Carbon losses from peatlands

6.3. Carbon losses from peatlands

Carbon losses from peatlands take place in the form of C-gases and as dissolved and particulate material.

Gaseous carbon flux largely takes place in the form of carbon dioxide and methane. CO 2 exchange is determined by the balance of carbon fixation by photosynthesis and carbon released through plant respiration and mineralization of peat carbon. The latter is strongly controlled by water table and temperature (Charman 2002). Peat fires may lead to enormous emissions of carbon gases. Methane production by bacteria under anaerobic conditions is controlled by temperature (Dise et al. 1993) and water table (Bubier et al. 1993).

Loss of Dissolved Organic Carbon (DOC) from peatlands is well-studied (Urban et al. 1989, Sallantaus 1992, 1995, Dosskey & Bertsch 1994, Sallantaus & Kaipainen 1996), also because of the impact of associated water colour problems on the water supply industry (Hope et al. 1997a, Freeman et al. 2001, Neal et al. 2005, Worrall et al. 2004a, b, Evans et al. 2005c). Export from temperate peatlands ranges between 10 and 500 kg DOC ha -1

a -1 (e.g. Dillon & Molot 1997). The aerobic zone of drained peat soils is a significant source of DOC

(Kalbitz & Mutscher 1993, Chow et al. 2006). A portion of the DOC is transported downstream and

some is oxidized and lost to the atmosphere as CO 2 (Billet et al. 2004, Dawson et al. 2004). Part also precipitates in the mineral subsoil under the peatland (Turunen et al. 1999).

Dissolved Inorganic Carbon (DIC) is the least studied component of the carbon flux and is a

relatively small component of the overall flux (Worrall et al. 2003b). Modelling suggests that DIC production is largely controlled by temperature Worrall et al.

Figure 6.5: Components of the peat carbon

(2003b).

cycle (from Faubert 2004) Fluxes of Particulate Organic Carbon (POC) are largely caused by physical erosion of the peat

surface and controlled by vegetation cover (Evans et al. 2005a, Holden et al. 2006). POC release from UK upland peatlands ranges from 1 kg ha -1 a -1 in intact (Hope et al. 1997b) to 100 kg ha -1 a -1 in heavily eroding peatlands (Evans et al. 2006). In severely

eroding peatlands, fluvial POC losses may be the Table 6.3: Carbon balance for a small blanket largest single component of carbon mass budgets

peat Trout Beck catchment in the Northern (Holden et al. 2006). POC may be buried in anoxic

Pennines (UK) (after Worrall et al. 2003b). The conditions in lakes or reservoirs, but may also be

catchment has been significantly affected by rapidly oxidised in the fluvial system (Pawson et

gully erosion, and parts are affected by land al. 2006).

drainage.

Areal Flux Range budget of a small upland blanket peat catchment

Table 6.3 presents the complete carbon

Flux

gC m -2 a -1 from the North Pennines (UK). The largest single

1.1 component of the budget is the fixation of carbon.

Rainfall DIC

Rainfall DOC 3.1 Peat sequestration thus depends on the CO 2 55 40 to 70

delicate balance between production and

CH 4 -7.1 -1.5 to -11.3

decay and other losses of organic material.

DOC

-9.4 -9.4 to -15

Therefore natural peatlands may shift between

POC

-19.9 -2.7 to -31.7 carbon sink and source on seasonal and inter- Dissolved CO 2 -3.8

-2 to -3.8

annual time scales and show variability in

DIC

-5.9 -4.1 to - 5.9

carbon accumulation rates during the

Weathering DIC

1.8 0 to 1.8

Holocene. The long-term natural balance is

As only a small proportion of the peatland biomass enters the anaerobic zone, peat sequestration is a matter of a delicate imbalance of production and decay. Many peatlands may be close to the tipping

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