210
growth of weeds and eutrophication of the wetlands.
It is also a limiting nutrient in northern bogs,
freshwater marshes, etc. However, P is an important mineral in other types of wetlands, e.g. agricultural wetlands. In such wetlands, it may not be
considered as a limiting factor because of its relative abundance and biochemical stability. Further,
P retention is considered as one of the most
important attributes of natural and constructed
wetlands, particularly those which receive nonpoint source of pollution.
Concomitant to above, P occurs as soluble
and insoluble complexes in both organic and
inorganic forms in wetland soils. Inorganic
forms include the ions PO 4
3−
, HPO 4
2−
and
H 2 PO 4
−
. P also has an affinity towards Ca, Fe
and Al. It forms complexes with those elements
when they are readily available. A major portion of P in wetlands is tied up in organic litter
and peat and in inorganic sediments at any one
time. The former generally dominates the peatlands, and the latter usually dominates the
mineral soil wetlands. Orthophosphate is the
main inorganic form. P is usually categorised
by its differential solubility in various chemical
extractants (Reddy et al. 1999). The analytical
measure of biologically available orthophosphates is sometimes called the ‘soluble reactive
phosphorus (SRP)’.
Notwithstanding the above, there are three
general conclusions about the tendency of P to
precipitate with selected ions: (a) P is fixed as Al
or Fe phosphates in acid soils, (b) P is bound by
Ca and Mg in alkaline soils, and (c) P is most
bioavailable at slightly acidic or neutral pH
(Reddy et al. 1999).
In addition to the above, it may be noted here
that high algal productivity may pull CO 2 out of
the water, shift the whole CO 3 equilibrium and
drive the pH as high as 9.0 or 10.0 in many surface water wetlands. Under these conditions, P is
co-precipitated as it adsorbs onto calcite and precipitates as calcium phosphate (Reddy et al.
1999). Further, the sorption of P onto clay particles is believed to involve both the chemical
bonding of the negatively charged phosphates to
the positively charged edges of the clay and the
substitution of phosphates for silicate in the clay
matrix. This clay–P complex is particularly
important for many wetlands, including riparian
wetlands. Most of the macrophytes of the wetlands obtain their P from the soil. The sedimentation of P sorbed on to clay particles is an indirect
way in which P is made available to the biotic
components of the wetland. The essence is that
the plants transform inorganic P to organic forms.
The latter are then stored in organic peats, mineralised by microbial activity or exported from the
wetland.
A number of changes in the availability of
P result when soils are flooded and conditions
become anaerobic. A well-documented phenomenon occurs in the hypolimnion of lakes.
It is the increase in soluble P when the hypolimnion and the sediment–water interface become
anoxic. A similar phenomenon may generally
also occur in the wetlands but on a compressed
vertical scale. P, which is in a specific ferric
phosphate state (analytically known as reductant-soluble P), is released into solution as ferric
iron is reduced to more soluble form (Faulkner
and Richardson 1989).
13.7 Chemical Transport
into Wetlands
The inputs of materials into wetlands generally
occur through geologic, hydrologic and biologic pathways. The geologic inputs from the
weathering of parent rocks may be important
in some wetlands. On the other hand, biologic
inputs include photosynthetic uptake of carbon, nitrogen fixation and biotic transport of
materials by mobile animals, such as birds.
Further, most of the elemental inputs into the
wetlands are generally dominated by hydrologic inputs.
Concomitant to above, climate influences surface water quality through the balance of precipitation and evapotranspiration. Further, the
amount of dissolved and suspended materials
which enter streams, rivers and wetlands also
depends on the size of the watershed, the slope of
the landscape, the soil texture and the variety of
topography.
13 Wetland Biogeochemistry
growth of weeds and eutrophication of the wetlands.
It is also a limiting nutrient in northern bogs,
freshwater marshes, etc. However, P is an important mineral in other types of wetlands, e.g. agricultural wetlands. In such wetlands, it may not be
considered as a limiting factor because of its relative abundance and biochemical stability. Further,
P retention is considered as one of the most
important attributes of natural and constructed
wetlands, particularly those which receive nonpoint source of pollution.
Concomitant to above, P occurs as soluble
and insoluble complexes in both organic and
inorganic forms in wetland soils. Inorganic
forms include the ions PO 4
3−
, HPO 4
2−
and
H 2 PO 4
−
. P also has an affinity towards Ca, Fe
and Al. It forms complexes with those elements
when they are readily available. A major portion of P in wetlands is tied up in organic litter
and peat and in inorganic sediments at any one
time. The former generally dominates the peatlands, and the latter usually dominates the
mineral soil wetlands. Orthophosphate is the
main inorganic form. P is usually categorised
by its differential solubility in various chemical
extractants (Reddy et al. 1999). The analytical
measure of biologically available orthophosphates is sometimes called the ‘soluble reactive
phosphorus (SRP)’.
Notwithstanding the above, there are three
general conclusions about the tendency of P to
precipitate with selected ions: (a) P is fixed as Al
or Fe phosphates in acid soils, (b) P is bound by
Ca and Mg in alkaline soils, and (c) P is most
bioavailable at slightly acidic or neutral pH
(Reddy et al. 1999).
In addition to the above, it may be noted here
that high algal productivity may pull CO 2 out of
the water, shift the whole CO 3 equilibrium and
drive the pH as high as 9.0 or 10.0 in many surface water wetlands. Under these conditions, P is
co-precipitated as it adsorbs onto calcite and precipitates as calcium phosphate (Reddy et al.
1999). Further, the sorption of P onto clay particles is believed to involve both the chemical
bonding of the negatively charged phosphates to
the positively charged edges of the clay and the
substitution of phosphates for silicate in the clay
matrix. This clay–P complex is particularly
important for many wetlands, including riparian
wetlands. Most of the macrophytes of the wetlands obtain their P from the soil. The sedimentation of P sorbed on to clay particles is an indirect
way in which P is made available to the biotic
components of the wetland. The essence is that
the plants transform inorganic P to organic forms.
The latter are then stored in organic peats, mineralised by microbial activity or exported from the
wetland.
A number of changes in the availability of
P result when soils are flooded and conditions
become anaerobic. A well-documented phenomenon occurs in the hypolimnion of lakes.
It is the increase in soluble P when the hypolimnion and the sediment–water interface become
anoxic. A similar phenomenon may generally
also occur in the wetlands but on a compressed
vertical scale. P, which is in a specific ferric
phosphate state (analytically known as reductant-soluble P), is released into solution as ferric
iron is reduced to more soluble form (Faulkner
and Richardson 1989).
13.7 Chemical Transport
into Wetlands
The inputs of materials into wetlands generally
occur through geologic, hydrologic and biologic pathways. The geologic inputs from the
weathering of parent rocks may be important
in some wetlands. On the other hand, biologic
inputs include photosynthetic uptake of carbon, nitrogen fixation and biotic transport of
materials by mobile animals, such as birds.
Further, most of the elemental inputs into the
wetlands are generally dominated by hydrologic inputs.
Concomitant to above, climate influences surface water quality through the balance of precipitation and evapotranspiration. Further, the
amount of dissolved and suspended materials
which enter streams, rivers and wetlands also
depends on the size of the watershed, the slope of
the landscape, the soil texture and the variety of
topography.
13 Wetland Biogeochemistry