PHOSPHORUS CYCLE
K. C. Ruttenberg, Woods Hole Oceanographic
Institution, Woods Hole, MA, USA
Copyright & 2001 Elsevier Ltd.
Introduction
The global phosphorus cycle has four major components: (i) tectonic uplift and exposure of phosphorus-bearing rocks to the forces of weathering;
(ii) physical erosion and chemical weathering of
rocks producing soils and providing dissolved and
particulate phosphorus to rivers; (iii) riverine transport of phosphorus to lakes and the ocean; and
(iv) sedimentation of phosphorus associated with
organic and mineral matter and burial in sediments
(Figure 1). The cycle begins anew with uplift of
sediments into the weathering regime.
Phosphorus is an essential nutrient for all life
forms. It is a key player in fundamental biochemical
reactions involving genetic material (DNA, RNA)
and energy transfer (adenosine triphosphate, ATP),
and in structural support of organisms provided by
membranes (phospholipids) and bone (the biomineral hydroxyapatite). Photosynthetic organisms
utilize dissolved phosphorus, carbon, and other essential nutrients to build their tissues using energy
from the sun. Biological productivity is contingent
upon the availability of phosphorus to these organisms, which constitute the base of the food chain in
both terrestrial and aquatic systems.
Phosphorus locked up in bedrock, soils, and
sediments is not directly available to organisms.
Conversion of unavailable forms to dissolved orthophosphate, which can be directly assimilated, occurs
through geochemical and biochemical reactions at
various stages in the global phosphorus cycle. Production of biomass fueled by phosphorus bioavailability results in the deposition of organic matter in
soil and sediments, where it acts as a source of fuel
and nutrients to microbial communities. Microbial
activity in soils and sediments, in turn, strongly
influences the concentration and chemical form
of phosphorus incorporated into the geological
record.
This article begins with a brief overview of the
various components of the global phosphorus cycle.
Estimates of the mass of important phosphorus reservoirs, transport rates (fluxes) between reservoirs,
and residence times are given in Tables 1 and 2. As is
clear from the large uncertainties associated with
these estimates of reservoir size and flux, there remain many aspects of the global phosphorus cycle
that are poorly understood. The second half of the
article describes current efforts underway to advance
our understanding of the global phosphorus cycle.
These include (i) the use of phosphate oxygen isotopes (d
18 O-PO 4 ) as a tool for identifying the role of
microbes in the transformer of phosphate from one
reservoir to another; (ii) the use of naturally occurring cosmogenic isotopes of phosphorus (
32 P and
33 P) to provide insight into phosphorus-cycling
pathways in the surface ocean; (iii) critical evaluation
of the potential role of phosphate limitation in
coastal and open ocean ecosystems; (iv) reevaluation
of the oceanic residence time of phosphorus; and
(v) rethinking the global phosphorus-cycle on geological timescales, with implications for atmospheric
oxygen and phosphorus limitation primary productivity in the ocean.
The Global Phosphorus Cycle:
Overview
The Terrestrial Phosphorus Cycle
In terrestrial systems, phosphorus resides in three
pools: bedrock, soil, and living organisms (biomass)
(Table 1). Weathering of continental bedrock is the
principal source of phosphorus to the soils that
support continental vegetation (F 12 ); atmospheric
deposition is relatively unimportant (F 82 ). Phosphorus is weathered from bedrock by dissolution of
phosphorus-bearing minerals such as apatite
(Ca 10 (PO 4 ) 6 (OH, F, Cl) 2 ), the most abundant primary phosphorus mineral in crustal rocks. Weathering reactions are driven by exposure of minerals
to naturally occurring acids derived mainly from
microbial activity. Phosphate solubilized during
weathering is available for uptake by terrestrial
plants, and is returned to the soil by decay of litterfall
(Figure 1).
Soil solution phosphate concentrations are maintained at low levels as a result of absorption of
phosphorus by various soil constituents, particularly
ferric iron and aluminum oxyhydroxides. Sorption is
considered the most important process controlling
terrestrial phosphorus bioavailability. Plants have
different physiological strategies for obtaining
phosphorus despite low soil solution concentrations.
567
K. C. Ruttenberg, Woods Hole Oceanographic
Institution, Woods Hole, MA, USA
Copyright & 2001 Elsevier Ltd.
Introduction
The global phosphorus cycle has four major components: (i) tectonic uplift and exposure of phosphorus-bearing rocks to the forces of weathering;
(ii) physical erosion and chemical weathering of
rocks producing soils and providing dissolved and
particulate phosphorus to rivers; (iii) riverine transport of phosphorus to lakes and the ocean; and
(iv) sedimentation of phosphorus associated with
organic and mineral matter and burial in sediments
(Figure 1). The cycle begins anew with uplift of
sediments into the weathering regime.
Phosphorus is an essential nutrient for all life
forms. It is a key player in fundamental biochemical
reactions involving genetic material (DNA, RNA)
and energy transfer (adenosine triphosphate, ATP),
and in structural support of organisms provided by
membranes (phospholipids) and bone (the biomineral hydroxyapatite). Photosynthetic organisms
utilize dissolved phosphorus, carbon, and other essential nutrients to build their tissues using energy
from the sun. Biological productivity is contingent
upon the availability of phosphorus to these organisms, which constitute the base of the food chain in
both terrestrial and aquatic systems.
Phosphorus locked up in bedrock, soils, and
sediments is not directly available to organisms.
Conversion of unavailable forms to dissolved orthophosphate, which can be directly assimilated, occurs
through geochemical and biochemical reactions at
various stages in the global phosphorus cycle. Production of biomass fueled by phosphorus bioavailability results in the deposition of organic matter in
soil and sediments, where it acts as a source of fuel
and nutrients to microbial communities. Microbial
activity in soils and sediments, in turn, strongly
influences the concentration and chemical form
of phosphorus incorporated into the geological
record.
This article begins with a brief overview of the
various components of the global phosphorus cycle.
Estimates of the mass of important phosphorus reservoirs, transport rates (fluxes) between reservoirs,
and residence times are given in Tables 1 and 2. As is
clear from the large uncertainties associated with
these estimates of reservoir size and flux, there remain many aspects of the global phosphorus cycle
that are poorly understood. The second half of the
article describes current efforts underway to advance
our understanding of the global phosphorus cycle.
These include (i) the use of phosphate oxygen isotopes (d
18 O-PO 4 ) as a tool for identifying the role of
microbes in the transformer of phosphate from one
reservoir to another; (ii) the use of naturally occurring cosmogenic isotopes of phosphorus (
32 P and
33 P) to provide insight into phosphorus-cycling
pathways in the surface ocean; (iii) critical evaluation
of the potential role of phosphate limitation in
coastal and open ocean ecosystems; (iv) reevaluation
of the oceanic residence time of phosphorus; and
(v) rethinking the global phosphorus-cycle on geological timescales, with implications for atmospheric
oxygen and phosphorus limitation primary productivity in the ocean.
The Global Phosphorus Cycle:
Overview
The Terrestrial Phosphorus Cycle
In terrestrial systems, phosphorus resides in three
pools: bedrock, soil, and living organisms (biomass)
(Table 1). Weathering of continental bedrock is the
principal source of phosphorus to the soils that
support continental vegetation (F 12 ); atmospheric
deposition is relatively unimportant (F 82 ). Phosphorus is weathered from bedrock by dissolution of
phosphorus-bearing minerals such as apatite
(Ca 10 (PO 4 ) 6 (OH, F, Cl) 2 ), the most abundant primary phosphorus mineral in crustal rocks. Weathering reactions are driven by exposure of minerals
to naturally occurring acids derived mainly from
microbial activity. Phosphate solubilized during
weathering is available for uptake by terrestrial
plants, and is returned to the soil by decay of litterfall
(Figure 1).
Soil solution phosphate concentrations are maintained at low levels as a result of absorption of
phosphorus by various soil constituents, particularly
ferric iron and aluminum oxyhydroxides. Sorption is
considered the most important process controlling
terrestrial phosphorus bioavailability. Plants have
different physiological strategies for obtaining
phosphorus despite low soil solution concentrations.
567