Agriculture Reference
In-Depth Information
during water absorption (Papendick and Campbell‚ 1981). Characteristic draining curves
also vary with temperature; at any given potential‚ less gravimetric water will be
present at higher temperatures than at low‚ irrespective of whether the soil is taking
up or losing water.
2.2.3
RESPONSE OF THE SOIL BIOTA TO WATER STRESS
A large part of the adaptive strategies of the soil biota is defined by the ability of these
organisms to live either in the water- or air-filled soil pores‚ and by their capacity to
survive desiccation. This is addressed further in Chapter III.
2.2.3.1
Microbial tolerance of water stress
Micro-organisms‚ as with higher plants‚ are highly sensitive to changes in water poten-
tial and considerable variation in water-stress tolerance exists between and within
groups. The movement of many propagules and of such unicellular organisms as
bacteria is dependent on the presence of free water or water held at very high potentials:
bacterial movement‚ for example‚ becomes negligible at matric potentials less than -0.01
MPa ( i.e .‚ when pores of a diameter greater than ca. 30
are not completely filled
with water).
As soils dry‚ the respiration of micro-organisms declines below certain ranges of
matric or osmotic potential‚ characteristic for the various groups. In wet but not saturated
soils‚ most organisms may be active although as soils progressively dry the bacteria
rapidly cease to be active. Soil bacterial respiration‚ for example‚ may be minimal at
water potentials less than -1.5 MPa. Soil fungi are much more tolerant of low matric
potentials than soil bacteria and may remain active to quite low potentials. Papendick and
Campbell (1981) state that some micro-organism activity may still be detectable where
soil air relative humidity exceeds 70 per cent.
The growth rates of fungi in vitro are clearly related to the water potential
although the limitations to growth below certain potentials may be as much due to their
inability to absorb nutrients as water (Jennings‚ 1995). Responses to matric and osmotic
potentials have different forms with growth declining nearly linearly with
but that
with showing an increase to an optimum followed by an approximately linear decline
(Griffin‚ 1981a).
Griffin (1981a) defined five overlapping groups of micro-organisms defined by the
osmotic potential ranges within which they grow. Overall‚ this range includes poorly-
tolerant species that fail to grow at less than -0.2 MPa to the highly xerotolerant species
that continue to grow at -40 MPa or less and which may fail to grow at high potentials.
Soil micro-organisms belong to several groups but are largely included in the range of
values from -0.1 MPa to those that grow little at osmotic potentials less than -15 MPa.
The reactions of micro-organisms over their active ranges of water potentials are
complicated by other factors than the matric and osmotic potentials. Antagonisms occur
between and within groups and both fungi and bacteria may grow at much lower
potentials in pure than in mixed cultures‚ or in the soil. In the natural environment‚
fungal growth may not be closely related to immediate water potentials due to the
Search WWH ::




Custom Search