Environmental Engineering Reference
In-Depth Information
Table 8.4
Mathematical expressions for soil phosphorus transformations
P pool
P transformation equation
Fresh organic P (orgP
frs
)
d
ð
Dz
orgP
frs
Þ
dt
¼
OP
min
j
imb
OP
dec
OP
frs
;
e
þ
OP
frs
;
s
Active organic P (orgP
act
)
d
ð
Dz
orgP
act
Þ
dt
¼
OP
dec
OP
min
OP
trn
OP
act
;
e
þ
OP
act
;
s
Stable organic P (orgP
sta
)
d
ð
Dz
orgP
sta
Þ
dt
¼
OP
dec
þ
OP
trn
OP
sta
;
e
þ
OP
sta
;
s
Labile (soluble) P (P
sol
)
d
ð
Dz
P
sol
Þ
dt
¼
IP
min
IP
sol
j
act
IP
up
R
DIP
;
e
þ
IP
s
Active inorganic P (minP
act
)
d
ð
Dz
minP
act
Þ
dt
¼
IP
sol
j
act
IP
act
j
sta
IP
act
;
e
þ
IP
act
;
s
Stable inorganic P (minP
sta
)
d
ð
Dz
minP
sta
Þ
dt
¼
IP
act
j
sta
IP
sta
;
e
þ
IP
sta
;
s
A schematic representation of the watershed phosphorus transport and transfor-
mation processes involved in the phosphorus cycle is given in Fig.
8.2
b.
8.3.2.1 Phosphorus Transformations in Soils
, particulate
phosphorus, organic phosphorus, or in phosphorus minerals. Many reactions and
mechanisms regulate and control the composition and forms of phosphorus present
in the soil. Phosphorus is generally much less mobile than nitrogen, being strongly
adsorbed to soil particles as well as organic matter. Phosphorus transformations in
the soil include decomposition and mineralization of organic phosphorus, immo-
bilization of labile phosphorus, and sorption of labile phosphorus to/from soil
particles, and plant uptake. Soil phosphorus cycling is simulated by NSM for the
six pool state variables for each of the soil layers. The mass balance equations used
to describe the phosphorus cycle in soils are summarized in Table
8.4
.
HPO
4
;
or, H
2
PO
4
Phosphorus can exist in the soil as phosphate
8.3.2.2 Phosphorus Transformations in Surface Runoff
The same process occurs within the aquatic ecosystem as for that in soils. Phos-
phorus is not highly soluble, binding tightly to soil particles. Therefore it mostly
reaches waters by traveling with runoff soil particles. Phosphorus enters surface
water primarily as particulate matter and secondarily as dissolved inorganic
phosphorus (phosphate and its conjugate base forms). In NSM, dominant trans-
formation processes are simulated for Particulate Organic Phosphorus (POP),
Dissolved Organic Phosphorus (DOP), Particulate Inorganic Phosphorus (PIP),
and Dissolved Inorganic Phosphorus (DIP). Transformation processes in surface
runoff include mineralization of DOP to DIP, plant uptake of DIP, soil mass
transfer of DIP and DOP, adsorption/desorption of DIP onto suspended sediments,
and hydrolysis of POP to DOP. The mass balance equations used to simulate the
phosphorus cycle in surface runoff are summarized in Table
8.5
.
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