Principles of terrestrial ecosystem ecology.pdf

218 9. Terrestrial Nutrient Cycling
Percentage distribution
100
50
0
Fixation by
hydrous
oxides of Fe,
Al, and Mg
Fixation
by soluble Fe,
Al, and Mn
Relatively available
phosphates
Silicate reactions
4.0 5.0 6.0
Soil pH
7.0 8.0
Al 3+ + H2PO4 - +
2H2O ´ 2H + + Al(OH)2H2PO4 (9.8)
soluble insoluble
Fixation mostly
as calcium
phosphate
Figure 9.11. Effect of pH on the major forms of
phosphorus present in soils. The low solubility of
phosphorus compounds at low and high pH result in
a relatively narrow window of phosphate availability
near pH 6.5. (Redrawn from Nature and Properties
of Soils, 13 th edition by N.C. Brady and R.R. Weil ©
2001, by permission of Pearson Education, Inc.,
Upper Saddle River, NJ; Brady and Weil 2001.)
Very little of the phosphorus in soils is
soluble at any time because many inorganic and
organic mechanisms retain phosphorus in insoluble
forms. Inorganic mechanisms are strongly
pH dependent. At low pH, phosphorus can be
sorbed onto the surfaces of clays and oxides of
iron and aluminum. Phosphate is initially electrostatically
attracted to positively charged sites
on minerals through anion exchange. Once
there, phosphate can become increasingly
tightly bound (and correspondingly unavailable
to plants) as it forms one or two covalent bonds
with the metals on the mineral surface. Phosphorus
can also bind with soluble minerals
(especially iron oxides) to form insoluble precipitates.
Chemical precipitates of phosphorus
with these oxides and phosphate sorption on
oxide surfaces, explain why highly weathered
tropical soils (Oxisols and Ultisols) have
extremely low phosphorus availability and why
the growth of forests on those soils is typically
phosphorus limited (see Chapter 3).The silicate
clay minerals that dominate temperate soils fix
phosphate to a lesser extent than do the oxides
of tropical oxisols.
In soils with high concentrations of
exchangeable calcium and calcium carbonate
(CaCO3), which typically occur at high pH,
calcium phosphate precipitates, reducing phosphate
availability in solution:
Ca(H2PO4)2 + 2Ca2+ Æ Ca3(PO4)2 + 4H + (9.9)
soluble insoluble
At high pH, phosphate combines with Ca to
form (in order of decreasing availability)
monocalcium, dicalcium, and tricalcium phosphates.
Precipitation of calcium phosphate is
one of the main reasons that phosphate
fertilizer immediately becomes unavailable in
calcium-rich temperate agricultural ecosystems.
Due to the precipitation reactions that
occur at high and low pH, phosphorus is most
available in a narrow range around pH 6.5 (Fig.
9.11). Organic compounds in the soil also regulate,
both directly and indirectly, phosphorus
binding and availability. Charged organic
compounds, for example, can compete with
phosphate ions for binding sites on the surfaces
of oxides, thereby decreasing phosphorus fixation.
Organic compounds can also chelate
metals and prevent their reaction with phosphate.
On the other hand, organic compounds
form complexes with iron, aluminum, and
phosphate that protect these compounds
from enzymatic attack. In tropical allophane
soils, these complexes form a major sink for
phosphorus.
Much of the phosphorus that precipitates as
iron, aluminum, and calcium compounds is
essentially unavailable to plants and is referred
to as occluded phosphorus. During soil development,
primary minerals gradually disappear
as a result of weathering and erosional loss.The
mass of phosphate in soils tends to shift from
mineral, organic, and nonoccluded forms to
occluded and organically bound forms, causing
a shift from nitrogen to phosphorus limitation
in ecosystems over long time scales (see Fig.
3.4) (Crews et al. 1995).
The tight binding of phosphate to organic
matter or to soil minerals in most soils causes
90% of the phosphorus loss to occur through
surface runoff and erosion of particulate phosphorus
rather than through leaching of soluble
phosphate to groundwater (Tiessen 1995). Two
thirds of the dissolved phosphorus that enters
groundwater is organic and therefore less reac-