biodiversity of microorganisms involved explain why the
nitrogen cycle is expressed in most ecosystems.
14.3.7.1 Nitrogen Cycle and Agriculture
Naturally present in the soil, nitrates play a central role in the
nitrogen cycle. Nitrates are natural and soluble compounds.
They diffuse in soils and groundwaters and are discharged in
running waters, but are also provided from nitrogen
fertilizers. Nitrates are one of the main causes of the deterioration of the quality of natural waters. European Framework Directive on water (2000/60/EC) is a directive of the
European Parliament which establishes a framework for a
comprehensive management of European waters. This is the
major European policy concerning the overall protection of
natural fresh-, brackish, salt-, ground-, and coastal water
resources. State members should encourage consultation
and active participation of all parties involved in the implementation of this directive, including the development of
management plans. The European Directive on “nitrate” is
the main regulatory instrument in the fight against pollution
related to agricultural nitrogen enrichments in Europe. It
concerns nitrogen from different sources: chemical
fertilizers, breeding, and food wastes. The nitrogen of urea
was used and is still used as a natural fertilizer. However,
today, the agricultural fertilizers are mainly produced industrially from chemical synthesis. Nitrogen is one of the
important nutrients for agricultural crop growth and yields.
Indeed, an optimum amount of nitrogen in soil enhances the
absorption of other nutrients and promotes the development
of plants and roots. In contrast, an excess of nitrogen can
lead to yield losses, favor diseases and infestations by
insects, and increase pollution of rivers and groundwaters.
It is therefore important to optimize the nitrogen fertilization
in order to obtain expected levels of agricultural yields while
avoiding these disadvantages.
Mineralization of organic compounds presents a set of
fundamental processes in the functioning of agroecosystems.
Microorganisms in soils produce extracellular enzymes that
transform polymers into soluble and easily assimilable
compounds such as amino acids. This soluble fraction
represents an important source of nitrogen even in cultivated
soils receiving large inputs of inorganic nitrogen. The
nitrates from anthropogenic (fertilization) or natural (produced by nitrification) sources will be: absorbed by plants,
immobilized by the microflora, leached in surface or deep
waters, or denitrified and therefore eliminated from the
ecosystem.
The only biological process allowing the retention of
nitrogen in soils is the biological fixation. Some fixing
micro-organisms are saprotrophic, others live in association
with the roots of plants. This is the case of bacteria belonging
to the genus Rhizobium that live in association with the roots
of Fabaceae. These bacteria cause the formation of nodules
on the roots that are sites of intense symbiotic activity
wherein the plant provides carbon sources and energy from
photosynthesis and, in turn, benefits from amino acids which
are produced by the dinitrogen-fixing bacteria. Other microorganisms are involved in the fixation of dinitrogen in association with other types of plants. Bacteria of the genus
Frankia belonging to the class of Actinobacteria, fix
dinitrogen in symbiosis with a wide range of dicotyledonous
plants called actinorhizal, including 24 genera in 8 families
such as Betulaceae (alder), Myricaceae (Myrica or wood
feels good), Casuarinaceae (Casuarinas). These plants, with
their symbiotic bacteria are collectively responsible for
approximately 15 % of nitrogen inputs from biological fixation on the Earth. They are found in different ecosystems
where nitrogen is limiting, e.g., aged ice moraines, volcanic
ash, mine backfills, or burning.
14.3.7.2 Nitrogen Cycle in Forest Ecosystems
Nitrogen cycle in most forest ecosystems can be
characterized by a relative balance between inputs (precipitation, biological fixation) and outputs of nitrogen (nitrification/denitrification, volatilization, and leaching).
14.3.7.3 Nitrogen Cycle in Wetlands
In the form of nitrate or ammonia, nitrogen is highly soluble
in water and mobile in ecosystems. Runoff and rains carry
nitrogen compounds into lakes and oceans. Wetland or
riparian zone is the zone that surrounds the riverbed between
the low water level and high water. These areas have specific
characteristics due to the mixing of aquatic and terrestrial
factors that influence their structure and functioning. They
play an important role in the maintenance of the water
quality in agricultural and urban areas, by acting as significant sinking sites for diffuse pollution, nutrients (especially
nitrogen), and micropollutants (pesticides).
The flows of particulate and dissolved matter within
the alluvial system are multiple and complex: surface and
subsurface cross flows from the adjacent hillsides, surface
(rivers, streams) and groundwater longitudinal flows, and
vertical flows (water exchanges along the continuum
groundwater–soil–atmosphere). They are affected by
physico-chemical and biological processes, some of which
are specific to these areas or are expressed there with particular
intensity (dilution, sedimentation, adsorption, absorption by
plants, or denitrification). The transfer of nitrogen in river
slopes occurs mainly in its dissolved oxidized form (nitrate)
which is hardly retained by soils. Wetlands in valley floors,
located between farmlands and running waters, are recognized
to be true “nitrate filters” against diffuse inputs from groundwater or surface inputs from watersheds. The purifier potential
of these areas can often be expressed only partially because it
is attenuated by the presence of natural (deep circulation or
runoff, macropores) or artificial (ditches, drains) short circuits.
14 Biogeochemical Cycles
579
nitrogen cycle is expressed in most ecosystems.
14.3.7.1 Nitrogen Cycle and Agriculture
Naturally present in the soil, nitrates play a central role in the
nitrogen cycle. Nitrates are natural and soluble compounds.
They diffuse in soils and groundwaters and are discharged in
running waters, but are also provided from nitrogen
fertilizers. Nitrates are one of the main causes of the deterioration of the quality of natural waters. European Framework Directive on water (2000/60/EC) is a directive of the
European Parliament which establishes a framework for a
comprehensive management of European waters. This is the
major European policy concerning the overall protection of
natural fresh-, brackish, salt-, ground-, and coastal water
resources. State members should encourage consultation
and active participation of all parties involved in the implementation of this directive, including the development of
management plans. The European Directive on “nitrate” is
the main regulatory instrument in the fight against pollution
related to agricultural nitrogen enrichments in Europe. It
concerns nitrogen from different sources: chemical
fertilizers, breeding, and food wastes. The nitrogen of urea
was used and is still used as a natural fertilizer. However,
today, the agricultural fertilizers are mainly produced industrially from chemical synthesis. Nitrogen is one of the
important nutrients for agricultural crop growth and yields.
Indeed, an optimum amount of nitrogen in soil enhances the
absorption of other nutrients and promotes the development
of plants and roots. In contrast, an excess of nitrogen can
lead to yield losses, favor diseases and infestations by
insects, and increase pollution of rivers and groundwaters.
It is therefore important to optimize the nitrogen fertilization
in order to obtain expected levels of agricultural yields while
avoiding these disadvantages.
Mineralization of organic compounds presents a set of
fundamental processes in the functioning of agroecosystems.
Microorganisms in soils produce extracellular enzymes that
transform polymers into soluble and easily assimilable
compounds such as amino acids. This soluble fraction
represents an important source of nitrogen even in cultivated
soils receiving large inputs of inorganic nitrogen. The
nitrates from anthropogenic (fertilization) or natural (produced by nitrification) sources will be: absorbed by plants,
immobilized by the microflora, leached in surface or deep
waters, or denitrified and therefore eliminated from the
ecosystem.
The only biological process allowing the retention of
nitrogen in soils is the biological fixation. Some fixing
micro-organisms are saprotrophic, others live in association
with the roots of plants. This is the case of bacteria belonging
to the genus Rhizobium that live in association with the roots
of Fabaceae. These bacteria cause the formation of nodules
on the roots that are sites of intense symbiotic activity
wherein the plant provides carbon sources and energy from
photosynthesis and, in turn, benefits from amino acids which
are produced by the dinitrogen-fixing bacteria. Other microorganisms are involved in the fixation of dinitrogen in association with other types of plants. Bacteria of the genus
Frankia belonging to the class of Actinobacteria, fix
dinitrogen in symbiosis with a wide range of dicotyledonous
plants called actinorhizal, including 24 genera in 8 families
such as Betulaceae (alder), Myricaceae (Myrica or wood
feels good), Casuarinaceae (Casuarinas). These plants, with
their symbiotic bacteria are collectively responsible for
approximately 15 % of nitrogen inputs from biological fixation on the Earth. They are found in different ecosystems
where nitrogen is limiting, e.g., aged ice moraines, volcanic
ash, mine backfills, or burning.
14.3.7.2 Nitrogen Cycle in Forest Ecosystems
Nitrogen cycle in most forest ecosystems can be
characterized by a relative balance between inputs (precipitation, biological fixation) and outputs of nitrogen (nitrification/denitrification, volatilization, and leaching).
14.3.7.3 Nitrogen Cycle in Wetlands
In the form of nitrate or ammonia, nitrogen is highly soluble
in water and mobile in ecosystems. Runoff and rains carry
nitrogen compounds into lakes and oceans. Wetland or
riparian zone is the zone that surrounds the riverbed between
the low water level and high water. These areas have specific
characteristics due to the mixing of aquatic and terrestrial
factors that influence their structure and functioning. They
play an important role in the maintenance of the water
quality in agricultural and urban areas, by acting as significant sinking sites for diffuse pollution, nutrients (especially
nitrogen), and micropollutants (pesticides).
The flows of particulate and dissolved matter within
the alluvial system are multiple and complex: surface and
subsurface cross flows from the adjacent hillsides, surface
(rivers, streams) and groundwater longitudinal flows, and
vertical flows (water exchanges along the continuum
groundwater–soil–atmosphere). They are affected by
physico-chemical and biological processes, some of which
are specific to these areas or are expressed there with particular
intensity (dilution, sedimentation, adsorption, absorption by
plants, or denitrification). The transfer of nitrogen in river
slopes occurs mainly in its dissolved oxidized form (nitrate)
which is hardly retained by soils. Wetlands in valley floors,
located between farmlands and running waters, are recognized
to be true “nitrate filters” against diffuse inputs from groundwater or surface inputs from watersheds. The purifier potential
of these areas can often be expressed only partially because it
is attenuated by the presence of natural (deep circulation or
runoff, macropores) or artificial (ditches, drains) short circuits.
14 Biogeochemical Cycles
579