FUNCTIONING OF THE SOIL SYSTEM - Soil Ecology - page 453

water and transport it to the roots (Read et al ., 1979; Boyd et al ., 1985; Mousain, 1983).
Duddridge et al. (1980) have shown that mycelial rhizomorphs can absorb water up to
20 cm away from the root surface and transport it to the root at a rate equivalent to that
of xylem vessels ( i.e., Mycorrhizal plants have a greater resistance to
water stress (Bowen and Theodorou, 1973; Dixon et al., 1980) due to the higher
tolerance of low water potentials shown by certain fungi and through a possibly
diminished resistance during water flow to the roots. For example, the resistance to water
flow into soybean ( Glycine soja ) plants infected with arbuscular mycorrhizal fungi is
50 % less than that of uninfected plants (Safir et al ., 1972). Finally, changes in stem
histology following mycorrhizal colonisation may increase the resistance of trees to
drought or water-logging (Mason et al., 1977).
Phosphorus uptake. Mycorrhizae have a remarkable ability to take up phosphorus
from soils within which this element is poorly available. In experimental studies, the
zone of phosphate uptake around the roots of Trifolium repens (White Clover) colonised
by the arbuscular mycorrhizal fungus Glomus mossae was estimated to extend to 11.5
cm from the rhizoplane compared with 1 cm in uncolonised plants (Li et al., 1991).
The increased soil volume explored was reflected in the greater mass of the colonised
plants at harvest and their lower ratios of root- to shoot mass. These authors calculated
that in colonised plants, phosphorus uptake was increased by more than four times
and that 76 to 79 % of this uptake occurred through the fungal hyphae. Similarly, in
ectomycorrhizal Pinus pinaster plants, Mousain and Lamond (1978) found that the phos-
phorus content was 2 to 3.5 times greater than in non-colonised plants. Plant phosphorus
uptake rates differ greatly, depending on the type of mycorrhiza, the species and strain
of the fungal partner and soil and environmental factors (Marschner and Dell, 1994).
In plants colonised by arbuscular mycorrhizal fungi, this improved phosphorus uptake
appears to result from a more complete exploitation of soluble phosphorus sources
and, to a much lesser extent, from the exploitation of less soluble phosphorus sources.
The increased volume of soil explored and longer life spans of mycorrhizal as compared
with non-infected roots, result in a more complete exploitation of slowly diffusing
phosphorus. The phosphatases of mycorrhizal fungi may play an important role in
the release of phosphorus fixed in organic forms allowing its transport and absorption by
the plant root. However, the quantitative significance of this requires more attention
(Haselwandter and Bowen, 1996).
Nitrogen uptake. Mycorrhizal formation generally improves nitrogen uptake in plants
although high soil nitrogen concentrations can depress their formation. Differences exist
between the major mycorrhizal types in the forms of nitrogen that they may access.
Both arbuscular and ectomycorrhizal fungi appear to take up inorganic N as
although at least some ectomycorrhizal species may also utilise both and organic
(amino acids) forms of N and transfer them to the plant (Marschner and Dell, 1994;
Smith and Read, 1997). Ericoid mycorrhizal fungi have well developed capacities to
utilise organic nitrogen sources (Smith and Read, 1997).
As eukaryotes, mycorrhizal fungi do not fix nitrogen although free-living, nitrogen-
fixing micro-organisms ( e.g., Beijerinckia or Clostridium ) may be active in the mycor-