Agriculture Reference
In-Depth Information
The global contribution of earthworm burrows to mass transfer does not describe the way that
they interact with the soil structure. The effectiveness of short circuits caused by earthworm burrows
is shown poorly by increases in the gas diffusion coefficient. This effectiveness is demonstrated
clearly when the evolution of tracer gas concentrations with time is compared with the theory of
gas diffusion following the non-steady-state equation (= second FickÔs law). In homogeneous soil
cores, the data fit perfectly with the theoretical diffusion calculations. When macropores were
present in the cores, the ÑdistanceÒ between the observed data and theory showed the effects of
macropores. Gaseous diffusion in the presence of macropores no longer followed the Fickian
diffusion process (Kretzschmar 1997). Modeling and simulations of specific gaseous diffusion
because of macropores associated with basic diffusion caused by the matrix are not yet available.
I
E
B
O
S
B
NTERACTIONS
BETWEEN
ARTHWORM
URROWS
AND
THER
OIL
IOLOGICAL
C
OMPONENTS
The interactions between roots and macropores, especially earthworm burrows, are easy to observe
in a soil profile; nevertheless, it is not so easy when an attempt is made to follow these interactions
in experimental conditions (Hirth et al. 1998). Observations of the soil profile have led to overes-
timates of the presence of roots in earthworm burrows because, although roots in burrows are easy
to see, almost the whole root system is concealed in the soil matrix. Statistical estimations of spatial
interactions between roots and fine cracks have been attempted on polished blocks of soil (i.e., in
two dimensions) (Krebs et al. 1994). There is a large variability in these estimations within the
same soil horizon and for the same plant species; moreover, it has been shown neatly that the
interaction between roots and fine cracks is specific for each plant species, even when they are
mixed, as in a pasture
Unpublished data from work by Brown and Kretzschmar (2001) showed at
least that the global effects of earthworms seem to randomize root densities and then to favor the
efficiency of soil exploration by roots
.
A three-dimensional estimation of the probability of crossing
between earthworm burrow systems and root systems is not available. Earthworm burrows have
been said to pave the way for plant roots; however, it is more realistic to expect that roots have a
scouting function in exploring soil depth, and that earthworms follow the way and stabilize and
develop the root-originated structures. The actual soil organization observed in an aged soil structure
depicts an equilibrium reached through the joint development of roots and earthworms, each
interacting with the other in response to soil conditions.
Interactions between earthworm burrows and microbial activity have also been described either
by measuring the microbial biomass in the vicinity of burrow walls (Loquet et al. 1977) or by
showing the CO
.
release associated with the presence of earthworm burrows. The chemical and
organic properties of burrow walls are responsible for the development of microbial activities,
together with the interactions of burrows with mass transfer, between the soil surface and soil
matrix, through earthworm burrow systems. Water, solutes, and gas contents or concentration
gradients are higher in the macropores than in the soil matrix, and the dynamics of microbial
activities are closely related to the dynamics of these gradients.
Observed effects of earthworms on CO
2
release are dependent on the interactions of macropores
with the surrounding porosity (Kretzschmar and Ladd 1993). CO
2
release results from the combined
effects of macropores on gaseous diffusion because of their connections to the matrix pore and to
the soil surface and the enrichment of organic material along the earthworm burrow walls. The
microbial population distribution interacts with that of the burrow system by way of both active
and passive transport. Microorganisms are washed down and along the burrow walls when water
flows intensively in vertical burrows; they are also transported into the guts of earthworms and
deposited at the place where the earthworms cast.
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