Environmental Engineering Reference
In-Depth Information
shift from low-productivity fen vegetation to high-
productivity reeds (Olde Venterink
et al.
2002,
Wheeler
et al.
2002). This process is often associated
with an influx of nutrient-rich surface water into the
fens (Harding 1993).
An increase in the sulphate concentration in the
surface water can also increase nutrient cycling in
the peat soils due to microbial sulphate reduction
under anoxic conditions (Smolders & Roelofs 1993,
Koerselman & Verhoeven 1995). During sulphate
reduction organic material is used as a carbon source
and sulphide is produced, which is toxic for most
vascular plants in relatively high concentrations
(Lamers
et al.
2002). Also an increase in alkalinity can
be observed during sulphate reduction (Stumm &
Morgan 1981). In most groundwater-fed fen and
fen meadow soils sulphide will not reach toxic
concentrations since it is chemically bound by
iron. However, when sulphide production exceeds
the availability of iron, free sulphide concentrations
increase (Caraco
et al.
1989). A side effect of increased
sulphide concentrations is that phosphate concentra-
tions can also increase considerably in the soil pore
water. In soils phosphate is normally bound to organic
matter, clay, but also to iron, calcium or aluminium
(Stumm & Morgan 1981, Scheffer & Schachtschabel
1992, Brady & Weil 1999). Phosphate sorption to iron
hydroxides is particularly important in wetland soils
that are not very acid or very calcareous. Sulphide can
interfere with phosphate binding to iron hydroxide. It
can bind with iron complexes forming iron sulphides
such as FeS and FeS
2
(pyrite). Under limited iron avail-
ability phosphates are released and become available
for plant growth (Koerselman
et al.
1990, Smolders
et al.
1995, Lucassen
et al.
2004). Such changes in
nutrient cycling, called
internal eutrophication
, can
be very harmful for fens and fen meadows that are
phosphorus-limited (Verhoeven
et al.
1996, Lamers
et al.
1998). In summary, it can be concluded that
anoxic iron-rich groundwater tends to slow down
nutrient cycling, while oxic surface water tends to
stimulate nutrient cycling.
wetland ecosystems. Between 1930 and 1980 the
annual atmospheric N deposition in the Netherlands
increased from 8 to 40 kg of N ha
−1
yr
−1
. In areas with
intensive cattle farming up to 80 kg were measured.
Increased N loads tend to increase the speed of
succession. Fast-growing species start to dominate
the vegetation and replace many characteristic and
protected plant and animal species, even in nature
reserves. The concept of critical N load has been
developed to indicate the annual amount of N
deposition where the ecosystem involved is stable
with respect to essential ecosystem functions. Critical
loads for bogs are 5 -10 kg of N ha
−1
yr
−1
, for moist
acid grasslands 10 -15 kg, for mesotrophic terrestrial-
ization mires 20 -35 kg (unmanaged) and for wet
heathlands 17-20 kg (Bobbink
et al.
1998). When the
deposition of N increases the
Sphagnum
species start
to accumulate N in amino acids or mineral N is not
taken up and becomes available for vascular plants
such as
M. caerulea
and tree species such as
B.
pubescens
(Lamers
et al.
2000, Tomassen
et al.
2004a)
.
9.5 Restoration approaches, successes
and failures
Apart from aesthetic reasons, economic reasons have
become important in initiating restoration projects in
peatland areas. Most peatlands have become unsuit-
able for modern agricultural production (Pfadenhauer
& Grootjans 1999), and in many peat areas in Europe
the maintenance costs for keeping such areas suitable
for agricultural production are simply too high to
sustain agricultural production in an economic way.
Environmental considerations are also important.
Deeply drained peatlands which are used heavily by
agriculture suffer badly from shrinking and peat loss
through deflation and mineralization. This may
amount to up to 2 cm yr
−1
and results in the release
of enormous amounts of CO
2
into the atmosphere
(release of up to 40 kg of N
2
O ha
−1
yr
−1
and up to
10 t of CO
2
; Armentano & Menges 1986, Augustin
et al.
1996). So there is a high societal need to restore
high water levels in degrading peat areas, which for
biodiversity reasons should be fed again with clean
groundwater from restored hydrological systems,
and which are capable of reducing nutrient loads of
polluted surface-water systems.
9.4.3 Atmospheric deposition
Atmospheric nitrogen and sulphur deposition have a
very negative effect on almost all low-productivity
