Environmental Engineering Reference
In-Depth Information
Table 14.1
Average nitrogen content at peak standing crop (yield; mean
S.D.) and nitrogen removal in the fi rst half of
July in different cutting regimes and different years. Nitrogen content: 1975,
n
±
=
1; 1983,
n
=
3; 1999,
n
=
5; yield all years,
n
=
10. After Bakker
et al
. (2002a) .
Year
Cutting regime
N concentration (%)
Yield (g of dw m
−
2
)
N removal (kg ha
−
1
)
1975
July
1.31
625
±
92
82
July and September
1.39
810
±
97
113
1983
July
1.49
723
±
64
108
July and September
1.77
641
±
69
113
1999
July
1.89
269
±
23
51
July and September
1.41
291
±
43
41
dw, dry weight.
soils that had been used for arable cropping without
the application of phosphorus on a sandy loam. The
rationale was that the ammonium-nitrogen and potas-
sium would help increase crop yield and hence enforce
a greater phosphorus offtake, and the sulphate would
help acidify the limed soil. The yields of the above-
ground material were lower than under normal agri-
cultural practice, and less nitrogen and potassium were
removed than measured inputs. With phosphorus,
there was indeed a net removal. However, there was no
reduction of extractable (plant-available) phophorus
in the soil in 7 years (Marrs
et al
. 1998 ). There was a
slight reduction in exchangeable calcium but not in
soil pH in 7 years. On clay soils, it had taken 70 years
of cropping to reduce soil phosphorus concentrations
to levels suitable for restoration of nutrient-poor condi-
tions (Marrs 1993). Cropping is, therefore, likely to be
a useful long-term strategy to modify a nutrient-rich
soil so that ecological restoration of communities char-
acteristic of nutrient-poor soil can begin thereafter.
Whether the crop can be sold is another matter that
needs to be addressed, but that question is outside the
scope of this chapter.
(mowing and leaving the cut grass where it lay) pro-
duced a different tall forb community (Bakker
et al
.
2002a ).
Although haymaking is a successful restoration
practice by reducing standing crop from 800 to 200 g
dry weight m
− 2
, it appeared not enough to create the
oligotrophic target grassland, which has a lower stand-
ing crop. The nitrogen offtake in haymaking is too low
to compensate for inputs from atmospheric deposition
(Table 14.1; Bakker
et al
. 2002a ).
Mulching is generally not successful if the aim is to
restore oligotrophic communities in the Netherlands.
However, this treatment produced similar results
to haymaking in a 25-year experiment in southern
Germany (see comparable changes in species composi-
tion between 1975 and 1999 for #2 (haymaking) and
#3 (muching) in Figure 14.4). High ambient tempera-
tures in late summer may encourage faster decomposi-
tion of the litter in Germany compared to the
Netherlands where the climate is cooler (Kahmen
et al
.
2002). However, these results confi rm the complexity
in vegetation management and the pitfall of trying to
transfer techniques between sites and regions without
testing them previously. This could also hold for other
techniques mentioned.
14.4.2
Mowing
Haymaking (mowing and removal of the mown grass)
is a common technique that prevents succession and
removes both seeds and nutrients from the site. A com-
parison of different cutting regimes showed that hay-
making twice a year resulted in a plant community
closer to the oligotrophic target grassland community
than haymaking once a year in the sandy uppercourse
of a brook valley in the Netherlands on a site that had
been intensively farmed. Annual or biennual mulching
14.4.3
Burning
Burning is a relatively cheap way to remove above-
ground biomass and some nutrients. However, the
effects on species richness are not positive. Collins
(1992) found that in the absence of grazers the appli-
cation of prescribed fi re in North American tallgrass
prairie tends to homogenize the vegetation by uni-
formly removing the above-ground biomass and litter.
