Environmental Engineering Reference
In-Depth Information
discovered P, isolating P
o
from urine. By the 1800s, P
o
was being used to make matches
and weapons. The interest in and utility of P
o
led to many studies of P abundance in
tissues of animals and plants and to methods to extract P from these tissues, particularly
P-rich bone. By the mid- to late 1800s, the extraction of P moved from these tissues to rock
P. These rocks, especially apatite (a term that refers to a variety of tri-calcium phosphate
minerals) are still the major source of P for human use. The P extracted from rocks is now
primarily used in the oxidized form (phosphates) in fertilizer, feed additives, and
detergent additives. These human uses of P have profoundly influenced P cycling on the
planet.
P is rarely found in its elemental form. Instead, it typically exists as some form of
phosphate, including PO
3
2
, HPO
2
2
,H
2
PO
4
2
, and H
3
PO
4
. Phosphorus has a number of
potential oxidation states ranging from
5(P
o
is, of course, 0). However, unlike
nitrogen, carbon, and sulfur, the cycles of which could not occur in the absence of
changes in oxidation state, P exists almost exclusively in its most oxidized form, PO
4
3-
.
This is true whether P is dissolved or particulate, organic or inorganic. That is, whether
in apatite deposits, soils, sediments, bones, living organisms, or detrital organic matter, P
is in the
3to
2
1
5 (oxide) form. Another difference between P and many other biologically
important elements is that the gaseous phase is extremely rare in nature.
Despite the fact that P does not itself have an active redox cycle and has only a minor
gaseous form (phosphine), the cycling of P in both terrestrial and aquatic systems can be
complex due to the large number of biotic, physical, and chemical pathways controlling its
movement and form. Phosphorus readily binds to siliceous clays, humic material, and iron
and aluminum oxides. This chemical binding is sufficient under many conditions to
strongly inhibit phosphorus release from soils and sediments; thus, P often moves physi-
cally within and across ecosystems attached or incorporated into soils and sediment
particles (see Chapter 5). The chemical reactivity of P, combined with rapid biotic uptake,
partly explains the low concentration of P in most ground water and surface water.
1
THE IMPORTANCE OF PHOSPHORUS IN
TERRESTRIAL ECOSYSTEMS
Soil P is an important plant nutrient that plays a critical role in limiting productivity in
terrestrial environments. A recent meta-analysis by Elser et al. (2007) summarized patterns
of P limitation developed through fertilization experiments conducted in over 1000 sites,
including nearly 200 terrestrial ecosystems. Contrary to expectations that nitrogen supply
typically limits plant growth in temperate forests, grasslands, and the coastal ocean while
P limits freshwater ecosystems and tropical forests, Elser et al. (2007) found the mean
effect of P (added alone) to be substantial—similar to that of nitrogen added alone—across
terrestrial ecosystems. Most P in terrestrial ecosystems is derived from weathering of
parent material, and is affected by climate, topography, time, and biota (
Jenny 1941
).
Ecosystems are ultimately limited by the amount of P available in the local parent
material. Because of its role in productivity, P is particularly important to people as an
important fertilizer in agricultural ecosystems.
