Environmental Engineering Reference
In-Depth Information
7.6.5 Phosphorous Mineralization
Chemical extraction procedures produce an estimate of available P at one point in
time. P mineralization studies produce estimates of available P over time. Phos-
phorous mineralization rates can be assessed either
in situ
with litter bags (see
Litter
Bags
above) or in the lab using an incubation approach. In the incubation approach,
soil samples are kept under controlled climate conditions in the lab and leached at
set time intervals. The two mineralization approaches would not be expected to
produce the same results. The benefit to the litter bag approach is that mineraliza-
tion is assessed under natural fluctuating conditions of moisture and temperature.
It does not totally reflect real life conditions as the substrate is usually fresh plant
material and the material is physically separated from the soil. Incubation studies
typically use soil samples, a more realistic substrate, but employ optimum temper-
ature and moisture conditions. In addition, soil structure is disturbed as the soil
samples are typically mixed with sand to facilitate drainage during leaching. The
benefit of the incubation approach is that it allows the comparison of different soils
under identical environmental conditions so that inherent differences in minerali-
zation potential can be determined.
Bridgham et al. (
1998
) utilized the following incubation procedure to estimate
P mineralization potential for a series of wetland soils. One advantage to their
approach is that they could estimate mineralization potential under both aerobic
and anaerobic conditions. This allowed them to determine maximum and mini-
mum mineralization rates for the given temperature. Field moistsoilwasmixed
with acid washed sand to promote drainage during leaching. Samples were placed
in 150 ml Falcon filter units and incubated at 30
C. For aerobic incubations, the
samples were exposed to ambient air. For anaerobic incubations, the duplicate
sampleswereplacedinfilterunitswhichinturnwereplacedin500mlMasonjars
filled with water. Samples were leached with 0.001 M CaCl
2
at ten dates ranging
from 2 to 59 weeks. Both the leachate and the Mason jar incubation water were
analyzed for PO
4
3
.
7.6.6 Phosphorous in Water
There are many tests for P in water, most use a chemical or physical fractionation
scheme as presented below. The P fraction analyzed in each step is based on whether
the sample is digested and/or filtered, and the nature of the digestion. In each step, P
concentration is determined by the ascorbic acid method. Particulate P is separated
from the dissolved (or soluble) fraction by passing the water sample through a filter,
typically a 0.45
m cellulose (Millipore) filter. In each step, the resulting extract is
assayed for OP by the ascorbic acid method. Phosphorous measured on undigested
samples is considered to be inorganic, predominantly OP. Acid hydrolysis digestion
converts inorganic P (primarily condensed phosphates) to OP. Some of the
μ
