Agriculture Reference
In-Depth Information
100
Conv
Legume
Manure
80
60
40
20
0
0
10
20
30
40
50
60
70
80
Soil Depth
Figure 7.2
Levels of propagules of AM fungi at six soil depths: 0-5, 5-10, 10-20, 20-30, 30-60, and
60-80 cm. Soil samples were collected from three farming systems at the Rodale Institute: conven-
tional, organic with legume cover crops, and organic with animal manure addition. Results of most
probable number bioassays. Means of 3 years of data, each year having three replicates per farming
system × depth combination. (D. D. Douds et al., unpublished results.)
the top 20 cm of soil than below
(
Figure 7.2
)
. Inversion of soil with deep moldboard till-
age would bury the propagule-dense soil and bring soil with fewer propagules up to the
surface, contributing further to lower early season colonization of seedlings in tilled soils.
In addition to the quantitative effects tillage has on AM fungi, it has qualitative effects
on the AM fungus community both in the soil and within roots of the crop plant. Generally,
the diversity of the AM fungus community has been observed to decline with increasing
disturbance, whether from native grassland/forest to cultivation (Helgason et al., 1998)
or from no-till to conventional tillage (Boddington and Dodd, 2000). However, no change
in diversity of AM fungus spore communities in response to tillage was observed in a
wheat field in Argentina (Schalamuk et al., 2006). Cultivation can select for certain mem-
bers of a genus versus others, which can be more prominent in fallow soils (Rosendahl
and Matzen, 2008). Communities of AM fungi in tilled soils generally have been found
to be dominated more by members of the genus
Glomus
, while representation by mem-
bers of the genera
Gigaspora
and
Scutellospora
are greater in no-till or uncultivated soils
(Jansa et al., 2002; Blaszkowski, 1993). This observation also was reflected in the AM fungi
colonizing the roots of the crop. For example, tillage reduced the colonization of maize
roots by
Scutellospora
spp. and increased colonization by
Glomus
spp. (Jansa et al., 2003).
These differences may be due to differences in the tolerance of the ERM of various groups
of AM fungi to disturbance or the impact of tillage on other factors that affect AM fun-
gus species diversity (e.g., weed community structure) (Jansa et al., 2003). Members of the
Gigasporaceae are sensitive to soil disturbance: Root pieces colonized by these fungi and
ERM fragments generally do not function well as inoculum (Klironomos and Hart, 2002).
These species rely more heavily on spores as inoculum than do
Glomus
spp. Members of
Glomus
and other genera of AM fungi produce lipid-rich vesicles as well as arbuscules
within roots, which enable colonized root fragments from the previous crop or weed to
function as propagules (Biermann and Linderman, 1983).
Qualitative impacts of tillage on the AM fungus community are potentially important
from the standpoint of the functioning of the symbiosis. Functional diversity has been
reported among AM fungus species (Smith et al., 2000; Stampe and Daehler, 2003). For
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