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Title:
Tillage or no-tillage: Impact on mycorrhizae
Author:
Kabir, Zahangir, University of California, Davis
Publication Date:
01-01-2005
Publication Info:
Postprints, UC Davis
Permalink:
http://escholarship.org/uc/item/6bv5h1nv
Keywords:
arbuscular mycorrhizal fungi, conservation tillage, conventional tillage, P uptake, soil aggregate
stability, cover crops, crop yield
Abstract:
Arbuscular mycorrhizal (AM) fungi are ubiquitous in agricultural soils. These fungi play important
roles in plant nutrition and soil conservation. The persistence of AM fungi in ecosystems depends
on the formation and survival of propagules (e.g., spore, hyphae and colonized roots). While
spores are considered to be resistant structure that may be view as "long-term" propagules when
viable host plants are not present, hyphae are considered to be the main source of inocula when
host plants are present and the soil is not disturbed. Tillage is an integral part of modem agriculture
that can modify the physical, chemical and biological properties of a soil. Consequently, tillage
practices may also affect AM fungi. The various tillage practices used in the management of soil
for maximum crop production may negatively impact the survival of AM fungal propagules. In
tilled soil, certain AM species may survive while others may disappear. Because AM fungi are
more abundant in the topsoil, deep plowing may dilute their propagules in a greater volume of
soil, thereby reducing the level of infection of a plant root. Tillage is particularly detrimental to
AM hyphae if the soil is tilled in the fall and the hyphae are detached from the host plant. Under
no-till (NT), AM fungi survive better, particularly when they are close to the host crop on which
they developed. There is speculation that in NT systems, plants may follow old root channels
and potentially encounter more AM fungal propagules than plants growing in soil that has been
tilled. Management of AM fungi in NT soil is essential to maximizing benefits to crops. This review
reports how tillage practices affect AM fungi species richness, survivability and infectivity, and how
conservation tillage can increase AM fungi survival, consequently improving plant phosphorus
uptake and soil aggregate stability.
Tillage or no-tillage: Impact on mycorrhizae
Zahangir Kabir
Department of Land, Air and Water Resources, 1150 Plant and Environmental Sciences
Building, University of California, Davis, One Shields Ave., Davis, California 95616, USA
(e-mail: kabir@ucdavis.edu). Received 14 October 2003, accepted 9 September 2004.
Kabir, Z. 2005. Tillage or no-tillage: Impact on mycorrhizae. Can. J. Plant Sci. 85: 23–29. Arbuscular mycorrhizal (AM) fungi
are ubiquitous in agricultural soils. These fungi play important roles in plant nutrition and soil conservation. The persistence of
AM fungi in ecosystems depends on the formation and survival of propagules (e.g., spore, hyphae and colonized roots). While
spores are considered to be resistant structure that may be view as “long-term” propagules when viable host plants are not present,
hyphae are considered to be the main source of inocula when host plants are present and the soil is not disturbed. Tillage is an inte-
gral part of modern agriculture that can modify the physical, chemical and biological properties of a soil. Consequently, tillage
practices may also affect AM fungi. The various tillage practices used in the management of soil for maximum crop production
may negatively impact the survival of AM fungal propagules. In tilled soil, certain AM species may survive while others may dis-
appear. Because AM fungi are more abundant in the topsoil, deep plowing may dilute their propagules in a greater volume of soil,
thereby reducing the level of infection of a plant root. Tillage is particularly detrimental to AM hyphae if the soil is tilled in the
fall and the hyphae are detached from the host plant. Under no-till (NT), AM fungi survive better, particularly when they are close
to the host crop on which they developed. There is speculation that in NT systems, plants may follow old root channels and poten-
tially encounter more AM fungal propagules than plants growing in soil that has been tilled. Management of AM fungi in NT soil
is essential to maximizing benefits to crops. This review reports how tillage practices affect AM fungi species richness, surviv-
ability and infectivity, and how conservation tillage can increase AM fungi survival, consequently improving plant phosphorus
uptake and soil aggregate stability.
Key words: Arbuscular mycorrhizal fungi, conservation tillage, conventional tillage, P uptake,
soil aggregate stability, cover crops, crop yield
Kabir, Z. 2005. Travail ou non-travail du sol : incidence sur les mycorhizes. Can. J. Plant Sci. 85: 23–29. Les mycorhizes à
arbuscules (MA) sont des champignons omniprésents dans les sols agricoles. Ces champignons jouent un rôle important pour la
nutrition des plantes et la conservation du sol. Leur persistance dans l’écosystème dépend de la formation et de la survie des
propagules (les spores, les hyphes et les racines colonisées). Bien que les spores soient considérées comme des propagules « à long
terme » à cause de leur résistance en l’absence de plantes hôtes, les hyphes demeurent la principale source d’inoculum quand il y
a des plantes hôtes et que le sol n’est pas perturbé. Les labours font partie intégrante des pratiques agricoles modernes et peuvent
modifier les propriétés physiques, chimiques et biologiques du sol. De telles pratiques affectent donc aussi les MA. Diverses pra-
tiques employées pour parvenir à la production maximale d’une culture ont une incidence négative sur la survie des propagules
des MA. Certaines espèces de champignons survivront dans le sol retourné alors que d’autres périront. Les MA étant plus abon-
dants dans le sol de surface, un labour en profondeur diluera leurs propagules dans un plus grand volume, donc réduira le taux
d’infection des racines de la plante hôte. Les labours sont particulièrement néfastes quand le travail s’effectue à l’automne et que
les hyphes des MA se détachent de la plante hôte. Les MA survivent mieux avec le non-travail du sol, surtout quand ils se trou-
vent à proximité de la culture qui a servi à leur développement. On se demande si les plantes n’empruntent pas les anciens canaux
radiculaires dans les champs non travaillés, si bien qu’elles trouvent plus de propagules de MA que celles poussant dans un sol
travaillé. Une gestion des MA dans le sol non travaillé est essentielle si l’on veut que les cultures en profitent au maximum. La
présente étude explique comment les pratiques en matière de travail du sol affectent la richesse des espèces de MA, leur capacité
de survie et leur pouvoir infectieux et comment les pratiques de conservation accroissent la survie de ces cryptogames, donc
améliorent l’absorption du phosphore par les plantes et la stabilité des agrégats du sol.
Mots clés: Mycorhizes à arbuscules, conservation du sol, travail du sol classique, absorption du P,
stabilité des agrégats, cultures abris, rendement des cultures
Tillage, the mechanical manipulation of soil, is a common
practice in modern agriculture. Tillage is performed to
enhance decomposition of crop residues through physical
breakdown and incorporation into soil. Tillage is also used
to level soil, prepare seedbeds for planting, and incorporate
fertilizers, manures and pesticides. Additionally, it can serve
as a method of post-emergence weed control and as a man-
agement tool to disrupt or reduce the incidence of diseases
and pests. While tillage is necessary in many situations, it
may also lead to soil degradation and environmental pollu-
tion. There are two main types of tillage systems, conven-
Abbreviations: AM, arbuscular mycorrhizal; CT, con-
ventional tillage; NT, no-till; RT, reduced tillage
23
tional (CT) and conservation (at least 30% residue left on
the soil surface; Conservation Technology Information
Center 1995). The general category of conservation tillage
includes specific practices such as no-till (NT), ridge-tillage,
reduced tillage (RT), shallow tillage and strip tillage.
Reduced tillage systems are characterized by a reduction in
the intensity or number of tillage operations compared to CT
(generally autumn plowing plus spring disking). In RT sys-
24 CANADIAN JOURNAL OF PLANT SCIENCE
tems most of the crop residues remain on the soil surface,
and the tillage operation is normally done only in spring.
Thus, the soil remains undisturbed throughout the winter, as
is the case under NT systems. Concerns of environmental
degradation through the transport of sediments, nutrients
and pesticides from farmlands to surface waters, as well as
the need to conserve soil water in dry areas, have prompted
a switch to conservation tillage practices. These practices
may improve soil physical properties at the macroscopic
level, which in turn affects chemical and biological proper-
ties of soil at the microscopic level including AM fungi.
Arbuscular mycorrhizal fungi form symbiotic relation-
ships with plants. In these associations plants provide car-
bohydrates to the fungi in exchange for mineral nutrients
from the fungi. The name “arbuscular” is derived from the
characteristic structures, arbuscules, which occur within
host-plant cortical cells and are thought to be the primary
site for fungus/plant metabolic exchanges (Scannerini and
Bonfante-Fasolo 1983; Barea 1991). AM hyphae proliferate
externally from colonized roots. The extra-radical hyphae
increase the volume of soil that can be exploited for nutri-
ents and make a fundamental bridge between plants and soil.
While improved nutrient acquisition is considered to be the
primary benefit of the AM symbiosis, other benefits such as
protection against root pathogens (Thygesen et al. 2004),
improved soil structure (Bethlenfalvay and Barea 1994,
Kabir and Koide 2002), increased vegetation in polluted soil
(Vivas et al. 2003) and enhanced water use efficiency
(Caravaca et al. 2004) can also be gained from the presence
of AM fungi.
Arbuscular mycorrhizal fungi are ubiquitous in both nat-
ural and agricultural soils. In agricultural field soils up to 50
m of AM hyphae per gram of soil have been observed
(Smith and Read 1997) and hyphae can extend more than 9
cm beyond the roots (Camel et al. 1991). Phosphorus is a
primary plant nutrient, however, in soil solution the concen-
tration of P is usually very low. Phosphorus is transported to
roots from the soil mainly via diffusion, however, the diffu-
sion coefficient of P is very low, about 1/10th of that of K
and NH4and 1/100th of that of NO3. Consequently, P is eas-
ily depleted in the root zone (Harley 1989). Barber (1995)
calculated that since an annual crop’s root system occupies
less than 1% of the soil volume and since the P depletion
zone of the plant root is between 1 and 2 mm in width, the-
oretically, plant roots should take up only 1–2% of applied
P to the soil. In practice, however, plants take up 10–15% of
applied P (Brady and Weil 1995). The presence of AM
hyphal networks in soil seems to improve efficiency of plant
P uptake from the soil. Nonetheless, tillage and other forms
of soil disturbance can alter the ability of AM fungi to colo-
nize roots, thereby reducing P uptake. Reducing tillage
appears to be an effective means of minimizing AM hyphal
network destruction, ensuring optimal plant nutrient uptake,
and reducing soil erosion.
From the perspective of sustainable crop production, it is
very important to understand the dynamics of AM fungi in
agricultural soil as influenced by tillage. The purpose of this
paper is to review how soil management practices affect the
development of AM fungal associations with crop plants.
EFFECTS OF SOIL DISTURBANCE ON
MYCORRHIZAL FUNCTIONING
Impacts on AM Fungal Development
Soil disturbance has a negative effect on AM fungi and there-
by reduces the benefits to crops and soil quality that are
derived from mycorrhizae. Schenk et al. (1982) in Florida,
USA, reported an increase of mycorrhizal spores and root
colonization on several agronomic crops with minimum
tillage compared with conventional tillage. Mulligan et al.
(1985) in Michigan, USA, proposed that the negative impact
of tillage on root colonization was due to lower root growth
of dry bean (Phaseolus vulgaris L.) caused by increased soil
bulk densities in tilled soils. In contrast, O’Halloran et al.
(1986) in eastern Canada found greater root growth but lower
P uptake of corn in CT soils, suggesting that the negative
effects of tillage were not the result of reduced root growth.
O’Halloran et al. (1986) reported that disturbance of previ-
ously NT soils decreased early plant growth, P uptake and
AM colonization in corn. Their other studies indicated that
the effects of soil disturbance did not occur if the AMF had
been eradicated by γ-radiation. Furthermore, when a non-
mycorrhizal canola plant (O’Halloran et al. 1986) or spinach
(Evans and Miller 1988) was grown, soil disturbance did not
have any effect on P absorption of those plants. The fungi-
cide Benomyl minimized the effect of soil disturbance on P
uptake by corn (Evans and Miller 1988). Collectively, these
results indicate that the negative effect of disturbance on P
uptake is likely due to impaired AM association.
Extraradical hyphae are thought to be the main source of
inoculum in soil (Sylvia 1992) especially when host plants
are present and soil is not tilled for crop production. Evans
and Miller (1990) in eastern Canada observed that distur-
bance of root-free soil containing only AM hyphae detached
from host plants reduced the AM colonization of corn roots
planted later in this soil, and decreased plant growth and
nutrient uptake. This suggested that if the AM hyphal net-
work is not disrupted, the next crop will be more rapidly
connected to the network and nutrient absorption capacity
would be enhanced. Jasper et al. (1989) in Australia found a
reduction of AM colonization of clover after soil distur-
bance and suggested that most of this reduction was due to
decreased hyphal viability. However, contrasting results
were obtained by McGonigle et al. (1990) in eastern Canada
in a study on the effect of soil disturbance on AM coloniza-
tion and corn growth. Unlike previous studies, decreases in
P uptake and plant growth were not accompanied by a
decrease in AM colonization. McGonigle et al. (1990) pro-
posed that if AM fungi were an important component of the
disturbance effect, it would have to be through the disman-
tling of a potential though dynamic hyphal network rather
than through reduction in the mycorrhizal colonization
potential of the soil. The role of extraradical hyphae as prin-
cipal propagules for AM colonization might be of consider-
able importance, particularly in cool climates where
population of the viable spores in agricultural soils may be
extremely low following winter (Dalpé Y., unpublished
data; Addy et al. 1997). Most of these early studies suggest
that AM were involved in P uptake and were negatively
affected by soil disturbance, but conclusive experimentation
KABIR — TILLAGE IMPACT ON MYCORRHIZAE 25
was hampered by lack of methodologies to examine AM
hyphae directly in the soil. Current research supports this
hypothesis and addresses various mechanisms.
Impacts on AM Fungal Community Composition
In a research field (Kabir et al. 1997a, 1998a) in Quebec,
Canada, the AM species diversity in a soil under 12 yr of CT
practice was significantly lower than that of the NT soil
(unpublished data). Hamel et al. (1994) in Quebec, Canada,
reported the disappearance of Gigaspora margarita and G.
caledonium 3 yr after plowing and putting a previously
uncultivated field into cultivation. Similarly, Boddington
and Dodd (2000) observed a decrease in AM fungal species
richness in tilled soil, relative to untilled soil, growing
Gliricidia sepium in Indonesia; Scutellospora sp. disap-
peared after soil disturbance. Significantly more AM spores
were also observed by Jansa et al. (2002) in soil growing
wheat under NT than under CT in Switzerland. Jansa et al.
(2003) also found that Scutellospora was more dominant in
low-tillage fields, whereas Glomus was dominant in highly
tilled fields. Douds et al. (1995) found more G. occultum-
like spores under NT corn-soybean-wheat rotations and
more G. etunicatum-like and other Glomus spp. spores in
soils under cultivation in Pennsylvania, USA. Sieverding
(1991) observed that G. scintillans was sporulating earlier
and was able to produce more spores than other AM species
in plowed soils in Columbia. The author reported that 75%
of the spores in plowed soils were belonging to G. scitillans
while this species accounted for only 5% of the total spores
in NT soils. This suggests that tillage practices may select
AM fungi with certain characteristics and eliminate others.
For example, soil disturbance created by tillage may favor
fast-growing species that might be less mutualistic and less
efficient in improving host plant nutrients uptake (Johnson
and Pfleger 1992).
Differentiating Effects of Disturbance on Nutrient
Acquisition and AM Functioning
The most dramatic effect of AM fungal proliferation in the
soil is an increase in P absorption by the host plants (Koide
1991). Phosphorus is found in very low concentration in the
soil solution as it has a high affinity for fixation onto soil
minerals (Lambert et al. 1984). The distribution of nutrients,
especially P, in the soil profile is affected by tillage intensi-
ty (Dick 1983) and thus may impact P availability to crop
roots. Research on the importance of soil disturbance on
AM colonization has been inconsistent, but P absorption by
plants in disturbed soil has always been lower than those
grown in undisturbed soil (Miller 2000). This indicates the
importance of the AM hyphal network in potential tillage
operations.
Kabir et al. (1998a) in Quebec, Canada, found that in a
sandy loam soil P concentration in corn plants growing
under NT and RT was greater than under CT at the 12- to
14-leaf and silking stages. At the grain filling stage, howev-
er, plant P concentration was greater only under NT, and the
difference between P concentrations obtained under CT and
RT had disappeared. In another year in the same soil, NT
increased P concentration in the corn plant only at the12-
to14-leaf stage. In the clay soil, however, P concentration
was greater under NT both at the 12- to14-leaf and the silk-
ing stage. These results are in accordance with those of
O’Halloran (1982) who also observed higher P absorption
by corn in a NT system. McGonigle and Miller (1993), in
eastern Canada, observed that corn shoot P concentrations
were significantly greater under NT and RT than under CT.
In addition to P, Kabir et al. (1998a) found that Zn and Cu
concentrations in corn plants were sometimes significantly
greater under NT than under CT plots. A similar effect of
soil disturbance on Zn and Cu was also observed for corn in
pot studies (McGonigle and Miller 1996). Mozafar et al.
(2000) reported that the concentrations of P, Zn and Cu in
corn and those of P, K, Mn and Zn in wheat grown in
Switzerland were greater in plants under NT than under CT
at most of their sampling dates. While NT systems allow for
greater nutrient uptake, in certain circumstances, crop yields
are reduced with NT. Thus the relatively minor economical
benefits that derived from improved nutrient acquisition are
therefore, often dwarfed by the losses in crop yield.
However, considering the cost and benefit ratio, one could
argue that the profitability of NT is similar to CT even
though yields are reduced under NT. Furthermore, consider-
ing soil health and the environment, growing crops in an NT
system far outweigh any short-term benefit of CT systems.
Relationships among Soil Disturbance, AM Fungi
and Aggregate Stability
Soil structure quality and aggregate stability in agricultural
fields are influenced by agricultural practices. Tillage gradu-
ally reduces aggregate stability making soil more vulnerable
to wind and water erosion. Arbuscular mycorrhizal fungi
make direct contributions to aggregation and aggregate sta-
bility (Bethlenfalvay and Barea 1994; Kabir and Koide 2000,
2002) and therefore play an important role in soil conserva-
tion. AM hyphae have been positively correlated with soil
aggregate stability (Kabir and Koide 2002). Because AM
hyphal networks remain intact In NT soils, the density of
active hyphae is greater than under CT soils (Kabir et al.
1997a). Hence, the importance of AM fungi for aggregation
is greater in NT than in CT systems. Tisdall (1991) speculat-
ed that extracellular polysaccharides of fungi and bacteria
provide a cementing agent for aggregates. Wright and
Upadhyaya (1996) discovered “glomalin”, a glycoprotein on
the surface of active AM hyphae, which appears to act as a
cementing agent for soil particles. The more abundant AM
mycelium under NT may lead to a more abundant production
of glomalin under NT than CT. In contrast, in CT regimes,
disruption of the hyphal network due to tillage operations,
would likely lead to reduced glomalin production and
reduced aggregate stability. For example, Bethlenfalvay and
Barea (1994) found an isolate of Glomus mosseae, which
improved soil aggregation by 50% when associated with pea,
in a yellow clay-loam soil, and by 400% in a gray silt-loam
soil. Wright et al. (1999) reported that both aggregate stabil-
ity and total glomalin were greater under NT than under CT
in the top 0 to 5 cm of the soil. They also found that when
soil was collected from the grassland adjacent to the tillage
experiment, the structure of the top 0–10 cm of the grassland
26 CANADIAN JOURNAL OF PLANT SCIENCE
soil was more stable than that of the cultivated soil after sev-
eral years under NT and 4 yr under CT. The production of
glomalin was also greater in the grassland than under NT.
Collectively, these results indicate that activities of AM
fungi are greater in NT than CT, and when mycotrophic
plants are present, leading to greater hyphal densities, glo-
malin production, and aggregate stability.
OPTIMIZING THE AM FUNGAL BENEFIT VIA
CHNANGES IN CROPPING SYSTEMS
Tillage Practices
Some AM fungi are capable of free-living growth after the
death of their host plant (Tommerup and Abbott 1981).
However, questions remain concerning how long the
hyphae remain viable in the absence of a living host plant,
and how soil disturbance may affect the survivability of
these AM hyphae especially in the field condition when dif-
ferent tillage operations take place.
Experiments were undertaken to determine effects of the
timing of tillage on the survival of AM hyphae (Kabir et al.
1997a). In these experiments, fall tillage severely reduced
AM hyphal viability, but spring tillage had little affect on
AM hyphal viability. This research revealed that timing of
tillage is critical to the survival of AM hyphae. McGonigle
et al. (1990) reported that as the number of tillage operations
is increased, AM fungal benefits to host plants are gradual-
ly decreased. Kabir et al. (1997a) concluded that the reduc-
tion of the AM fungal benefits is due to the reduction of
viable AM hyphae. McGonigle and Miller (1999) demon-
strated that extraradical AM hyphae, which over wintered in
the field remained viable as inoculum in spring and that dis-
turbance of these hyphae in spring reduced colonization and
P uptake in the following crop.
For row crops, the influences of tillage on AM develop-
ment are likely related to spatial and temporal distributions
of the AM fungi. Kabir et al. (1998a) studied AM fungal dis-
tributions under CT, RT and NT corn. The greatest season-
al fluctuation of hyphal density was observed under the row,
where a sharp increase occurred at the silking stage and
decreased thereafter. The least variation and least overall
hyphal densities were observed between the rows.
Densities of total and metabolically active hyphae were
greatest directly in the crop row and decreased with distance
from the row. About half of the hyphae were observed in the
row in both soils, whereas less than 20% of hyphae were
observed between the two rows in both clay and sandy loam
soils (Kabir et al. 1998a), again suggesting a prevalence of
AM hyphae in the row. Plants growing near the previous
year’s row are likely to receive more benefits from AM fungi
than plants growing between the two rows. For example, ridge
tillage near the previous year’s row could increase the benefits
of AM fungi to the crops. Hyphal densities in the row were
greater under NT than under CT, but between rows there was
no difference between NT and CT. Mulligan et al. (1985)
observed that excessive secondary tillage reduced AM colo-
nization of Phaseolus vulgaris L. Mycorrhizal root coloniza-
tion of corn growing in NT and ridge till plots was greater than
that in CT plots (McGonigle and Miller 1993).
The vertical distribution is also influenced by tillage prac-
tices (Kabir et al. 1998b). Mycorrhizal growth was mea-
sured within the top 0–25 cm of soil in NT and CT fields
under corn cultivation. Arbuscular mycorrhizal hyphae and
spores were more abundant in the top 0- to15-cm layer of
the soil profile and decreased dramatically below this depth.
Similar results were reported for AM spores by An et al.
(1990) in Kentucky, USA, under soybean, and by Smith
(1978) in an Australian wheat field under NT and CT oper-
ations. This suggests that tilling the soil to a depth of 15 cm
would affect most of the AM fungi and that plowing below
this depth would dilute the AM propagules in the zone of
seedling establishment.
Kabir and O’Halloran (unpublished data) observed lower
hyphal density under CT than under NT in the top 0- to
15-cm soil depth at the early stage of corn growth (5- to
6-leaf stage), in the 0- to10-cm depth at the 10- to12-leaf
stage, and in the 0- to 5-cm depth at the silking stage (Fig.
1). These differences disappeared at maturity. The number
of AM spores was significantly greater in NT than in CT in
the top 0- to10-cm of soil through the 10- to12-leaf stage,
but tillage effects differences disappeared at the silking
stage (Fig. 2). The negative effects of soil disturbance on
AM hyphae and spores changed over time under CT soil and
are ephemeral. Hyphal densities gradually increased from
the 5- to 6-leaf stage to the silking stage of corn and
decreased thereafter. The distribution of spores, however,
did not follow the same seasonality as the hyphal densities
under both tillage systems. The number of spores gradually
increased up to plant maturity, indicating that spores are the
final product of the AM fungal growth cycle (Fig. 2).
Cover Crops
Since AM fungi are biotrophic, viability of AM hyphae grad-
ually decreases in the absence of host plants such as during a
fallow, even in NT systems. Arbuscular mycorrhiza hyphal
survival and inoculum potential depends on the presence of the
host plants during the fallow period. Harinikumar and
Bagyaraj (1988) in India reported 40% reduction of AM
inoculum in field soil after leaving the land fallow for one sea-
son. Long-fallow periods (more than a year) in northern
Australia were associated with a decline in mycorrhizal colo-
nization and AM sporulation in various crops (Thompson
1987). This reduction in AM fungal inoculum may be exacer-
bated by adverse winter conditions (Kabir et al. 1997a). In a
NT system in eastern Canada, winter alone caused a reduction
of approximately 31 and 40% of total and metabolic active
hyphae, respectively (Kabir et al. 1997b).
It is important to maintain the level of AM inoculum in
soil over winter to maximize the benefits of AM fungi on the
following crop. Mycotrophic cover crops capable of surviv-
ing freezing winter conditions may help maintain the AM
inoculum potential in soil. Whereas a mycorrhizal cover
crop may improve P uptake and eventually increase crop
yield, a non-mycorrhizal cover crop in the cropping sched-
ule of a NT or CT systems may reduce propagules of AM
fungi in the soil. An experiment was conducted by Kabir
and Koide (unpublished) in Pennsylvania, USA, to verify
the effect of growing mycorrhizal and non-mycorrhizal
cover crops over winter in NT system. At 31 d after planti-
KABIR — TILLAGE IMPACT ON MYCORRHIZAE 27
ng sweet corn, root colonization and shoot P content of the
plants were significantly greater in mycorrhizal cover
cropped (oats and winter wheat) plots than in fallow plots or
non-mycorrhizal cover cropped (buckwheat) plots. This
indicates that the mycorrhizal cover crop increased or main-
tained AM fungal inoculum in soil. Accordingly, sweet corn
shoot dry weight (14 and 31 days after planting) and plant
height (87 d after planting) were significantly greater in the
mycorrhizal cover cropped plots. Similarly, sweet corn yield
was also greater in the mycorrhizal cover cropped plots than
in the fallow or non-mycorrhizal cover cropped plots
(Fig. 3). Boswell et al. (1998) and Kabir and Koide (2000)
demonstrated that mycotrophic winter cover cropping with
wheat or dandelion increased subsequent sweet corn yield.
Kabir and Koide (2002) observed that either single or mixed
mycotrophic cover crops increased the following cash
crop’s P status, and plant P status positively correlated with
vegetative growth, reproductive maturity and yield of sweet
corn. These results suggest that management of indigenous
AM fungi is important to maintain or improve AM fungal
propagules by using cover crops for succeeding crops
improvement either under NT or CT operations.
CONCLUSIONS
Conventional tillage practices reduce AM hyphal survival and
proliferation thus reducing benefits of the symbiosis to associ-
ated plants and soils. Under temperate climates, it is beneficial
to optimize survival of AM hyphae from fall to spring often in
the absence of a living host plant. Fall tillage has been shown
to adversely affect hyphal viability and host plant benefits in
the following year. Reduced tillage or ridge tillage systems
have less negative effects than CT on the abundance of AM
propagules because tillage operation in these systems are per-
formed in the spring and AM fungi remain intact throughout
the winter. In my studies the greatest amounts of hyphae were
found in the crop rows and hyphal abundance decreased loga-
rithmically to the inter-row, suggesting that growing crops
close to the previous years’ rows optimizes AM fungal bene-
fits. Arbuscular mycorrhizal fungi were abundant in the upper
15 cm of the soil irrespective of tillage practices suggesting that
Fig. 1. Seasonal and vertical distribution of mycorrhizal hyphae in four corn-growing periods: 5- to 6-leaf stage (1), 10- to12-leaf stage (2),
silking stage (3), and mature stage (4) in conventional (CT) and no tillage (NT) systems.
Fig. 2. Seasonal and vertical distribution of mycorrhizal spores in four corn-growing periods: 5- to 6-leaf stage (1), 10- to12-leaf stage (2),
silking stage (3), and mature stage (4) in conventional (CT) and no tillage (NT) systems.
28 CANADIAN JOURNAL OF PLANT SCIENCE
deep plowing (<15 cm) would dilute the AM propagules into a
greater volume of soil. Inoculum dilution, however, is not the
main mechanism for the tillage effect since researchers have
found that colonization is not always decreased by soil distur-
bance, although nutrient uptake is usually reduced. Since AM
fungi are biotrophic, the combination of fallowing and tillage
substantially reduces the density of AM infective propagules
(Kabir et al. 1999). This problem is aggravated by adverse
weather conditions. More research is needed to define the inter-
active effects of soil disturbance and fallowing, especially for
field and vegetable crops other than corn. No-tillage along with
mycotrophic winter cover crops improves AM hyphal densities
and inoculum potential, which subsequently stabilizes soil and
increases crop yield (Boswell et al. 1998; Kabir and Koide
2000, 2002). By introducing a mycotrophic cover crop after fall
tillage, it may be possible to increase the benefits of AM fungi
in CT systems as well.
ACKNOWLEDGMENTS
This paper was presented at a Joint Symposium of The
Canadian Society of Agronomy, the Canadian Society of
Soil Science and the Fourth International Conference on
Mycorrhizae held in Montreal, QC, 11 August 2003. I grate-
fully acknowledge the financial support from the
International Conference on Mycorrhizae for attendance at
the symposium and the Canadian Society of Agronomy for
publication costs. I would like to thank Tom Forge, C.
Hamel, K. Reed and anonymous reviewers for their valuable
suggestions and criticisms to improve the manuscript.
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