5.8.1.2. Carbon Sink
The importance of peat-accumulating wetlands to global change is via the large
carbon store accumulated over millennia—and the risk that this store would
be released to the atmosphere in conditions modified by global change (e.g.,
fires). The carbon store in boreal and subarctic peatlands alone has been estimated
at 455 Gt with an annual sink of slightly less than 0.1 Gt (Gorham, 1991). Tropical
peatlands also are a considerable store (total of 70 Gt), containing as much
as 5,000 t C ha-1, compared with an average of 1,200 t C ha-1
for peatlands globally (Immirzi et al., 1992; Diemont et al., 1997). Estimates
of the annual sink in tropical peatlands vary from 0.01 Gt (Sorensen, 1993)
and 0.06 Gt (Franzen, 1994) to 0.09 Gt (Immirzi et al., 1992)—emphasizing
the lack of reliable data.
Optimal conditions for carbon sequestration appear to be in areas with mean
annual temperatures between 4 and 10°C (Clymo et al., 1998), which prevail
in much of the southern boreal and temperate zones. The present carbon accumulation
rate for boreal and subarctic bogs and fens is estimated as 0.21 t C ha-1
yr-1 (Clymo et al., 1998). Rapid carbon accumulation rates also have
been estimated for some tropical peatlands (Neuzil, 1997); retrospective values
of Indonesian peatlands range from 0.61 to 1.45 t C ha-1 yr-1
(Neuzil, 1997).
Another important long-term (>100 years) sink for carbon in forested wetlands
is wood biomass. Based on growth data (Shepard et al., 1998) and conversion
factors (Turner et al., 1995) for bottomland hardwood forests, southern U.S.
swamps, for example, sequester 0.011 Gt C yr-1.
A small but significant proportion of organic matter in wetland soils is transformed
into methane in the metabolism of methanogenic bacteria. Methane production
is a characteristic feature in all wetland soils; the rate is governed largely
by substrate availability and temperature (Shannon and White, 1994; Mikkelä
et al., 1995; Schimel, 1995; Bergman et al., 1998; Komulainen et al., 1998).
Part of the methane produced in anoxic soil is oxidized by methanotrophic bacteria
in aerobic surface layers.
The ratio of methane production to consumption determines the magnitude of
the flux from the soil to the atmosphere; this rate is governed largely by the
depth of the aerobic layer (Roulet et al., 1993; Shannon and White, 1994). The
role of vascular plants in providing continuous substrate flux for methanogenesis
and as a transport pathway to the atmosphere has been stressed (Whiting and
Chanton, 1993; Schimel, 1995; Frenzel and Rudolph, 1998). Slow fermentation
of organic matter in growing peat layers of bogs has been cited as the factor
that theoretically limits the final volume a bog may reach during its development
(Clymo, 1984).
|