Wetland Biogeochemistry Techniques - Wetland Techniques - page 391

phosphate from organic compounds found in the water may be released but this can
be reduced by selecting the right acid strength, the right boiling temperature and
the hydrolysis time. Oxidative digestion converts both organic and inorganic forms
of P to OP. Organic P is calculated as the difference between total P and inorganic
P. Analytical details for this scheme are presented in Eaton et al. ( 2005 ).

1. Dissolved acid hydrolyzable P (DAHP): OP determined on filtered samples after
acid hydrolysis.
2. Total acid hydrolyzable P (TAHP): OP determined on unfiltered samples after
acid hydrolysis.
3. Total dissolved P (TDP): OP determined on filtered samples after oxidative
digestion.
4. Total P (TP): OP determined on unfiltered samples after oxidative digestion.
5. Dissolved organic P (DOP): DOP ¼ TDP DAHP.

7.6.7 Biologically Available Phosphorous

7.6.7.1 Overview

The term “biologically available P” (BAP) is used in a number of disciplines
without a standardized meaning. However, in part due to the recognized importance
of P in eutrophication of surface waters, BAP has been operationally defined as
“the amount of inorganic P a P-deficient algal population can utilize over a period
of 24 h or longer” (Sonzogni et al. 1982 ). The amount of BAP in soil, sediment, and
water has been routinely quantified by algal assays or chemical extractions (see
Sharpley ( 2009 ) for a review of methods). Algal assays require long incubation
periods and chemical extraction times. Results from the chemical extraction
approach are impacted by the nature of the extractant. Weaker chemical extractants
(e.g., NH4F and NaOH) approximate P that is bioavailable under aerobic
conditions, whereas stronger extractants (citrate-dithionite-bicarbonate) represent
P that may become bioavailable under reducing conditions (Sharpley 2009 ).
Because of these limitations, there has been increased interest in a different
approach-one that utilizes P sinks.
The P-sink approach to assessing BAP is based on a chemical sink that attracts
only P in forms that would be available to macrophytes and algae. In theory because
the P sinks continuously remove dissolved P from the soil solution (Kuo 1996 )
which creates a concentration gradient, they simulate P removal from soil or water
by plant roots and algae. This is not a completely accurate analogy because a
chemical sink cannot approximate the rhizosphere influence as root exudates and
mycorrhizal fungi can alter P availability. Two of the more common P-sink
methods utilize anion exchange resins and Fe oxide impregnated filter paper.
These are addressed below.