Agriculture Reference
In-Depth Information
3.3.2 BIOLOGICAL TRANSLOCATION: BIOTURBATION
The translocations considered above result from the expenditure of physical energy (grav-
ity, capillarity, osmosis) within the soil. Movements of soil materials also result from
the expenditure of biological (photosynthetic) energy by roots and soil animals, largely
invertebrates but sometimes mediated by the burrowing and other soil-disturbing effects
of vertebrates (see authors in Meadows and Meadows, 1991; Butler, 1995). Such effects
may be direct (effective transportation) or indirect, for example, the translocation of soil
materials through galleries and other channels (biopores) formed by soil invertebrates.
Faunal pedoturbation is most apparent when it involves the deposition of soil materials
at the surface. However, much faunal pedoturbation is less apparent since it involves the
vertical and horizontal movement of ingested and transported materials within the soil.
The materials displaced may simply be deposited behind the animal as it moves through
the soil, as with certain earthworms or packed into pre-existing voids, as occurs with
certain termites, ants and some burrowing vertebrates.
Because they are unable to move through existing cracks and crevices in the soil, the
larger invertebrates and vertebrates must rearrange the soil to accommodate their general
life patterns and movements. Johnson et al. (1987) recorded the contrasting effects of
two burrowing rodents in California. During burrow construction, the Ground Squirrel
Ostospermophylus beecheyi causes considerable profile homogenisation extending to
the C horizon. However, it deposits much of the soil it excavates into abandoned burrows.
In contrast, the Pocket Gopher Thomomys bottae deposits most of the materials exca-
vated during burrow construction on the surface and therefore acts to promote profile
differentiation. Soil materials deposited at the surface become available for erosional
detachment and transport and may contribute substantially to hillslope processes.
Some of the larger invertebrates simply eat their way through the soil, thereby trans-
porting soil upwards or downwards, depending on the net direction of their movements.
Most selectively ingest the finer soil particles which are thereby concentrated in
their faeces. The main agents of such biological translocations are the earthworms and
certain termites. Both termites and ants also transport fine particles of an upper size limit
related to that of their mouthparts. Members of these three groups frequently effect a net
transport of finer particles to the surface during the excavation and construction of nests,
burrows and other structures.
Humphreys (1994) quantitatively described a number of micro-morphological features
of a southeastern Australian ultisol heavily faunally-turbated by ants, termites and other
animals (Figure II.8). It is clear that most activity occurs in the coarse-textured A and E
horizons with about 8 % of the soil volume being represented by faunal constructs.
However, the surface of the B horizon was being mined for clay and biologically-formed
structures (pedotubules) were distributed throughout the C horizon, although in diminished
amounts within the lower Cr horizon. Of particular note is the large proportion (80-98 %)
of the A and E horizons comprising macular colour since this represents the remnants of
reworked pedotubules. Faunal deposition of soil materials on the surface exposes them
to rainwash with the finer particles being transported away in runoff waters leaving the
coarser particles to form the surface soil horizons. It is clear that the A and E horizons of
this soil owe much of their structure to faunal activity and therefore comprise a biomantle.
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