Environmental Engineering Reference
In-Depth Information
and Narev Rivers in Poland, the Peene Haffmoor in Ger-
many and also the Odra National Park on the border
between Poland and Germany. Very often large canals
have replaced the original streams. Dikes were built
to prevent flooding of agricultural and urban areas,
which led to increased fluctuations in the water table
in the remnants of floodplain areas: high water
levels in the wet periods and low water levels in the
summer. Floodplain systems that have vegetation
types which resemble natural peat-forming vegetation
types are either groundwater-fed in addition to regu-
lar flooding, or they are supplied with additional water
from surface-water systems (Wassen et al. 1996).
water (Plate 9.3). This process is generally promoted
by drainage activities in the landscape (Grootjans et al .
1988), but it can also be a natural phenomenon,
when floating mats of vegetation in terrestrializing
mires increase in thickness and retain rainwater in
the topsoil (Schot et al. 2004). Such processes mark
the beginning of bog formation. In autumn and
winter a rainwater lens develops in the profile and
can reach a depth of over 1 m. In areas with a large
precipitation surplus this leads to acidification of the
topsoil, because practically all base cations are being
exchanged for H + and Al 3+ (Steinberg & Wright 1994,
Brady & Weil 1999). In the long run the soil may
even become iron depleted. Over a period of several
decades, this acidification leads to a drop in soil
fertility. Moist sub-neutral grassland types, in particu-
lar, are very susceptible to acidification, because buffer
mechanisms are weak in such ecosystems. Species such
as Arnica montana , Antennaria dioica , Botrychium
lunaria and many others have declined in north-west
Europe, even in well-managed nature reserves, due
to drainage and atmospheric deposition. This acid
rain has often depleted the buffer mechanisms in the
topsoil, thus increasing the availability of Al 3+ and also
NH 4 + at the cost of Ca 2+ and NO 3 . De Graaf et al.
(1998b) have shown that A. montana and A. dioica
are very vulnerable to high concentrations of NH 4 + and
Al 3+ in the topsoil. They found that the absolute val-
ues of Al 3+ were not important, but that the Ca 2+ :Al 3+
ratio in the soil solution should be above 1.
9.4 Causes of further deterioration in
semi-natural wetlands
Remnants of semi-natural vegetation are now situated
in a landscape that has completely different propert-
ies compared to the time in which these ecosystems
evolved. Deep drainage in agricultural areas and
abstraction of groundwater for the public water supply
and for industrial purposes has left wet nature areas
with considerable water shortages, especially during
the summer. This process is sometimes called desicca-
tion, but in areas with a large precipitation surplus
lack of groundwater does not so much lead to water
shortage for plant growth, but to increased mineral-
ization of the peat and replacement of groundwater
by rainwater (Grootjans et al. 1996). In order to
compensate for these water losses additional surface
water was sometimes supplied to these wetlands. The
nutrient content of surface water usually is much higher
than that of groundwater and it also has different con-
centrations of iron, sulphate and chloride (Roelofs 1991,
Barendregt et al. 1995). Apart from that, the intens-
ive agricultural land use emits high amounts of NH 3
into the atmosphere, which also reaches nature areas
through dust particles and precipitation. In some
regions in Europe this atmospheric N deposition can
be very high (25 - 40 kg of N ha −1 yr −1 ).
9.4.2 Eutrophication: increased influence
of surface water systems
In natural fens nitrogen is often the primary limiting
nutrient, while harvesting eventually results in phos-
phorus limitation (Verhoeven et al. 1996). Experi-
mental research had shown that Ca 2+ can limit P
availability in very calcareous fens (Boyer & Wheeler
1989) but in less calcareous fens Fe 2+ /Fe 3+ and Al 3+
play a more important role (Boeye & Verheyen 1994,
Wassen et al. 1996).
The decline of groundwater discharge in wetlands
has lead to widespread eutrophication in most Euro-
pean wetlands, in north-west Europe in particular.
Eutrophication means an increase in availability of
nutrients and is expressed in the vegetation by a
9.4.1 Acidification
Acidification of the topsoil occurs when calcareous and
iron-rich groundwater in the topsoil is replaced by rain-
 
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