152 T.P. McGonigle, M.H. Miller Applied Soil Ecology 14 2000 147–155
Soil biomass-C decreased in proportion with in- creases in content of pasteurized soil in the soil blends
Fig. 3, but it was not affected by soil disturbance. Considering each of the inoculum density treatments
separately, soil biomass-C levels were similar at the three-, four-, and five-leaf stages Fig. 3. For the data
as in Fig. 3 but pooled across soil-blend treatments and times, extrapolation gave a biomass carbon value
of 338 mg kg
− 1
for non-pasteurized soil: biomass car- bon mg kg
− 1
=338–2.2107 pasteurized soil , with r
2
= 0.48 and n=96.
4. Discussion
In contrast to the prediction of our hypothesis, the impact of soil disturbance on colonization was great-
est under the highest of the inoculum densities tested here. On the basis of our results we reject our hypoth-
esis, and conclude that high inoculum density does not prevent a difference in colonization from occur-
ring between U and D systems, at least for the soil studied here. The present study is in agreement with
the data of Yano et al. 1998, who also reported a stronger impact of soil disturbance under conditions
of higher inoculum density. Yano et al. 1998 inoc- ulated wheat Triticum aestivum L. plants with Gi-
gaspora margarita
Becker and Hall and grew them in a sub-soil in pots in the open for 6 months. Soils
were then either broken up by hand or left intact, and pigeon pea Cajanus cajun L. Huth. was grown
for 90 days. The root length colonized for plants in the uninoculated pots was not affected by soil distur-
bance, with mean values of 16 and 17 in the U- and D-treatments, respectively. Thus, there must have
been a low level of some indigenous AM fungi in
Table 1 Length densities in soil of roots colonized and not colonized by AM fungi for ecosystems studied in Western Australia WA and for the
soil from Ontario ON, Canada, as used here but prior to pasteurization Site
Land use Length density of root cm 100 g
− 1
dry soil Reference
Not colonized Colonized
Jarrahdale, WA Jarrah forest
61 26
Jasper et al., 1991 Eneabba, WA
Heathland 135
8 Jasper et al., 1991
Capel, WA Pasture
711 585
Jasper et al., 1991 Elora, ON
Field crops 59
121 Present study
the sub-soil at the time of wheat planting, or some fungi must have invaded during the experiment. For
the soils that were inoculated, colonization of plants in the U treatment was 56, which was significantly
higher than that of the plants in the D treatment at 41 Yano et al., 1998. Thus, high inoculum density
acted to facilitate, rather than obscure, the develop- ment of a difference in colonization in response to soil
disturbance.
The effect of soil disturbance on shoot P and growth in our experiment was successively greater as the pro-
portion of pasteurized soil was reduced. This pattern can be seen clearly in terms of the values correspond-
ing to 90, 60, 35, and 10 pasteurized soil for the ratios of UD of shoot-P content mg per plantmg
per plant: namely, 1.45, 1.83, 3.34, and 3.66, respec- tively. Although at a relatively low level, colonization
did develop in the treatment with 90 of pasteurized soil, and so an extraradical mycelium can be expected
to have been present in all treatments at the end of the first growth cycle. In previous work with this soil,
100 pasteurized soil produced zero arbuscules and no response of shoot-P to soil disturbance McGonigle
and Miller, 1996a.
With the low inoculum densities of the treatments with 90 and 65 pasteurized soil used here, coloniza-
tion was in most cases slight in both U and D soil. For the pasture studied by Jasper et al. 1991, in-
oculum density was sufficiently high that AM fungi extensively colonized roots in both U and D soils,
whereas for the heathland and forest it was lower, and reduced colonization of roots was seen in re-
sponse to disturbance. Compared to the pasture and the other ecosystems of Jasper et al. 1991, the soil used
here appears to have an intermediate inoculum density Table 1. The appearance or not, following distur-
T.P. McGonigle, M.H. Miller Applied Soil Ecology 14 2000 147–155 153
bance, of a difference in colonization of roots by AM fungi in the Elora soil with its intermediate inoculum
density probably depends on the interaction between the environmental conditions and the type of plants
and fungi involved.
It is plant-P status rather than soil-P level that is generally thought to have the greater regulating effect
on colonization Sanders, 1975; Menge et al., 1978. More specifically, there is evidence that the P status
of the root may be the controlling factor de Miranda et al., 1989; Braunberger et al., 1991. Although in-
creases in plant-P status have generally resulted in de- creases in colonization, there are reports of increases
in colonization with increased shoot P when the ini- tial level is very low Amijee et al., 1989; de Miranda
et al., 1989. Such an effect could have played a part in the study we report here. The shoot-P concentra-
tions for plants in the D soil here were consistently less than 1.0 mg g
− 1
, while those for plants in the U soil ranged from about 1.5 mg g
− 1
with the lowest inoculum density, to more than 2.0 mg g
− 1
with the highest. In this way, greater colonization in the U treat-
ment at the two higher inoculum densities could have been related to relief from severe P shortage. While
we cannot reject this explanation for the results of this study, it cannot explain the data of a related study
that used the same soil as here. Fairchild and Miller 1990 observed a consistently greater colonization of
roots by AM fungi in U compared to D soil, even though the shoot-P concentration in the plants in the
D treatment increased from about 1.5 mg g
− 1
to more than 4.0 mg g
− 1
in response to P fertilizer. Averaged across disturbance treatments, colonization decreased
continually through this range of increasing P fertility Fairchild and Miller, 1990.
The increase in shoot-N concentration in plants in the U treatment was small relative to those seen for
shoot-P, and it was seen only as a main effect of dis- turbance. Thus, it is likely that this response of N
was a secondary effect caused by improved plant-P nutrition.
Although any given isolate of AM fungus can be ex- pected to colonize any AM plant that it is challenged
with in culture Mosse et al., 1975, a picture has re- cently emerged of a degree of specificity in AM asso-
ciations. Various combinations between a selection of AM fungi and rice varieties generated a wide range of
extents of colonization of roots Dhillion, 1992. From among the pool of AM fungal species available in soil,
certain combinations of fungus and plant were seen to develop more extensively than others in the mycor-
rhizae of a grassland McGonigle and Fitter, 1990 and a forest Merryweather and Fitter, 1998b. Moreover,
species of AM fungi differ in the ways in which they distribute their mycelia, both between the inside and
outside of the roots Abbott et al., 1992, and for the extraradical mycelium, with increasing distance from
the rhizoplane Jakobsen et al., 1992. In turn, different pairings of type of AM fungus and plant species can
lead to widely different plant growth responses van der Heijden et al., 1998. There is a good chance that
different pairings of plant and AM fungus have oc- curred among the various treatments and experiments
on soil disturbance conducted through the 1980s and 1990s in our laboratory and others. In addition, dif-
ferences in types of colonist AM fungi may have con- tributed to the variability seen in effects of soil dis-
turbance on extent of colonization. Certainly, differ- ent AM fungi simultaneously colonizing the same root
systems can respond in different ways to soil distur- bance Merryweather and Fitter, 1998a. However, as
yet the role of the selection of different plant-fungus pairings in generating the variability seen in this ef-
fect is not well understood. Indeed, in one experiment Addy et al., 1994 the use of particular AM fungal
inocula as the sole source of colonization did not elim- inate variability in the occurrence from one trial to the
next of a difference in colonization between U- and D-treatments.
Values for biomass-C in the soil blend with the lowest proportion of pasteurized soil was typical com-
pared to those for agricultural soils Martens, 1995. Biomass-C was reduced in proportion to the amount
of pasteurized soil, but it did not respond to soil dis- turbance, and it did not increase in association with a
70 increase of root-length density from the three- to the five-leaf stage in the second growth cycle. Even
though AM hyphae and spores develop extensively in this soil even when pasteurized McGonigle and
Miller, 1999 and densities of hyphae in this soil can respond to disturbance McGonigle and Miller,
1996a, biomass-C was unaffected by anything other than the initial pasteurization. It therefore appears
that soil biomass-C reflects microbial populations in the bulk soil and not those associated with rhizo-
sphere processes. Biomass-C measurement using the
154 T.P. McGonigle, M.H. Miller Applied Soil Ecology 14 2000 147–155
fumigation-extraction method is not recommended for future studies on extraradical hyphae related to
rhizosphere processes.
5. Conclusion
