Agriculture Reference
In-Depth Information
where
k
is a rate constant and
C
eq
an apparent equilibrium P concentration.
Va l u e s o f
k
=
10
−
6
s
−
1
and
C
eq
=
0
.
5mm were used for the calculations in
Figures 2.13 and 2.14. The values of the other parameters used were
θ
L
=
0
.
6,
f
L
=
0
.
4,
b
=
100,
B
=
0,
r
1
=
0
.
5mm,
r
2
=
1
/
√
πN
, where the values of
N
are given in the figures and
L
=
3 cm. It will be seen that the tubificids have a
large effect and the flux of P to the floodwater increased several fold for realistic
numbers and dimensions. For comparison, the fluxes of P from the soil required
to sustain typical rates of primary production in the floodwater in ricefields are
in the range 0
.
05-0
.
25 mmol m
−
2
day
−
1
, calculated from measured primary pro-
duction and the P contents of likely photosynthetic organisms given by Roger
(1996). Because the tubificids depend upon the photosynthetic organisms for their
carbon, there will be a positive feedback between mixing by tubificids and net
primary production in the floodwater.
Note that the sensitivity of the net flux between the soil and water to the
worms' activities depends on the relation between the rate
R
and the solute con-
centration. For the calculations in Figures 2.13 and 2.14,
R
varies linearly with
concentration as specified in Equation (2.40), and the flux is sensitive to worm
activity. But where the rate is independent of concentration, as for NH
4
+
forma-
tion in Equation (2.39), the net flux, which in this case is roughly
R
0
/α
+
LR
1
,
is necessarily independent of worm activity, though the distribution of the flux
between burrows and the sediment surface and the concentration profile are not.
In practice the rate will always depend to some extent on concentration. But the
predictions here for the idealized steady state indicate the expected sensitivities.
