Agriculture Reference
In-Depth Information
used), and yields were the same as in a weed-free treatment as long as dry rye biomass was
high (>9,000 kg/ha).
The presence or absence of tillage can have an impact on the AM fungus community struc-
ture and function in several ways. Soil disturbance affects the functioning of the ERM,
the distribution of AM fungi throughout the soil profile, and the community of AM fungi
colonizing roots of the crop plant.
The ERM of the arbuscular mycorrhiza serves two primary functions. First, it serves
as the nutrient uptake organ of the symbiosis, exploring the soil for nutrients, which it
then transports back to the intraradical phase for release to the host root. Second, it grows
out into the soil to colonize other roots of the same or other potential host plants. Both of
these functions are impacted by tillage-induced soil disturbance (Kabir, 2005). This has
been demonstrated well in a series of greenhouse and growth chamber experiments con-
ducted by Miller and coworkers (Evans and Miller, 1990; Fairchild and Miller, 1990). Corn
(
Zea mays
L.) was grown in plastic cylinders containing field soil amended with various
levels of P, ranging from 0 to 160 mg P g
-1
soil. Three growth cycles were conducted in
which shoots of successive plantings of maize were harvested after 3 weeks of growth.
All soils were initially disturbed to fill the containers, but after the first harvest, half of the
containers were replanted without disturbing the soil. In the other containers, the soil was
removed, mixed, and repotted prior to sowing new seeds. Shoot growth was equivalent
among paired disturbance treatments, within P addition level, for the first cycle (
Figure 7.1
)
.
With successive cycles, however, the plants grown in undisturbed soil, at low P, exhibited
progressively greater shoot weight than those grown in disturbed soil. The soil distur-
bance had an impact on the ability of the ERM of the previously grown seedling to colonize
the subsequently grown plant. This was reflected in colonization parameters and shoot P
concentrations (Fairchild and Miller, 1990). In addition, soil disturbance between planting
had no effect on shoot growth in the high-P treatment (
Figure 7.1
)
. This was not surprising
because AM fungus colonization is expected to provide little benefit for nutrient uptake in
high-P soils. A high rate of AM fungus colonization is not necessary to receive the mycor-
rhiza-mediated contribution to the success of a reduced-tillage treatment. The important
factor is the connection to the previously established, intact ERM and its nutrient-absorbing
capacity (McGonigle et al., 1990; Miller, 2000). Maintenance of the attachment of the ERM
to the roots of the previous crop is not essential to the survival and infectivity of the ERM
(Jasper et al., 1989b). However, survival of the disturbed mycelium and the ability of dis-
turbed ERM fragments to function as propagules decrease with time after disturbance and
with increasing moisture level of the soil (Jasper et al., 1989a; Kabir et al., 1998).
Soil disturbance also has been shown to affect the interaction between AM fungi,
Bradyrhizobium japonicum,
and soybean plants in a greenhouse experiment (Goss and deVa-
rennes, 2002). Soil disturbance had the previous effects on shoot growth, P uptake, and
mycorrhiza development. In addition, the number of nodules and overall N fixation was
greater for plants growing in the undisturbed versus disturbed soil.
Do these findings in the greenhouse or growth chamber translate to the field? Field
experiments have demonstrated increased AM fungus colonization of maize, wheat, a
Vicia villosa
cover crop, and pasture grasses when sown into no-tilled versus tilled soils
(Miller, 2000; Galvez et al., 1995; Galvez, Douds, Drinkwater, et al., 2001; Galvez, Wagoner,
et al., 2001; Borie et al., 2006). These studies also showed increased early season P uptake
by plants in no-tilled soils relative to tilled soils (Miller, 2000; Galvez, Douds, Drinkwater,
et al., 2001; Galvez, Wagoner, et al., 2001), and this effect was lessened with increasing
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