Geoscience Reference
In-Depth Information
Effects on carbon budget of harvesting and
restoring ombrotrophic peat bogs
HUMAN IMPACT
Such is the value of organic matter as a determinant of soil fertility that harvesting sphagnum peat for composting
and use as a soil amendment is an important industry in Canada, Ireland and many European countries. In England
peat has been harvested for many decades at Wedholme Flow in the Solway mosses of Cumbria and in the two
largest lowland raised peat bogs in the United Kingdom, Thorne Moor and Hatfield Moor of Yorkshire. Harvesting
involves draining the peat with 2 m deep ditches 30 m apart, and then extracting the dried peat by digging or vacuum
techniques. After an agreement between the Department of the Environment, Food and Rural Affairs (DEFRA) and
the extraction company to cease peat harvesting, both Thorne Moor and Hatfield Moor became a National Nature
Reserve (NNR) in 2005, with restoration completed in 2007.
The peat industry is a major economic activity in Quebec and the Maritime provinces in eastern Canada, where
companies such as Sungro and Premier Horticulture exploit the large Canadian peat reserves for the US market.
After concerns that peat harvesting and its use in horticulture are a global source of carbon dioxide, because they
accelerate the rate of decomposition of the peat, Professor Moore and a team of researchers from McGill University,
Montreal, have been monitoring the fluxes of CO
2
, methane (CH
4
) and dissolved organic carbon (DOC) at pristine,
harvested and restoration sites in Quebec. Data from a pristine ombrotrophic bog with a Sphagnummoss and dwarf
shrub cover, Mer Bleu, near Ottawa, show that there is a small uptake or sink of CO
2
(
Plate 19.6a
).
At harvesting
rates of 10 cm yr
-1
, peat loses 5 kg C m
-2
through harvest, a further 200-300 g C m
-2
through respiration, plus an
unknown amount through wind erosion
(
Plate 19.6b
).
In the past, reclamation of abandoned harvested sites relied
on natural recolonization by vegetation. Nowadays, however, the peat industry has made a commitment to the
progressive restoration of mined sites. This is carried out by blocking the drainage ditches to raise the water table,
and mulching the peatland surface with a thin layer of peat and/or straw to provide seeds and plant material to stimulate
vegetation establishment
(
Plate 19.6c
).
These reclamation techniques have the effect of stimulating the loss of CO
2
through the wetter surface conditions and the 'priming' effect of vegetation, especially by cotton-grass sedges
(Eriophorum spp.)
(
Plate 19.6d
).
Even when vegetation cover has been established, there are still significant losses
of CO
2
. The phenomenon of increased CO
2
is greatest in the early stages of succession, and reduces as the cotton-
grass tussocks mature.
Fluxes of CH
4
show a strong seasonal pattern with maximum peaks in mid-summer. In addition to soil temperature,
the two controlling factors explaining differences of several orders of magnitude between bogs are the position of
the water table and the coverage of vegetation. Where the water table is low, whether pristine or harvested, CH
4
emissions are 1 g CH
4
-C m
-2
, whereas emission rates are 3 g CH
4
-C m
-2
at restored sites with a high water
table, especially where cotton grass is dominant. The role of cotton grass is related to root exudates and rhizosphere
effects as for CO
2
. Although cotton grass raises CO
2
and CH
4
emission rates, it is important in establishing an initial
vegetation cover, trapping seeds, providing a better micro-habitat for the establishment of shrubs and mosses, and
stimulating microbial activity and nutrient cycling. The results are disappointing because it seems that restoration of
peatland does not rapidly restore the C budget to a net sink, and it probably takes longer than the 15 years currently
available to study at restored sites.
and:
H
2
O = H
+
+ OH
-
pOH = -log
a
OH
-
= 7
At 25
C the product of the ionic activities equals 10
-14
g
ions (moles) litre
-1
, i.e. the activity of each ion is 10
-7
.
Hence:
At neutrality, pH = pOH = 7. Thus each division below
or above pH 7 represents a tenfold decrease or increase in
acidity. Natural rainwater has a pH value of about 5ยท5,
pH = - log
a
H
+
= 7
