Environmental Engineering Reference
In-Depth Information
ferric Fe and aluminum oxides and hydroxides. As these soils become first anaerobic
and then reduced, dissolution of the precipitates releases P into the soil pore water
and water column. In alkaline soils (pH
7), precipitation of insoluble Ca or Mg
phosphates or OP adsorption to carbonates occurs (Richardson and Craft
1993
;
Vepraskas and Faulkner
2001
). The soluble inorganic OP is considered the biologi-
cally available form of P. Organic forms of P are found in plants, partially
decomposed plant tissue, and found as OP bound in organic matter.
Phosphorous has a complex role in wetland ecology. Because of its dual role as both
a limiting nutrient and as a major pollutant, especially with its role in eutrophication of
surface waters, a number of approaches have been developed to assess different forms
of P or different P pools in wetlands. Most of the assays are specific to soil or water.
Sequestration is addressed by assays for soil P content. To address P as a limiting
nutrient, two approaches are commonly used-a chemical extraction scheme that
separates soil P into various fractions representing a range in bioavailability and a
mineralization assay. A similar fractionation approach is used to evaluate water quality.
As the major eutrophic surface water nutrient, both soil and water assays have been
developed that take a bioavailable P approach and target OP. We will not go into detail
on the wet chemistry techniques for assessing P. These methods are presented in detail
by Kuo (
1996
) for soils and Eaton et al. (
2005
) for water. Kovar and Pierzynski (
2009
)
ent methods for soil and water including methods specific to runoff water and
flooded soils.
>
7.6.2 Soil Phosphorous Content
7.6.2.1 Total P
Many methods have been developed to extract and analyze total P in soil; four
of the more common include: sodium carbonate (Na
2
CO
3
) fusion, perchloric
acid (HClO
4
) digestion, sulfuric acid-hydrogen peroxide-hydrofluoric acid
(H
2
SO
4
-H
2
O
2
-HF) digestion, and sodium hypobromite (NaOBr) oxidation
followed by H
2
SO
4
dissolution. All four methods convert soil organic P to inor-
ganic P (Kuo
1996
), but the Na2CO3 fusion method is considered to be the most
reliable (Syers et al.
1967
). It is believed that the acid digestion methods fail to
extract P from apatite inclusions or imbedded in the matrix of silicate minerals
(Syers et al.
1967
) and therefore underestimate total P. Three of the methods
(Na
2
CO
3
,H
2
SO
4
-H
2
O
2
-HF, and HClO
4
) are described by Kuo (
1996
).
7.6.2.2 Total Inorganic P
Inorganic P is extracted with 1 M hydrochloric acid (HCl) or H
2
SO
4
. Not all of the
mineral forms of P may be solubilized with this extraction leading to an
underestimation of inorganic P.
