208
conditions. This, in turn, has a marked influence
on a number of biochemical transformations
unique to anaerobic conditions.
Notwithstanding the above, an ecosystem is
considered to be biogeochemically ‘open’ when
there is an abundant exchange of materials with
its surroundings. Conversely, a wetland is considered to be biogeochemically ‘closed’ when there
is little movement of materials across the ecosystem boundary. A wetland could be either of the
two categories.
13.1 Chemical Transformations
in Wetlands
13.1.1 Oxygen and Redox Potential
Anaerobic conditions usually result when soils
(whether mineral or organic) are inundated with
water. The rate at which oxygen can diffuse
through the soil is drastically reduced when water
fills the pore spaces (Greenwood 1961). Further,
the rate at which oxygen is depleted depends on
the ambient temperature, availability of organic
substrates for microbial respiration and sometimes the chemical oxygen demand from reductants, such as ferrous iron. However, it is not
always true that oxygen is totally depleted from
the soil and water of wetlands. Usually, there is a
thin layer of oxidised soil, which is sometimes
only a few millimetres thick. The deeper layers of
wetland soils may generally remain reduced. This
thin oxidised layer is often very important in the
chemical transformations and nutrient cycling
which occur in wetlands. Oxidised ions, e.g. Fe
3+
,
Mn
4+
, NO 3
− and SO 4
2
− , are found in this microlayer. On the other hand, the lower anaerobic soils
are dominated by reduced forms such as ferrous
and manganous salts, ammonia and sulphides.
Notwithstanding the above, ‘redox potential’
or oxidation–reduction potential is a measure of
the electron pressure (or availability) in a solution. It is often used to quantify further the degree
of electrochemical reduction of wetland soils.
‘Oxidation’ occurs not only during the uptake of
oxygen but also when hydrogen is removed or,
more generally, when a chemical gives up an
electron. ‘Reduction’ is the opposite process of
releasing oxygen, gaining hydrogen or gaining
an electron.
In wetland soils, redox potential could be measured. It is a quantitative measure of the tendency
of the soil to oxidise or reduce substances.
13.2 Nitrogen Transformations
In wetlands, nitrogen occurs in a number of
oxidation states. Many of these are important in
a wetland’s biogeochemistry. Nitrogen often is
one of the most limiting nutrients in flooded
soils. Anoxic conditions occur in wetlands.
Microbial denitrification of nitrates to gaseous
forms of nitrogen in wetlands and their subsequent release to the atmosphere remain one of
the more significant ways, in which nitrogen is
lost from the lithosphere and hydrosphere to the
atmosphere.
Notwithstanding the above, nitrogen transformations in wetlands involve a number of microbiological processes. Some of these make the
nutrients less available for uptake by plants.
Further, nitrogen mineralisation refers to a series
of biological transformations which convert
organically bound nitrogen to ammonium nitrogen as the organic matter is being decomposed
and degraded. This pathway occurs both under
anaerobic and aerobic conditions and is often
referred to as ‘ammonification’.
13.3 Iron and Manganese
Transformations
The reduction of Mn and Fe comes below the
reduction of nitrate on the redox potential scale.
Fe and Mn are found in wetlands primarily in
their reduced forms (ferrous and manganous,
respectively). Both are soluble and readily
available to organisms in those forms. Iron
bacteria are believed to be responsible for the
oxidation of soluble ferrous iron to insoluble
ferr ic compounds. Further, Fe and Mn may
reach toxic proportions in their reduced forms
in wetland soils.
13 Wetland Biogeochemistry
conditions. This, in turn, has a marked influence
on a number of biochemical transformations
unique to anaerobic conditions.
Notwithstanding the above, an ecosystem is
considered to be biogeochemically ‘open’ when
there is an abundant exchange of materials with
its surroundings. Conversely, a wetland is considered to be biogeochemically ‘closed’ when there
is little movement of materials across the ecosystem boundary. A wetland could be either of the
two categories.
13.1 Chemical Transformations
in Wetlands
13.1.1 Oxygen and Redox Potential
Anaerobic conditions usually result when soils
(whether mineral or organic) are inundated with
water. The rate at which oxygen can diffuse
through the soil is drastically reduced when water
fills the pore spaces (Greenwood 1961). Further,
the rate at which oxygen is depleted depends on
the ambient temperature, availability of organic
substrates for microbial respiration and sometimes the chemical oxygen demand from reductants, such as ferrous iron. However, it is not
always true that oxygen is totally depleted from
the soil and water of wetlands. Usually, there is a
thin layer of oxidised soil, which is sometimes
only a few millimetres thick. The deeper layers of
wetland soils may generally remain reduced. This
thin oxidised layer is often very important in the
chemical transformations and nutrient cycling
which occur in wetlands. Oxidised ions, e.g. Fe
3+
,
Mn
4+
, NO 3
− and SO 4
2
− , are found in this microlayer. On the other hand, the lower anaerobic soils
are dominated by reduced forms such as ferrous
and manganous salts, ammonia and sulphides.
Notwithstanding the above, ‘redox potential’
or oxidation–reduction potential is a measure of
the electron pressure (or availability) in a solution. It is often used to quantify further the degree
of electrochemical reduction of wetland soils.
‘Oxidation’ occurs not only during the uptake of
oxygen but also when hydrogen is removed or,
more generally, when a chemical gives up an
electron. ‘Reduction’ is the opposite process of
releasing oxygen, gaining hydrogen or gaining
an electron.
In wetland soils, redox potential could be measured. It is a quantitative measure of the tendency
of the soil to oxidise or reduce substances.
13.2 Nitrogen Transformations
In wetlands, nitrogen occurs in a number of
oxidation states. Many of these are important in
a wetland’s biogeochemistry. Nitrogen often is
one of the most limiting nutrients in flooded
soils. Anoxic conditions occur in wetlands.
Microbial denitrification of nitrates to gaseous
forms of nitrogen in wetlands and their subsequent release to the atmosphere remain one of
the more significant ways, in which nitrogen is
lost from the lithosphere and hydrosphere to the
atmosphere.
Notwithstanding the above, nitrogen transformations in wetlands involve a number of microbiological processes. Some of these make the
nutrients less available for uptake by plants.
Further, nitrogen mineralisation refers to a series
of biological transformations which convert
organically bound nitrogen to ammonium nitrogen as the organic matter is being decomposed
and degraded. This pathway occurs both under
anaerobic and aerobic conditions and is often
referred to as ‘ammonification’.
13.3 Iron and Manganese
Transformations
The reduction of Mn and Fe comes below the
reduction of nitrate on the redox potential scale.
Fe and Mn are found in wetlands primarily in
their reduced forms (ferrous and manganous,
respectively). Both are soluble and readily
available to organisms in those forms. Iron
bacteria are believed to be responsible for the
oxidation of soluble ferrous iron to insoluble
ferr ic compounds. Further, Fe and Mn may
reach toxic proportions in their reduced forms
in wetland soils.
13 Wetland Biogeochemistry