Environmental Engineering Reference
In-Depth Information
60
(a)
(b)
(c)
50
40
30
20
10
0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0
20
40
60
80
100
0
10
20
30
40
N
total
(%)
K (mg 100 g
-1
)
P
2
O
5
(mg 100 g
-1
)
Fig. 8.6
Relation between total nitrogen, percentage of soil-extractable potassium, soil-extractable phosphorus and
the number of species per 100 m
2
in 281 grassland sites in western Europe. After Janssens
et al
. (1998).
8.6.8 Turning the soil profile
depleted, but so is that of non-target species. The pro-
cess is also mimicked by local topsoil removal and local
accumulation of soil within a site. The results so far
show that nutrient removal is successful. However, in
some sites a considerable amount of phosphorus is
found in the C horizon as a result of heavy fertilizer
input in the past. Together with the continuous input
of nitrogen from atmospheric deposition this will
cause problems for the future development of oligo-
trohic communities. The highest species diversity in
grasslands over north-west Europe is more related to
low phosphorus levels in the soil than to the levels
of potassium and nitrogen (Fig. 8.6; Janssens
et al
.
1998). The phosphate saturation in agricultural soils
in the Netherlands increased strongly during the last
few decades (www.minlenv.nl).
After topsoil removal, vegetation studied by
Verhagen
et al
. (2001) revealed a range of 0 -30%
similarity with target plant communities of oligo-
trophic grassland and heathland. Many target species
occurred in the same grid cell (1 km
2
) or adjacent grid
cells (Table 8.2). However, after nearly 10 years many
target species were still absent. This again indicates
that dispersal is a limiting factor for restoration. As
mentioned above, seed dispersal by the large her-
bivores is not always successful when still-existing
oligotrophic grassland and heathland are fenced in
together with locally more productive communities.
When sod cutting or topsoil removal are too expens-
ive or otherwise not feasible, grasslands can be
ploughed. This will activate the persistent seed bank
of target species when present. A site that was
reclaimed from heathland in the 1950s by ploughing
up to 40 cm was ploughed again up to 20 cm and up
to 60 cm. In both cases the seed bank of target heath-
land species was activated. However, after 10 years the
cover of heathland species was higher in the area
ploughed up to 60 cm than in that ploughed up to
20 cm (Fig. 8.5). This must be due to bringing the
oligotrophic C horizon to the surface. After plough-
ing to 20 cm the topsoil remains productive, which
enables competition from non-target species such as
the grass
A. capillaris
(Fig. 8.5).
8.7 Concluding remarks: key issues
8.7.1 Nutrient reduction
As dry grassland and heathland depend on low nutri-
ent availability, the removal of nutrients is a key issue.
Not only the absolute amount, but also the balance
between the limiting nutrients phosphorus, potas-
sium and nitrogen is important. Pastor
et al.
(1984)
