Environmental Engineering Reference
In-Depth Information
reversible. Phosphorus can also be dissolved, in which case it moves with surface and sub-
surface flows of water on its way to the ocean. Dissolved P cycles through terrestrial and
aquatic ecosystems, being assimilated into plants and algae as they take up P from soils
and water through their roots. This P can move into herbivores that eat plants containing
P, and into carnivores that eat these herbivores. P taken up by plants and animals is
returned to the soil through the excretion of urine and feces as well as the mineralization
that happens upon death and decomposition as fungi and decomposers break down
tissues, releasing phosphate back into the soils.
While this shorter cycle-within-a-cycle of P movement through organisms is happening,
P is completing a long journey from the place of initial weathering to the oceans, where,
over geologic timescales, it will be reformed into rocks and uplifted to terrestrial settings.
A P atom might cycle for hundreds of thousands of years among soils, water, plants, and
animals before it finally reaches the bottom of the ocean. Inland running waters play an
obvious role in this cycle by being conduits of terrestrial P to coastal oceans through other
aquatic systems such as lakes and wetlands. Wetlands, lakes, and, more recently, reser-
voirs can add a substantial time-lag to this delivery as a substantial part of the P being
weathered can be stored temporarily in sediments of these inland waters rather than
being delivered immediately to coastal waters. The P carried by rivers to the oceans con-
sists of both dissolved and particulate forms. In general, particulate forms make up the
majority of the P being transported. Much of this P is very low reactivity and may never
enter the biotic cycle before being buried in deltaic and coastal sediments. Once in the
oceans, P can remain in sediments for hundreds of millions of years before being lifted by
geological forces to Earth's surface, where the cycle will continue, beginning anew with
weathering.
HUMAN ALTERATION OF THE GLOBAL
PHOSPHORUS CYCLE
Today's P cycle is dominated by human activity, especially agriculture (
Figure 8.2
).
Obvious human impact on the P cycle comes from mining P and distributing it around the
world in the form of fertilizers, animal feeds, and detergents. There is also international
movement of P in traded feed and food crops as well as traded livestock and animal
products. Less obvious, but still important, alterations are in the form of increased soil ero-
sion, and therefore increased P movement, due to land use change, as well as locally
increased movement of P from terrestrial to aquatic ecosystems through sewage release
and septic tank leaks. Thus, human activities impact many parts of the P cycle, and have
resulted in the redistribution of P. In fact, there is a concern that P supplies may become
limited in the future (
Box 8.2
). The human-dominated P cycle (
Figure 8.2
) releases more P
into the cycle annually (approximately triple the prehuman-dominated cycle; see
Bennett
et al. 2001
) and speeds the delivery of P to the oceans.
Humans directly mine P from rocks to make fertilizers and other compounds. In 2010,
we mined approximately 23.5 Tg (10
12
g) P, about 22.6 Tg of which was added to agricul-
tural systems in the form of
fertilizers and animal
feeds (
MacDonald et al. 2011
).
Weathering likely adds 15
20 Tg P/year to the soils, meaning that mining is currently a
