Applied Soil Ecology 14 2000 147–155
The inconsistent effect of soil disturbance on colonization of roots by arbuscular mycorrhizal fungi: a test of the inoculum density hypothesis
Terence P. McGonigle
∗
, Murray H. Miller
Department of Land Resource Science, University of Guelph, Guelph, Canada N1G 2W1 Received 28 April 1999; accepted 30 December 1999
Abstract
Reducing the tillage of agricultural soils can increase early-season crop-P uptake. Consistent increases in plant-P have been found in both field- and laboratory-systems with undisturbed U compared to disturbed D soil. A concomitant stimulatory
effect on colonization of roots in U soil by arbuscular mycorrhizal AM fungi has been found in some cases, but in others the colonization has been similar in U and D treatments. Disruption of the extraradical mycelium that remains from the previous
crop is the mechanism by which soil disturbance restricts mycorrhizally mediated P uptake for the subsequent crop, with a tandem change in colonization not necessary, but sometimes seen. Nonetheless, a complete account of these processes will
need an understanding of the conditions under which the extent of colonization is affected. Soil-P does not explain when a difference in colonization will appear. Among ecosystems in Western Australia, high inoculum density in a pasture was
reported previously to preclude the appearance of a difference in colonization in response to soil disturbance, whereas for other ecosystems with lower inoculum densities a difference in colonization was seen. Here, we determined if a similar mechanism
operates for an agricultural soil collected mid-season during the growth of a maize Zea mays L. crop in Ontario, Canada. Blending various proportions of pasteurized and non-pasteurized soil gave a range of inoculum densities. Maize was taken
through two 3-week growth cycles in pots, and for the D treatment the soil was passed through a 5 mm sieve between cycles. All plants became colonized with AM fungi. Reducing the inoculum density served to limit colonization to similar low levels
in both U and D soils. Stimulation of colonization and of shoot-P uptake in the U-compared with the D-treatment was greater for plants under the higher inoculum conditions tested. We conclude that the inoculum density during crop growth of the soil
studied here is moderate, and that this density makes it possible, if other conditions are met, for a reduction of colonization of roots in response to soil disturbance. Whether or not a difference in colonization will appear following disturbance of a soil
such as the one studied here probably depends on the interaction between the environment and the plant. Possible interactions are discussed. The high inoculum density of ecosystems such as the pasture studied in Australia likely overrides any effect of
soil disturbance and ensures roots of all plants become well-colonized by AM fungi. © 2000 Elsevier Science B.V. All rights reserved.
Keywords: Biomass carbon; Maize; Mycorrhiza; Pasteurization; Tillage; Undisturbed soil
∗
Corresponding author. Present address: Faculty of Agriculture, Gifu University, Gifu 501-1193, Japan. Tel.: +81-58-293-2842;
fax: 81-58-293-2842. E-mail address:
tmcgonigcc.gifu-u.ac.jp T.P. McGonigle
1. Introduction
Reduced tillage has recently been adopted exten- sively for field crops in North America in order to
conserve soil water and reduce soil erosion Tisdale et al., 1993. Reduced tillage causes cooler conditions
0929-139300 – see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 9 - 1 3 9 3 0 0 0 0 0 4 8 - 2
148 T.P. McGonigle, M.H. Miller Applied Soil Ecology 14 2000 147–155
for seedlings because of increased residue cover, and it changes soil structure and soil organic matter Blevins
et al., 1985. Tillage was shown in the 1980s to reduce colo-
nization of roots by arbuscular mycorrhizal AM fungi Mulligan et al., 1985 and early-season P up-
take O‘Halloran et al., 1986 by crops. Subsequent growth-chamber studies involving g-irradiation of
soil O‘Halloran et al., 1986, non-mycorrhizal crops Evans and Miller, 1988, a fungicide Evans and
Miller, 1988, and soil pasteurization McGonigle and Miller, 1996a established that it is reduced ef-
fectiveness of the AM symbiosis in disturbed D, as compared to undisturbed U soil, that leads to
reduced P uptake.
Greater P uptake from U than D soil has been consistent in four types of experimental systems.
First, tillage experiments in the field McGonigle and Miller, 1993, 1996b; Gavito and Miller, 1998. Sec-
ond, growth-chamber studies with U cores collected from the field Evans and Miller, 1988; Miller et al.,
1995. Third, studies in pots using initially D soil that was taken through various growth cycles Fairchild
and Miller, 1988, 1990. Last, experiments with hy- phae developed in root-free soils within nylon mesh,
which were then disturbed or not Evans and Miller, 1990; Addy et al., 1994, 1997; McGonigle and Miller,
1999.
Soil disturbance affects plant P by breaking up the extraradical mycelium of the AM fungi that are asso-
ciated with the roots of the previous crop Jasper et al., 1989; Evans and Miller, 1990. Available data are
consistent with the following interpretation. In U soil, the extraradical mycelium provides effective P acqui-
sition as soon as roots become connected, but if this mycelium is disrupted a new one must then be de-
veloped from colonized roots, which themselves must first be initiated by spores, or by fragments of either
hyphae or colonized roots.
Colonization of roots by AM fungi is sometimes reduced for plants in the D soil, while in other exper-
iments the U- and D-treatments have similar and rela- tively high colonization. Examples are as follows. At
a single field site, colonization was reduced for maize in conventionally tilled plots compared to no-till plots
in one study McGonigle and Miller, 1996b but not in another McGonigle et al., 1990a. Soil-P levels
could not explain this difference in outcome McGo- nigle and Miller, 1996b. U cores collected from one
established sand dune had reduced colonization fol- lowing disturbance, but no difference was found for
another site that was both close and similar Koske and Gemma, 1997. With D soil initially, disturbance of
soil between growth cycles in pots reduced coloniza- tion in some trials Fairchild and Miller, 1988, 1990
but not others McGonigle et al., 1990a; Miller and McGonigle, 1992. Again, soil-P was not the expla-
nation Fairchild and Miller, 1990. In another series of experiments, mesh pouches were used to produce
root-free zones with hyphae either in situ in the field McGonigle and Miller, 1999, or in growth rooms
followed by burial in the field Addy et al., 1994, 1997. Pouches were removed from the field at several
times, and the colonization of bioassay plants grown in the pouch soil in the growth chamber was deter-
mined. In one study Addy et al., 1994, disturbance reduced colonization in a bioassay conducted prior to
transferring pouches into the field in November, but not in those conducted on pouches collected from the
field the following spring. In a subsequent experiment Addy et al., 1997, disturbance reduced colonization
of bioassay plants for pouches collected at all times from November of 1 year to May of the next. In a
third experiment, McGonigle and Miller, 1999, dis- turbance did not reduce colonization of bioassay plants
in pouches collected in August, October, or May, but it did for those collected in April. Thus, month of the
year for collection of pouches could not explain when it is possible for a difference in colonization to appear
following disturbance.
A hypothesis that has been proposed to explain the conditional effects of disturbance on colonization
is based on inoculum density. Jasper et al. 1991 raised subterraneum clover Trifolium subterraneum
L. bioassay plants in U soil cores collected from various established ecosystems. Decreased coloniza-
tion for bioassay plants was found after disturbance of soils taken from a forest and from a heathland,
while colonization was high for bioassay plants in a soil from a pasture irrespective of whether the soil
was U or D Jasper et al., 1991. The interpretation made was that for the pasture a high inoculum density
prevented a decrease in colonization following distur- bance Jasper et al., 1991. Among experiments in our
laboratory, variability in inoculum density may have occurred and therefore contributed to the inconsistent
T.P. McGonigle, M.H. Miller Applied Soil Ecology 14 2000 147–155 149
effects of disturbance on colonization that have been found.
Tillage of soil can itself affect the inoculum den- sity in soil during the growth of the subsequent crop.
The densities in soil of numbers of AM spores, and of lengths of structurally and metabolically stained hy-
phae, was lower in the top 5 cm of plots in Quebec, Canada, following conventional tillage as compared to
no-till Kabir et al., 1998. No difference in coloniza- tion of maize roots in the field by AM fungi was found
Kabir et al., 1998. However, a difference in inoculum density following tillage may have led to a difference
in colonization earlier in the growth season. Soil and roots were sampled during grain filling Kabir et al.,
1998, whereas differences for colonization of maize by AM fungi in response to tillage are typically seen
only up to the six-leaf stage McGonigle and Miller, 1993, 1996b.
Our aim was to evaluate the role that inoculum den- sity plays in determining the impact of disturbance
on colonization of roots by AM fungi in agricultural soils. We used maize and the soil of the Elora Re-
search Station in Ontario, Canada, which we have studied extensively. The hypothesis was as follows:
colonization of roots in D soil will be reduced, rela- tive to that for plants sown in U soil, but only when
the inoculum potential of the soil is low. On this basis, high inoculum should produce well-colonized roots
in both U and D soils. The approach was to select a batch of soil for which a high inoculum density
was expected, and to lower it by manipulation. Soil was collected mid-season from the rooting zone in
a maize field. Plant growth and the colonization of roots by AM fungi after two cycles of growth in the
laboratory were determined, with or without distur- bance between cycles. As an exploratory exercise, soil
biomass-C was estimated for available samples using the fumigation–extraction technique, in order to see if
it would reflect the behavior of the mycorrhizal fungi, and by doing so aid our understanding of mycorrhizal
relationships.
2. Materials and methods
