Wetland Biogeochemistry Techniques - Wetland Techniques - page 388

7.6.2.3 Total Organic P
Three methods are commonly used to determine total organic P in soils (Reddy
and DeLaune 2008 ) as follows. The methods for quantifying P in extracts are found
in Kuo ( 1996 ).
1. Difference method: Organic P is calculated as the difference between total P and
inorganic P. This approach is best suited for soils high in organic matter; it tends
to overestimate organic P in samples high in mineral forms of P.
2. Acid-alkaline extraction method: First P i is extracted with acid (1 M HCl or
H 2 SO 4 ); this is followed by an extraction with alkali (0.5 M NaOH). The alkali
extracts are digested to solubilize organic P. This method may underestimate
organic P as the extraction procedure may not be sufficient to solubilize all forms
of organic P.
3. Ignition method: Organic P is removed by ashing the sample at 550 C.
Inorganic P in the residue is extracted with acid. Organic P is calculated as the
difference between total P in the unignited soil and inorganic P in the residue.
7.6.3 Soil Phosphorous Fractionation Schemes
to Assess Availability
Orthophosphorus is considered the most bioavailable form of P and it constitutes
the bulk of P found in soil pore water. Orthophosphate is readily adsorbed onto
surfaces of clay particles and organic matter, and OP can also substitute for silicate
within clays (Mitsch and Gosselink 2000 ). Adsorption of OP onto wetland soils and
organic matter is considered long-term P retention. Phosphorus availability is
characterized by the concentration of P in the soil solution and by the P buffering
capacity that governs the distribution of P between the solution and solid phases.
Some of the adsorbed P can be released (desorbed) into soil solution where it is
readily bioavailable. Therefore, it is useful to separate exchangeable P, which is
potentially bioavailable, from the non-exchangeable P. Because of the complex
processes that govern the distribution of P between the solution and the solid phase
within wetland soils, P tests have been developed in order to evaluate P availability.
A sequential chemical extraction scheme is commonly used to determine soil
P pools in soil to approximate bioavailabilty. This approach is based on differential
solubilities in a series of chemical extractants. A number of extraction schemes
have been developed for soil P. We offer a simplified scheme below that was
presented by Reddy and DeLaune ( 2008 ). Most are more complex than the example
presented here but they allow for further fractionation. For example, some schemes
utilize a filter to separate soluble P from particulate forms of P. Also, extraction
schemes have been developed to target soils with specific characteristics (e.g.,
calcareous soils or organic soils). In each step the resulting extract is assayed
for OP. Details on these analytical procedures are presented by Kuo ( 1996 ).