8
M. N. Khan and F. Mohammad
P inputs because of the difficulty in controlling the exchange
of nitrogen and carbon between the atmosphere and water
and the fixation of atmospheric nitrogen by some blue-green
algae. Therefore, P is often the limiting element, and its control is of prime importance in reducing the accelerated eutrophication of fresh waters.
Agriculture is regarded as an important source of P in environment. However, the rapid growth and intensification of
crop and animal farming in any areas has created regional
and local imbalances in P inputs and outputs. This has created regional surpluses in P inputs (mineral fertilizers and
feed) over outputs (crop and animal produce), built up soil
P in excess of crop needs, and increased the loss of P from
land to water. Recent research has shown that this loss of P
in both surface runoff and subsurface flow originates primarily from small areas within watersheds during a few storms.
These areas occur where high soil P, or P application in mineral fertilizer or manure, coincide with high runoff or erosion
potential (Sharpley et al. 2001).
Phosphorus is an essential element for all life forms. A
mineral nutrient orthophosphate is the only form of P that
autotrophs can assimilate (Corell 1998). Phosphorus does
not occur as abundantly in soils as N and K. Total phosphorus in surface soils varies between 0.005 and 0.15 %. The
average total P content of soils is lower in the humid southeast than in the Prairie and Western states. Unfortunately, the
quality of total P in soils has little or no relationship to the
availability of P to plants. Although Prairie soils are often
high in total P, many of them are characteristically low in
plant available P. Therefore, understanding the relationship
and interactions of the various forms of P in soils and the numerous factors that influence P availability was felt essential
to efficient P management (Tisdale et al. 1995).
Anthropogenic factors, viz. mining P and transporting
it in fertilizers, animal feeds, agricultural crops, and other
products are altering the global P cycle, causing P to accumulate in the soil. Increasing P levels in the soil elevate the
potential P runoff to aquatic ecosystems leading to eutrophication of fresh water ecosystems (Schroeder et al. 2004;
Djodjic and Bergstrom 2005; Vadas et al. 2005). Phosphorus generally enters aquatic ecosystems adhered to soil particles that are eroded into lakes, streams, and rivers (Daniel
et al. 1994; Sharpley et al. 1994). Much of this runoff occurs
during major erosion-causing storms (Pionke et al. 1997).
Phosphorus-induced pollution of aquatic ecosystems is thus
strongly influenced by watershed land use and the concentration of P in watershed soil. Any factor that increases erosion or the amount of P in the soil increases the potential
P runoff to downhill aquatic ecosystems (Daniel et al.1994;
Sharpley et al. 1994). Most of the eutrophication is caused by
water quality problems owing to phosphorus accumulation
in upland soils. Accumulation of P in soil and the appearance of adverse effects in freshwater ecosystems may take a
long period of many years. Soil P accretion could lead to
sudden and unanticipated changes in aquatic ecosystem
productivity. It could also cause lags between management
actions taken to control eutrophication and the time when
results of those actions are realized (Stigliani et al. 1991).
Information regarding human impact on P cycle is meager.
However, instances of human impact on the P cycle leading to
accumulation of P in upland systems have been discovered.
Lowrance et al. (1985) found that imports of P exceeded exports by 3.7 to 11.3 kg ha
−1
per year in four subwatersheds
of the Little River in the Georgia Coastal Plain. Similarly,
a P budget of the upper Potomac River Basin revealed that
over 60 % of imported P was retained within the watershed
(Jaworski et al. 1992). In this case, P retention was caused by
an excess of fertilizer and animal feed inputs over outputs of
agricultural products. In a Florida study, Fluck et al. (1992)
found that less than 20 % was P output, in agricultural and
other products, of P input to the Lake Okeechobee watershed in fertilizers was output. Runge-Metzger (1995) calculated net fertilization (fertilizer input minus crop removal)
of 0.7–57.2 kg P ha
−1
per year in 25 countries of Europe.
All the studies showed no country on the globe with net loss
of P; all countries were P accumulators. Natural imbalance
in P inputs and outputs are not the cause for P accumulation but inputs of fertilizer and animal feeds that exceeded
outputs in agricultural products contribute to P accumulation
(Runge-Metzger 1995). During 1950 and 1990, an eightfold
increase in average available P in the soils of Ireland was
recorded (Tunney 1990). In 1990, P inputs in fertilizers to
Ireland were more than double the outputs (Tunney 1990).
Isermann (1990) calculated the P surplus (total application
of fertilizers minus net withdrawal by agricultural products)
in the Netherlands and Germany to be 88 and 63 kg ha
−1
per
year, respectively.
Bennett et al. (2001) calculated a global agricultural P
budget to determine the amount of P accumulation that occurs in agricultural areas. This budget included only agricultural inputs (fertilizer and manure) and outputs (agricultural
products such as meat and eggs, and runoff). Fertilizer inputs
were calculated based on global estimates of fertilizer use
and P content of fertilizer (FAO 1950–1997). They calculated agricultural P budget from 1958 to 1998 at 5-year intervals, which indicated that the average annual P accumulation in agricultural areas of the world was 8 Tg per year.
The result of the study carried out by Bennett et al. (2001)
suggest that a considerable fraction of the excess P in the
current global budget is being stored in agricultural soils,
which occupy 11 % of the terrestrial area of the Earth (World
Resources Institute 1998).
Moreover, studies predict that fertilizer demand and use
will continue to increase to 208 million tons by 2020, with
greater increases in developing countries, further aggravat-
M. N. Khan and F. Mohammad
P inputs because of the difficulty in controlling the exchange
of nitrogen and carbon between the atmosphere and water
and the fixation of atmospheric nitrogen by some blue-green
algae. Therefore, P is often the limiting element, and its control is of prime importance in reducing the accelerated eutrophication of fresh waters.
Agriculture is regarded as an important source of P in environment. However, the rapid growth and intensification of
crop and animal farming in any areas has created regional
and local imbalances in P inputs and outputs. This has created regional surpluses in P inputs (mineral fertilizers and
feed) over outputs (crop and animal produce), built up soil
P in excess of crop needs, and increased the loss of P from
land to water. Recent research has shown that this loss of P
in both surface runoff and subsurface flow originates primarily from small areas within watersheds during a few storms.
These areas occur where high soil P, or P application in mineral fertilizer or manure, coincide with high runoff or erosion
potential (Sharpley et al. 2001).
Phosphorus is an essential element for all life forms. A
mineral nutrient orthophosphate is the only form of P that
autotrophs can assimilate (Corell 1998). Phosphorus does
not occur as abundantly in soils as N and K. Total phosphorus in surface soils varies between 0.005 and 0.15 %. The
average total P content of soils is lower in the humid southeast than in the Prairie and Western states. Unfortunately, the
quality of total P in soils has little or no relationship to the
availability of P to plants. Although Prairie soils are often
high in total P, many of them are characteristically low in
plant available P. Therefore, understanding the relationship
and interactions of the various forms of P in soils and the numerous factors that influence P availability was felt essential
to efficient P management (Tisdale et al. 1995).
Anthropogenic factors, viz. mining P and transporting
it in fertilizers, animal feeds, agricultural crops, and other
products are altering the global P cycle, causing P to accumulate in the soil. Increasing P levels in the soil elevate the
potential P runoff to aquatic ecosystems leading to eutrophication of fresh water ecosystems (Schroeder et al. 2004;
Djodjic and Bergstrom 2005; Vadas et al. 2005). Phosphorus generally enters aquatic ecosystems adhered to soil particles that are eroded into lakes, streams, and rivers (Daniel
et al. 1994; Sharpley et al. 1994). Much of this runoff occurs
during major erosion-causing storms (Pionke et al. 1997).
Phosphorus-induced pollution of aquatic ecosystems is thus
strongly influenced by watershed land use and the concentration of P in watershed soil. Any factor that increases erosion or the amount of P in the soil increases the potential
P runoff to downhill aquatic ecosystems (Daniel et al.1994;
Sharpley et al. 1994). Most of the eutrophication is caused by
water quality problems owing to phosphorus accumulation
in upland soils. Accumulation of P in soil and the appearance of adverse effects in freshwater ecosystems may take a
long period of many years. Soil P accretion could lead to
sudden and unanticipated changes in aquatic ecosystem
productivity. It could also cause lags between management
actions taken to control eutrophication and the time when
results of those actions are realized (Stigliani et al. 1991).
Information regarding human impact on P cycle is meager.
However, instances of human impact on the P cycle leading to
accumulation of P in upland systems have been discovered.
Lowrance et al. (1985) found that imports of P exceeded exports by 3.7 to 11.3 kg ha
−1
per year in four subwatersheds
of the Little River in the Georgia Coastal Plain. Similarly,
a P budget of the upper Potomac River Basin revealed that
over 60 % of imported P was retained within the watershed
(Jaworski et al. 1992). In this case, P retention was caused by
an excess of fertilizer and animal feed inputs over outputs of
agricultural products. In a Florida study, Fluck et al. (1992)
found that less than 20 % was P output, in agricultural and
other products, of P input to the Lake Okeechobee watershed in fertilizers was output. Runge-Metzger (1995) calculated net fertilization (fertilizer input minus crop removal)
of 0.7–57.2 kg P ha
−1
per year in 25 countries of Europe.
All the studies showed no country on the globe with net loss
of P; all countries were P accumulators. Natural imbalance
in P inputs and outputs are not the cause for P accumulation but inputs of fertilizer and animal feeds that exceeded
outputs in agricultural products contribute to P accumulation
(Runge-Metzger 1995). During 1950 and 1990, an eightfold
increase in average available P in the soils of Ireland was
recorded (Tunney 1990). In 1990, P inputs in fertilizers to
Ireland were more than double the outputs (Tunney 1990).
Isermann (1990) calculated the P surplus (total application
of fertilizers minus net withdrawal by agricultural products)
in the Netherlands and Germany to be 88 and 63 kg ha
−1
per
year, respectively.
Bennett et al. (2001) calculated a global agricultural P
budget to determine the amount of P accumulation that occurs in agricultural areas. This budget included only agricultural inputs (fertilizer and manure) and outputs (agricultural
products such as meat and eggs, and runoff). Fertilizer inputs
were calculated based on global estimates of fertilizer use
and P content of fertilizer (FAO 1950–1997). They calculated agricultural P budget from 1958 to 1998 at 5-year intervals, which indicated that the average annual P accumulation in agricultural areas of the world was 8 Tg per year.
The result of the study carried out by Bennett et al. (2001)
suggest that a considerable fraction of the excess P in the
current global budget is being stored in agricultural soils,
which occupy 11 % of the terrestrial area of the Earth (World
Resources Institute 1998).
Moreover, studies predict that fertilizer demand and use
will continue to increase to 208 million tons by 2020, with
greater increases in developing countries, further aggravat-