Agriculture Reference
In-Depth Information
Table 7.2
Nutrient balance for blanket peat bog in the Pennines, north-
ern England
Amount (kg ha
−
1
year
−
1
)
Na
K
Ca
P
N
Input
Precipitation
25.5
3.1
9.0
0.69
8.2
Output
Sale of sheep
0.0
0.0
0.0
0.01
0.1
Dissolved in stream
45.3
9.0
53.8
0.39
2.9
Peat erosion in stream
0.3
2.1
4.8
0.45
14.6
Net loss
20.0
8.0
49.7
0.15
9.5
Source
: data from Crisp (1966).
be dominated by transformations in the wetlands (Olson, 1992; Mitsch and Gos-
selink, 2000). There are large seasonal variations depending on the water flow and
the state of the riparian vegetation. But in general inorganic forms of nutrients
are transformed to organic forms and nitrate is denitrified. These transforma-
tions and losses have important consequences for the productivity of aquatic
systems downstream.
The terms in the nutrient balance of a riparian wetland are essentially the same
as those in a traditional wetland ricefield in a river floodplain or delta, though
of course the magnitudes differ. Inputs are delivered along the stream course as
dissolved material and sediment, and in lateral runoff from neighbouring upland;
dissolved and particulate material is filtered, absorbed, adsorbed and variously
transformed in the wetland, and flows out in runoff and percolation.
As for ricefields, additions from sediment vary very widely. The amount carried
and its composition will depend on the landscape through which the river has
flown and its soils and geology, and rainfall characteristics. The amount deposited
will in turn depend on the local landscape and conditions, including the nature of
the vegetation filtering and trapping the sediment. Table 7.3 compares additions
of P in sediment in riparian wetlands in North America with those estimated for
ricefields in Asia. The generally greater additions in North American sediments
and the very large variations are apparent.
Riparian wetlands are effective though not infinite sinks for nitrate and phos-
phate from agricultural runoff. Strips of wetland a few tens of metres wide have
been shown to remove the bulk of nitrate and phosphate entering in runoff and
groundwater, though the limits to this under different circumstances are not well
quantified (Baker and Maltby, 1995; Mitsch and Gosselink, 2000).
7.1.5 TIDAL WETLANDS
The balance in tidal wetlands is complicated by the tidal inflow and outflow of
water across the submerged sediments and the greater influence of subsurface
leaching under the large tidal head of water.
