Agriculture Reference
In-Depth Information
near complete removal of weatherable minerals. The effects are most marked in
soils derived from acidic rocks or strongly weathered sediments. Large wetland
areas in south and south-east Asia have soils of this sort (Brammer and Brinkman,
1977; Brinkman, 1977a, b), but doubt has recently been cast on the importance
of ferrolysis in European soils previously thought to have been formed by it (van
Ranst and De Coninck, 2002).
7.1.3 PEAT BOGS
At the opposite end of the fertility scale from ricefields are peat bogs in pluvial
landscapes. Nutrient inputs come almost entirely from rainfall, and the nutrient
reserves in the organic matter buffering the soil solution are small (Moore and
Bellamy, 1974). The chemistry of peat bogs is therefore precarious and changes
in the composition of the rainfall can have a large effect on the composition of
the soil solution.
Bogs are naturally acid. This is an inevitable consequence of their development.
The principal source of acidity is the intake of mineral nutrients by the vegetation.
Because the main forms of N absorbed under the anoxic conditions are NH
4
+
or
N
2
fixed from the air, the plants absorb more cations than anions and consequently
export H
+
from their roots to maintain electrical neutrality. At steady state the
input of organic matter from primary production at the bog surface is balanced by
loss of organic matter by decomposition throughout the profile (Clymo, 1984).
But because by definition bogs largely comprise undecomposed plant material,
at steady state there is a substantial and effectively permanent accumulation
of alkalinity in the organic matter and of acidity in the soil. The process is self
reinforcing: the greater the acidity that develops, the more the CEC of the organic
and mineral matter in the peat is dominated by H
+
and the weaker its ability
to retain nutrient cations. Deposition of nitric or sulfuric acids in rainfall will
add to the acidity. Subsequent denitrification and sulfate reduction generate an
equivalent amount of base, so this acidity is neutralized. The acidity of wetland
peats is discussed by Ross (1995).
An example of the fragile nutrient balance in peat bogs is given in Table 7.2 for
a blanket bog in northern England. The table shows the losses of N through ero-
sion of the peat, and the losses of nutrient cations through leaching and removal
in stream water.
7.1.4 RIPARIAN WETLANDS
Riparian wetlands are those lands that are periodically inundated with water
from adjacent rivers, streams, lakes or other freshwater bodies, and by runoff
from upland areas. Large fluxes of energy and nutrients pass through riparian
wetlands and they are important sinks and transformers of nutrients. In water-
sheds with extensive riparian wetlands, the composition of the river water may
