Agriculture Reference
In-Depth Information
dense, this may have a large effect on solute transfer between the soil and overly-
ing water. Oligochaete worms are often present in submerged soils in populations
exceeding several thousand per m
2
with burrows extending to several centimetres
(Chapter 5). Once the burrows are constructed, the worms remain in them feeding
with their heads downward and their posterior ends upward in the overlying water.
By waving their posteriors and moving their bodies in a peristaltic motion they
cause the water in the burrows to be mixed with the overlying water. Solid particles
also fall into the burrows and are mixed. Hence solutes diffusing into a burrow
will be rapidly transferred to the surface, and vice versa. The ecology of tubificids
and other organisms in the soil and floodwater are discussed in Chapter 5. I here
discuss approaches to modelling their effects on solute transfer between the soil
and floodwater.
Three approaches have been taken to the analogous problem of mixing by
invertebrates in marine sediments (Aller, 1980a; Berner, 1980). The simplest
approach has been to lump together all the processes involved and to assume
that mixing is random and complete to a specified depth. This has been applied
successfully to the long-term mixing of sediments under the combined action
of invertebrates and waves or currents, but is inappropriate for less perturbed
systems and short times. A second approach has been to express the effect of
burrowing as increased effective diffusion coefficients of solutes in the pore water,
derived by fitting diffusion equations to empirical data (Aller, 1980a; Berner,
1980; van Rees
et al
., 1996). But the physical basis of this approach is doubtful.
A third approach was developed by Aller (1980a, b) who studied solute fluxes
in near-shore marine sediments showing seasonal variation. In this approach, the
geometry of the burrow-sediment system is allowed for explicitly and trans-
port in the sediment between the burrows is described with appropriate diffusion
equations. It is assumed that the burrows are oriented normal to the sediment sur-
face and distributed uniformly or randomly in the horizontal plane (Figure 2.11).
Thereby a cylindrical zone of influence is ascribed to each burrow with a radius
burrow
(radius
=
r
1
)
zone of influence
(radius
=
r
2
=
1/
√π
N)
Figure 2.11
Distribution of worm burrows and cylinders of influence represented by
boundary conditions for Equation (2.37)
