the high-Arctic.
Tagesson, T., Mölder, M., Mastepanov, M., Sigsgaard, C., Tamstorf, M.P., Lund, M., Falk, J.M., Lindroth, A., Christensen, T.R., Ström, L., 2012. Land-atmosphere exchange of methane from soil thawing to soil freezing in a high-Arctic wet tundra ecosystem. Global Change Biology 18, 1928-1940
ReplyDeleteAbstract
The land-atmosphere exchange of methane (CH4) and carbon dioxide (CO2) in a high-Arctic wet tundra ecosystem (Rylekærene) in Zackenberg, north-eastern Greenland, was studied over the full growing season and until early winter in 2008 and from before snow melt until early winter in 2009. The eddy covariance technique was used to estimate CO2 fluxes and a combination of the gradient and eddy covariance methods was used to estimate CH4 fluxes. Small CH4 bursts were observed during spring thawing 2009, but these existed during short periods and would not have any significant effect on the annual budget. Growing season CH4 fluxes were well correlated with soil temperature, gross primary production, and active layer thickness. The CH4 fluxes remained low during the entire autumn, and until early winter. No increase in CH4 fluxes were seen as the soil started to freeze. However, in autumn 2008 there were two CH4 burst events that were highly correlated with atmospheric turbulence. They were likely associated with the release of stored CH4 from soil and vegetation cavities. Over the measurement period, 7.6 and 6.5 g C m−2 was emitted as CH4 in 2008 and in 2009, respectively. Rylekærene acted as a C source during the warmer and wetter measurement period 2008, whereas it was a C sink for the colder and drier period of 2009. Wet tundra ecosystems, such as Rylekærene may thus play a more significant role for the climate in the future, as temperature and precipitation are predicted to increase in the high-Arctic.