6
M. N. Khan and F. Mohammad
by microorganisms living principally not only in the soil but
also in aquatic environments before it is available to most
living organisms. In natural water, nitrogen is present as dissolved dinitrogen, ammonia, and salts of the nitrate and nitrite ions.
The supply of both nitrogen and phosphorus from atmosphere deposition is potentially a significant source of nutrients for aquatic ecosystem. However, nitrogen deposition
is of higher magnitude than that of phosphorus, it includes
dissolved dinitrogen, the products of chemical fixation,
and some organic compounds as well. Atmospheric inputs
of nitrogen have increased a great deal more than those of
phosphorus as a result of human activities. Gaseous nitrogen
pollutants may be ammonia from the application of fertilizers and the decomposition of animal and human wastes
and oxides of nitrogen from the combustion of fossils fuels
and the use of motor vehicles. Such compounds are released
into the long-range atmospheric circulation patterns and may
travel very long distances, according to atmospheric wind
patterns and meteorological conditions, before they are deposited onto a lake surface.
Crop and livestock agricultural systems are important
contributors to local, regional, and global budgets of NH 3 ,
NO X (NO + NO 2 ), and nitrous oxide (N 2 O). Emissions of
biologically and chemically active NH 3 into the atmosphere
serve to redistribute fixed N to local and regional aquatic
and terrestrial ecosystems that may otherwise be disconnected from the sources of the N gases. About 50–75 % of
N in terrestrial ecosystems emitted from animal excreta and
synthetic fertilizer applications (Mosier 2001).
Among nonpoint nutrient pollutions, fertilizer application on land remains a major contributor, and this source
is still increasing at a frightening rate in many regions (Vitousek et al. 1997). There is a direct relationship between
population development, fertilizer applications, and riverine
N and P fluxes (Caraco 1995; Smil 2001). Both industrial
and developing nations are using significantly higher loadings of fertilizer in agriculture, with global N usage increasing eightfold, since the early 1960s (Constant and Sheldrick
1992; Caraco 1995; Matson et al. 1997; Smil 2001). These
nutrient supplies, after reaching lower rivers, estuaries, and
coastal waters, are available for phytoplankton uptake and
growth. The nitrate component of fertilizers can travel long
distances. Mallin et al. (1993) demonstrated a significant relationship between nitrate, carried ca. 400 km downstream
to the lower Neuse estuary (over a 2-week period), and increased phytoplankton productivity. Biological transformations of N added to ponds in the form of inorganic or organic
fertilizers and formulated feeds were found to dominate the
nitrogen biogeochemistry of aquaculture ponds.
One of the most rapidly increasing sources of nutrients
to both freshwaters and the coastal zone is the atmosphere.
Nutrient inputs from runoff are influenced by several environmental factors and form of fertilizer in use. A dramatic
trend in world fertilizer production is the increased proportion of urea in N production, urea now comprises roughly
40 % of all N fertilizers produced (Constant and Sheldrick
1992). This is significant because data indicate that in some
areas this shift in fertilizer composition has resulted in a shift
in the nutrient composition of runoff. Nitrate derived from
particulate or oxidized nitric/nitrous oxides in wet and dry
deposition have long been recognized as important sources
of nutrients to streams and lakes, and can be major sources
especially for soft water, nutrient-poor freshwater systems
(Likens et al. 1979; Kilham 1982). It has been estimated that
in estuarine and coastal waters 20–40 % of N inputs can be of
atmospheric origin, from industrial, agricultural, and urban
sources (Duce 1986; Fisher and Oppenheimer 1991; Paerl
1995; Coale et al. 1996). Interactions of N and iron (Fe) can
influence the structure of plankton community (DiTullio
et al. 1993) and may act as a regulator of growth and encystment of dinoflagellates (Doucette and Harrison 1991) and
possibly in the toxicity of diatoms.
Nearly 81.7 million metric tons of commercial nitrogen
fertilizer account for approximately half of all N reaching
global croplands now a days and supplies basic food needs
for at least 40 % of the world population. The challenge is
to meet the increasing food requirements and minimize the
risk of negative environmental impacts through improved
N-use efficiency. Current N-efficiency and crop productivity are generally lower in several parts of Asia than in North
America, but they are improving (Fixen and West 2002).
The surplus nitrogen (N) from Japanese agriculture was
speculated to have strongly affected the environment. In a
study of N flow in agricultural production during 1980–1997
it was estimated that application of chemical fertilizer per
unit area of farmland in Japan peaked in 1985 and then declined. The amount of residual N on farmland (expressed as
the difference between N inputs and N outputs) was lowest in 1997 owing to the low input of chemical fertilizers
and manure, although the amount of nonutilized livestock
waste was highest. Therefore, the total amount of residual N
in farmland and nonutilized livestock wastes in Japan were
141, 163, 158, and 148 kg N ha
−1
in 1980, 1985, 1990, and
1997, respectively (Mishima 2001).
The breakdown of dissolved inorganic and organic nitrogen from readily available compounds such as NH 4
+ and urea
was felt to be the important process in the nitrogen nutrient
availability. The nitrogen was utilized by the phytoplanktons
and bacteria in the Lake Kinneret (Israel), the River Charante
estuary, and coastal water near Ile de Re the French Atlantic
Coast (Berman et al. 1999).
Nonpoint sources are considered as the major nutrient
contributors because they are larger and difficult to control.
However, point sources can be a major source of nutrients for
small watersheds. Phosphorus and nitrogen in runoff from
M. N. Khan and F. Mohammad
by microorganisms living principally not only in the soil but
also in aquatic environments before it is available to most
living organisms. In natural water, nitrogen is present as dissolved dinitrogen, ammonia, and salts of the nitrate and nitrite ions.
The supply of both nitrogen and phosphorus from atmosphere deposition is potentially a significant source of nutrients for aquatic ecosystem. However, nitrogen deposition
is of higher magnitude than that of phosphorus, it includes
dissolved dinitrogen, the products of chemical fixation,
and some organic compounds as well. Atmospheric inputs
of nitrogen have increased a great deal more than those of
phosphorus as a result of human activities. Gaseous nitrogen
pollutants may be ammonia from the application of fertilizers and the decomposition of animal and human wastes
and oxides of nitrogen from the combustion of fossils fuels
and the use of motor vehicles. Such compounds are released
into the long-range atmospheric circulation patterns and may
travel very long distances, according to atmospheric wind
patterns and meteorological conditions, before they are deposited onto a lake surface.
Crop and livestock agricultural systems are important
contributors to local, regional, and global budgets of NH 3 ,
NO X (NO + NO 2 ), and nitrous oxide (N 2 O). Emissions of
biologically and chemically active NH 3 into the atmosphere
serve to redistribute fixed N to local and regional aquatic
and terrestrial ecosystems that may otherwise be disconnected from the sources of the N gases. About 50–75 % of
N in terrestrial ecosystems emitted from animal excreta and
synthetic fertilizer applications (Mosier 2001).
Among nonpoint nutrient pollutions, fertilizer application on land remains a major contributor, and this source
is still increasing at a frightening rate in many regions (Vitousek et al. 1997). There is a direct relationship between
population development, fertilizer applications, and riverine
N and P fluxes (Caraco 1995; Smil 2001). Both industrial
and developing nations are using significantly higher loadings of fertilizer in agriculture, with global N usage increasing eightfold, since the early 1960s (Constant and Sheldrick
1992; Caraco 1995; Matson et al. 1997; Smil 2001). These
nutrient supplies, after reaching lower rivers, estuaries, and
coastal waters, are available for phytoplankton uptake and
growth. The nitrate component of fertilizers can travel long
distances. Mallin et al. (1993) demonstrated a significant relationship between nitrate, carried ca. 400 km downstream
to the lower Neuse estuary (over a 2-week period), and increased phytoplankton productivity. Biological transformations of N added to ponds in the form of inorganic or organic
fertilizers and formulated feeds were found to dominate the
nitrogen biogeochemistry of aquaculture ponds.
One of the most rapidly increasing sources of nutrients
to both freshwaters and the coastal zone is the atmosphere.
Nutrient inputs from runoff are influenced by several environmental factors and form of fertilizer in use. A dramatic
trend in world fertilizer production is the increased proportion of urea in N production, urea now comprises roughly
40 % of all N fertilizers produced (Constant and Sheldrick
1992). This is significant because data indicate that in some
areas this shift in fertilizer composition has resulted in a shift
in the nutrient composition of runoff. Nitrate derived from
particulate or oxidized nitric/nitrous oxides in wet and dry
deposition have long been recognized as important sources
of nutrients to streams and lakes, and can be major sources
especially for soft water, nutrient-poor freshwater systems
(Likens et al. 1979; Kilham 1982). It has been estimated that
in estuarine and coastal waters 20–40 % of N inputs can be of
atmospheric origin, from industrial, agricultural, and urban
sources (Duce 1986; Fisher and Oppenheimer 1991; Paerl
1995; Coale et al. 1996). Interactions of N and iron (Fe) can
influence the structure of plankton community (DiTullio
et al. 1993) and may act as a regulator of growth and encystment of dinoflagellates (Doucette and Harrison 1991) and
possibly in the toxicity of diatoms.
Nearly 81.7 million metric tons of commercial nitrogen
fertilizer account for approximately half of all N reaching
global croplands now a days and supplies basic food needs
for at least 40 % of the world population. The challenge is
to meet the increasing food requirements and minimize the
risk of negative environmental impacts through improved
N-use efficiency. Current N-efficiency and crop productivity are generally lower in several parts of Asia than in North
America, but they are improving (Fixen and West 2002).
The surplus nitrogen (N) from Japanese agriculture was
speculated to have strongly affected the environment. In a
study of N flow in agricultural production during 1980–1997
it was estimated that application of chemical fertilizer per
unit area of farmland in Japan peaked in 1985 and then declined. The amount of residual N on farmland (expressed as
the difference between N inputs and N outputs) was lowest in 1997 owing to the low input of chemical fertilizers
and manure, although the amount of nonutilized livestock
waste was highest. Therefore, the total amount of residual N
in farmland and nonutilized livestock wastes in Japan were
141, 163, 158, and 148 kg N ha
−1
in 1980, 1985, 1990, and
1997, respectively (Mishima 2001).
The breakdown of dissolved inorganic and organic nitrogen from readily available compounds such as NH 4
+ and urea
was felt to be the important process in the nitrogen nutrient
availability. The nitrogen was utilized by the phytoplanktons
and bacteria in the Lake Kinneret (Israel), the River Charante
estuary, and coastal water near Ile de Re the French Atlantic
Coast (Berman et al. 1999).
Nonpoint sources are considered as the major nutrient
contributors because they are larger and difficult to control.
However, point sources can be a major source of nutrients for
small watersheds. Phosphorus and nitrogen in runoff from