This role of filter is due to two biological processes favored by
the wetland ecology: plant assimilation which corresponds to
more or less long-term storage of nitrogen, and
microbiological denitrification which allows elimination of
nitrates (Forshay and Stanley 2005).
Potentially, these processes are active throughout the
wetland, although nitrogen recycling of the groundwater is
carried out in a few meters distance from its entrance in
wetlands. The boundary between agricultural parcels and
riparian areas appears to be the key point (active area) of
the buffering capacity against diffuse nitrogen pollution.
However, this recycling activity is heterogeneous in time
and space. This process of nitrogen elimination is difficult to
measure in situ. Little assessments of emitted gases are
available today. Most studies estimate that the denitrification
potential can reach up to 40 kg nitrogen ha
À1 day
À1 . Many
factors influence the denitrifying activity in the scale of
prokaryotes (temperature, oxygen, mineralizable carbon,
nitrate), soil (waterlogging of soils, organic material), the
targeted site (topography, vegetation), and wetland (geomorphology, hydrology, uses). The effectiveness of this phenomenon depends on two factors: the immersion time (few
days are necessary to remove oxygen from ground) and the
connectivity of water flows (which allow incomes of nitrate
and carbon). Denitrification is at its maximum in winter
(high water period) if temperatures are not too low (weak
activity of microorganisms).
14.3.7.4 Nitrogen Cycle in Marine Ecosystems
In the sea, as on the land, dissolved nitrogen is absorbed by
photosynthetic organisms and in the food chain. However,
important inputs of nitrogen in marine systems can cause
eutrophication*. This phenomenon seems to become more
frequent, taking different forms in the enclosed seas and in
gulfs or bays where water is not renewed: Blooms of planktonic or photosynthetic multicellular organisms occur there,
causing anoxic conditions. The risk is highest in areas where
agricultural nutrient inputs from estuaries or rains (dissolved
nitrogen) are excessive. The only sustainable solution is to
limit upstream agricultural nitrogen losses. The phenomenon
of eutrophication may occur in some muddy estuaries, at
certain times of the year, for example, as a result of riverine
inputs in autumn or after heavy rains with flooding. Waterproofing and agriculture have exacerbated these phenomena.
These episodes of eutrophication have limited duration,
often less than 15 days during which intense denitrifying
activity allows the removal of nitrogen in excess. Because of
their high productivity (>30 t ha
À1 year
À1 ), estuaries are
recognized as carbon sinks. When they are eutrophic or
dystrophic, CO 2 , N 2 O (via nitrification and denitrification)
and CH 4 can be emitted in very large quantities from
sediments. These emissions could correspond to 8 % of
total emissions of greenhouse gases in Europe.
Competition between denitrification and DNRA presents
a major ecological interest as denitrification led to the loss of
nitrogen from the ecosystem while DNRA converts nitrate to
ammonium which is available to the food chain. The competition between these two processes seems to be essentially
controlled by the amounts of electron donors and acceptors
(nitrate), DNRA being favored when carbon is limiting
while denitrification becomes the dominant process when
nitrate is limiting (Bonin et al. 1998). A study was conducted
in coastal marine sediments collected in Mediterranean Sea,
study in which different sampling stations were selected for
their
contrasting
physico-chemical
characteristics:
sediments from muddy areas rich in organic material
(2.5–7.3 %), sandy stations where organic matter content
were lower (0.3–0.8 %). Nitrate concentrations were comparable regardless of the stations. For the study period,
microbial activities are generally higher in muddy and
organic matter rich stations than in sandy sites. The seasonal
effect is indisputable: DNRA activity is maximum in autumn
when C:N ratios are high, due to the death and sedimentation
of a phytoplankton bloom; maximum denitrifying activities
are observed in winter when the nitrate concentration is
highest (period of heavy rains). In the samples which were
very rich in organic matter (15 %), up to 98 % of nitrate can
be reduced to ammonium and thus remain available to the
food chain. This process is therefore optimally expressed in
marine sediments, but has been little studied in soils. Similarly, the anammox process, initially reported in wastewater
treatment ponds, has also been observed in aquatic
ecosystems. This process which is in direct competition
with denitrification, contributes significantly to the production of molecular nitrogen in some aquatic ecosystems. In
sediments where the supply of organic matter is low, more
than 67 % of dinitrogen would be formed due to anammox
(against 33 % for denitrification) (Thamdrup and Dalsgaard
2002). Nevertheless, this percentage becomes negligible
(2 %) in eutrophic coastal areas, where the presence of
significant quantities of organic matter promotes dissimilatory reduction processes (denitrification and/or DNRA).
In the euphotic zone of coastal marine ecosystems,
magnoliophytes may affect the nitrogen cycling and in particular denitrification. Indeed, oxygen from photosynthesis
which passes from leaves to roots/rhizomes is released in
certain zones of the sediment, promoting nitrification and
denitrification in the adjacent anoxic areas. Depending on
the species, the percentage of oxygen emitted by the roots
compared with the total percentage emitted by the plant can
be variable. It is at 100 % for Lobelia sp. and at 1–4 % for
Zostera sp. Plants can also stimulate denitrifying activity by
increasing the amount of labile organic matter in the
sediments. This phenomenon occurs by excretion of
dissolved organic compounds through the roots of plants.
However, the presence of benthic algae covering the
580
J.-C. Bertrand et al.
the wetland ecology: plant assimilation which corresponds to
more or less long-term storage of nitrogen, and
microbiological denitrification which allows elimination of
nitrates (Forshay and Stanley 2005).
Potentially, these processes are active throughout the
wetland, although nitrogen recycling of the groundwater is
carried out in a few meters distance from its entrance in
wetlands. The boundary between agricultural parcels and
riparian areas appears to be the key point (active area) of
the buffering capacity against diffuse nitrogen pollution.
However, this recycling activity is heterogeneous in time
and space. This process of nitrogen elimination is difficult to
measure in situ. Little assessments of emitted gases are
available today. Most studies estimate that the denitrification
potential can reach up to 40 kg nitrogen ha
À1 day
À1 . Many
factors influence the denitrifying activity in the scale of
prokaryotes (temperature, oxygen, mineralizable carbon,
nitrate), soil (waterlogging of soils, organic material), the
targeted site (topography, vegetation), and wetland (geomorphology, hydrology, uses). The effectiveness of this phenomenon depends on two factors: the immersion time (few
days are necessary to remove oxygen from ground) and the
connectivity of water flows (which allow incomes of nitrate
and carbon). Denitrification is at its maximum in winter
(high water period) if temperatures are not too low (weak
activity of microorganisms).
14.3.7.4 Nitrogen Cycle in Marine Ecosystems
In the sea, as on the land, dissolved nitrogen is absorbed by
photosynthetic organisms and in the food chain. However,
important inputs of nitrogen in marine systems can cause
eutrophication*. This phenomenon seems to become more
frequent, taking different forms in the enclosed seas and in
gulfs or bays where water is not renewed: Blooms of planktonic or photosynthetic multicellular organisms occur there,
causing anoxic conditions. The risk is highest in areas where
agricultural nutrient inputs from estuaries or rains (dissolved
nitrogen) are excessive. The only sustainable solution is to
limit upstream agricultural nitrogen losses. The phenomenon
of eutrophication may occur in some muddy estuaries, at
certain times of the year, for example, as a result of riverine
inputs in autumn or after heavy rains with flooding. Waterproofing and agriculture have exacerbated these phenomena.
These episodes of eutrophication have limited duration,
often less than 15 days during which intense denitrifying
activity allows the removal of nitrogen in excess. Because of
their high productivity (>30 t ha
À1 year
À1 ), estuaries are
recognized as carbon sinks. When they are eutrophic or
dystrophic, CO 2 , N 2 O (via nitrification and denitrification)
and CH 4 can be emitted in very large quantities from
sediments. These emissions could correspond to 8 % of
total emissions of greenhouse gases in Europe.
Competition between denitrification and DNRA presents
a major ecological interest as denitrification led to the loss of
nitrogen from the ecosystem while DNRA converts nitrate to
ammonium which is available to the food chain. The competition between these two processes seems to be essentially
controlled by the amounts of electron donors and acceptors
(nitrate), DNRA being favored when carbon is limiting
while denitrification becomes the dominant process when
nitrate is limiting (Bonin et al. 1998). A study was conducted
in coastal marine sediments collected in Mediterranean Sea,
study in which different sampling stations were selected for
their
contrasting
physico-chemical
characteristics:
sediments from muddy areas rich in organic material
(2.5–7.3 %), sandy stations where organic matter content
were lower (0.3–0.8 %). Nitrate concentrations were comparable regardless of the stations. For the study period,
microbial activities are generally higher in muddy and
organic matter rich stations than in sandy sites. The seasonal
effect is indisputable: DNRA activity is maximum in autumn
when C:N ratios are high, due to the death and sedimentation
of a phytoplankton bloom; maximum denitrifying activities
are observed in winter when the nitrate concentration is
highest (period of heavy rains). In the samples which were
very rich in organic matter (15 %), up to 98 % of nitrate can
be reduced to ammonium and thus remain available to the
food chain. This process is therefore optimally expressed in
marine sediments, but has been little studied in soils. Similarly, the anammox process, initially reported in wastewater
treatment ponds, has also been observed in aquatic
ecosystems. This process which is in direct competition
with denitrification, contributes significantly to the production of molecular nitrogen in some aquatic ecosystems. In
sediments where the supply of organic matter is low, more
than 67 % of dinitrogen would be formed due to anammox
(against 33 % for denitrification) (Thamdrup and Dalsgaard
2002). Nevertheless, this percentage becomes negligible
(2 %) in eutrophic coastal areas, where the presence of
significant quantities of organic matter promotes dissimilatory reduction processes (denitrification and/or DNRA).
In the euphotic zone of coastal marine ecosystems,
magnoliophytes may affect the nitrogen cycling and in particular denitrification. Indeed, oxygen from photosynthesis
which passes from leaves to roots/rhizomes is released in
certain zones of the sediment, promoting nitrification and
denitrification in the adjacent anoxic areas. Depending on
the species, the percentage of oxygen emitted by the roots
compared with the total percentage emitted by the plant can
be variable. It is at 100 % for Lobelia sp. and at 1–4 % for
Zostera sp. Plants can also stimulate denitrifying activity by
increasing the amount of labile organic matter in the
sediments. This phenomenon occurs by excretion of
dissolved organic compounds through the roots of plants.
However, the presence of benthic algae covering the
580
J.-C. Bertrand et al.