Agriculture Reference
In-Depth Information
capacity of these organisms to translocate water and support growth into materials or
across sites where little water is present at potentials suitable for growth. The situation
is further complicated by additional interactions with temperature and nutrition. Griffin
(1981b) considers that water-retention energy alone is insufficient as a predictor of
microbial activity and that pore-size distribution‚ volumetric water content and other
soil factors are also of importance.
2.2.3.2
Faunal responses to water stress
Because of their eco-physiological adaptations to a soil-dwelling existence‚ the activities
of most soil animals are closely circumscribed by soil water status (Ghilarov‚ 1983).
With respect to soil water‚ common adaptations include the possession of permeable
integuments and a predominantly cutaneous respiration. Among the tracheate groups
living in the soil‚ spiracles are frequently absent; where present‚ their possessors are
often unable to regulate the rate of gas exchange through them.
Soil animals may be divided into two groups based on the ways they move through
soils (see Chapter III‚ section 4)‚ namely‚ those that are constrained to move through
existing interstices and those that can progress through the soil either by ingesting it or
transporting it out of their path. In terms of energy conservation‚ the reduced strength of
moist soils must be advantageous to members of the latter group. Four major macrofaunal
taxa belonging to this group are the earthworms‚ termites‚ ants and certain beetles.
Earthworms require high soil water potentials for activity and this is reflected in their
virtual exclusion from arid areas. In contrast‚ many species of termites and ants live in
the protected environment of the soil and members of the former group can at least
partially control the humidity of their nests by transporting water from deeper horizons.
For this reason‚ a range of species has been able to colonise dry tropical savanna
ecosystems and deserts successfully.
Despite the well-developed digging and burrowing capacities of a few groups‚ most
soil animals are constrained to move through existing soil pores and the size
distributions and inter-connectedness of these pores must constrain their mobilities.
Many small soil animals such as nematodes and protists are effectively aquatic and
also require water at high potentials for activity. For example‚ Wallace (1961) showed
that movement of the larvae of the nematode
Ditylenchus dipsaci
was optimal at poten-
tials between approximately -0.002 and -0.003 MPa‚ but also subject to satisfactory
pore sizes being available.
As soils dry‚ the larger mesofaunal species are affected by water availability only
below critical matric potentials specific to each species and beyond which they can no
longer maintain their internal water balance. For example‚ atracheate Isotomid Collembola
are affected at water potentials less than -1.5 MPa while the more desiccation-resistant
Cryptostigmata (Acari) are only affected below -9.8 MPa. A few Cryptostigmata may
sustain their internal water balances at water potentials down to -98 MPa‚ reflecting
the considerable variability present within some groups (Vannier‚ 1987).
