of atmospheric aerosols (F 84 ) is a minor flux.
Groundwater seepage to the coastal ocean is a potentially important but undocumented flux.
Riverine phosphorus derives from weathered
continental rocks and soils. Because phosphorus is
particle-reactive, most riverine phosphorus is associated with particulate matter. By most estimates,
over 90% of the phosphorus delivered by rivers to
the ocean is as particulate phosphorus (F 24(p) ). Dissolved phosphorus in rivers occurs in both inorganic
and organic forms. The scant data on dissolved organic phosphorus suggest that it may account for
50% or more of dissolved riverine phosphorus. The
chemical form of phosphorus associated with riverine particles is variable and depends upon the
drainage basin geology, on the extent of weathering
of the substrate, and on the nature of the river itself.
Available data suggest that approximately 20–40%
of phosphorus in suspended particulate matter is
organic. Inorganic forms are partitioned mainly between ferric oxyhydroxides and apatite. Aluminum
oxyhydroxides and clays may also be significant
carriers of phosphorus.
The fate of phosphorus entering the ocean via
rivers is variable. Dissolved phosphorus in estuaries
at the continent–ocean interface typically displays
nonconservative behavior. Both negative and positive
deviations from conservative mixing can occur,
sometimes changing seasonally within the same estuary. Net removal of phosphorus in estuaries is
typically driven by flocculation of humic-iron complexes and biological uptake. Net phosphorus release
is due to a combination of desorption from fresh
water particles entering high-ionic-strength marine
waters, and flux of diagenetically mobilized phosphorus from benthic sediments. Accurate estimates
of bioavailable riverine phosphorus flux to the ocean
must take into account, in addition to dissolved
forms, the fraction of riverine particulate phosphorus
released to solution upon entering the ocean.
Human impacts on the global phosphorus cycle The
mining of phosphate rock (mostly from marine
phosphorite deposits) for use as agricultural fertilizer
(F 72 ) increased dramatically in the latter half of the
twentieth century. In addition to fertilizer use,
deforestation, increased cultivation, and urban and
industrial waste disposal all have enhanced
phosphorus transport from terrestrial to aquatic
systems, often with deleterious results. For example,
elevated phosphorus concentrations in rivers resulting
from these activities have resulted in eutrophication in
some lakes and coastal areas, stimulating nuisance
algal blooms and promoting hypoxic or anoxic
conditions that are harmful or lethal to natural
populations.
Increased erosion due to forest clear-cutting and
widespread cultivation has increased riverine suspended matter concentrations, and thus increased the
riverine particulate phosphorus flux. Dams, in contrast, decrease sediment loads in rivers and therefore
diminish phosphorus-flux to the oceans. However,
increased erosion below dams and diagenetic mobilization of phosphorus in sediments trapped behind
dams moderates this effect. The overall effect has
been a 50–300% increase in riverine phosphorus flux
to the oceans above pre-agricultural levels.
The Marine Phosphorus Cycle
Phosphorus in its simplest form, dissolved orthophosphate, is taken up by photosynthetic organisms
at the base of the marine food web. When phosphate
is exhausted, organisms may utilize more complex
forms by converting them to orthophosphate via
enzymatic and microbiological reactions. In the open
ocean most phosphorus associated with biogenic
particles is recycled within the upper water column.
P mineral
P nonoccluded
P organic
P occluded
P total
Time
Weight of P/unit area of soil
Figure 2 The fate of phosphorus during soil formation can be
viewed as the progressive dissolution of primary mineral P
(dominantly apatite), some of which is lost from the system by
leaching (decrease in P total ), and some of which is reincorporated
into nonoccluded, occluded and organic fractions within the soil.
Nonoccluded P is defined as phosphate sorbed to surface of
hydrous oxides of iron and aluminum, and calcium carbonate.
Occluded P refers to P present within the mineral matrix of
discrete mineral phases. The initial build-up in organic P results
from organic matter return to soil from vegetation supported by
the soil. The subsequent decline in P organic is due to progressive
mineralization and soil leaching. The time-scale over which these
transformations occur depends upon the initial soil composition,
topographic, and climatic factors. Figure is after Walker and
Syers (1976). The fate of phosphorus during pedogenesis.
Geoderma 15: 1–19, with permission.
572 PHOSPHORUS CYCLE
Groundwater seepage to the coastal ocean is a potentially important but undocumented flux.
Riverine phosphorus derives from weathered
continental rocks and soils. Because phosphorus is
particle-reactive, most riverine phosphorus is associated with particulate matter. By most estimates,
over 90% of the phosphorus delivered by rivers to
the ocean is as particulate phosphorus (F 24(p) ). Dissolved phosphorus in rivers occurs in both inorganic
and organic forms. The scant data on dissolved organic phosphorus suggest that it may account for
50% or more of dissolved riverine phosphorus. The
chemical form of phosphorus associated with riverine particles is variable and depends upon the
drainage basin geology, on the extent of weathering
of the substrate, and on the nature of the river itself.
Available data suggest that approximately 20–40%
of phosphorus in suspended particulate matter is
organic. Inorganic forms are partitioned mainly between ferric oxyhydroxides and apatite. Aluminum
oxyhydroxides and clays may also be significant
carriers of phosphorus.
The fate of phosphorus entering the ocean via
rivers is variable. Dissolved phosphorus in estuaries
at the continent–ocean interface typically displays
nonconservative behavior. Both negative and positive
deviations from conservative mixing can occur,
sometimes changing seasonally within the same estuary. Net removal of phosphorus in estuaries is
typically driven by flocculation of humic-iron complexes and biological uptake. Net phosphorus release
is due to a combination of desorption from fresh
water particles entering high-ionic-strength marine
waters, and flux of diagenetically mobilized phosphorus from benthic sediments. Accurate estimates
of bioavailable riverine phosphorus flux to the ocean
must take into account, in addition to dissolved
forms, the fraction of riverine particulate phosphorus
released to solution upon entering the ocean.
Human impacts on the global phosphorus cycle The
mining of phosphate rock (mostly from marine
phosphorite deposits) for use as agricultural fertilizer
(F 72 ) increased dramatically in the latter half of the
twentieth century. In addition to fertilizer use,
deforestation, increased cultivation, and urban and
industrial waste disposal all have enhanced
phosphorus transport from terrestrial to aquatic
systems, often with deleterious results. For example,
elevated phosphorus concentrations in rivers resulting
from these activities have resulted in eutrophication in
some lakes and coastal areas, stimulating nuisance
algal blooms and promoting hypoxic or anoxic
conditions that are harmful or lethal to natural
populations.
Increased erosion due to forest clear-cutting and
widespread cultivation has increased riverine suspended matter concentrations, and thus increased the
riverine particulate phosphorus flux. Dams, in contrast, decrease sediment loads in rivers and therefore
diminish phosphorus-flux to the oceans. However,
increased erosion below dams and diagenetic mobilization of phosphorus in sediments trapped behind
dams moderates this effect. The overall effect has
been a 50–300% increase in riverine phosphorus flux
to the oceans above pre-agricultural levels.
The Marine Phosphorus Cycle
Phosphorus in its simplest form, dissolved orthophosphate, is taken up by photosynthetic organisms
at the base of the marine food web. When phosphate
is exhausted, organisms may utilize more complex
forms by converting them to orthophosphate via
enzymatic and microbiological reactions. In the open
ocean most phosphorus associated with biogenic
particles is recycled within the upper water column.
P mineral
P nonoccluded
P organic
P occluded
P total
Time
Weight of P/unit area of soil
Figure 2 The fate of phosphorus during soil formation can be
viewed as the progressive dissolution of primary mineral P
(dominantly apatite), some of which is lost from the system by
leaching (decrease in P total ), and some of which is reincorporated
into nonoccluded, occluded and organic fractions within the soil.
Nonoccluded P is defined as phosphate sorbed to surface of
hydrous oxides of iron and aluminum, and calcium carbonate.
Occluded P refers to P present within the mineral matrix of
discrete mineral phases. The initial build-up in organic P results
from organic matter return to soil from vegetation supported by
the soil. The subsequent decline in P organic is due to progressive
mineralization and soil leaching. The time-scale over which these
transformations occur depends upon the initial soil composition,
topographic, and climatic factors. Figure is after Walker and
Syers (1976). The fate of phosphorus during pedogenesis.
Geoderma 15: 1–19, with permission.
572 PHOSPHORUS CYCLE