Agriculture Reference
In-Depth Information
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Maize
Sm. grain
Gr. pepper
Crop rotation
Figure 7.3
Effect of crop rotation on populations of the AM fungus
Gigaspora gigantea
. Spores were
isolated from the soil in the autumn after final crop harvest. Each bar represents the mean of 3
years, 48 soil samples per year. (Adapted from Douds, D.D., L. Galvez, M. Franke-Snyder, C. Reider,
and L.E. Drinkwater. 1997. Effect of compost addition and crop rotation point upon VAM fungi.
Agriculture Ecosystems and Environment
65:257-266.)
in continuous monoculture (i.e., in the absence of crop rotation). For example, the average
increase in yield of wheat when grown in rotation, over that in continuous monoculture,
ranges from 14% in North America to 33% in Australia (Kirkegaard et al., 2008). Crop
monocultures can produce AM fungus communities that do not enhance the growth of
that crop (Schenck et al., 1989; Johnson et al., 1992). Johnson et al. (1992) proposed a model
that explains how this can occur. Basically, the AM fungi that proliferate on a given host
are not necessarily the most effective symbionts from the phytocentric perspective. They
are very effective at procuring carbon from that particular host for their own reproduc-
tion. The disproportionate representation of these species within the AM fungus com-
munity increases with successive cropping cycles of that plant, and crop vigor declines.
Crop rotation reduces the relative abundance of these fungi and increases that of other
members of the community. Interestingly, the community of AM fungi that was detrimen-
tal to maize, for example, was shown to be beneficial to soybean and vice versa (Johnson et
al., 1992). Crop rotation therefore presents a win-win situation. There also is evidence that
weeds can be beneficial in maintaining levels of potentially underrepresented members
of the AM fungus community, and that this can enhance the efficacy of the community
in enhancing crop growth (Feldmann and Boyle, 1999). This fits with a generalization that
the diversity of the AM fungus community is proportional to that of the associated plant
community (van der Heijden et al., 1998).
7.2.3.1
Agronomic background
7.2.3.1.1 History and definition.
Cover crops are usually not harvested but grown
between permanent crops such as trees and vines or in fields before or after main crops.
They have been used for centuries but are often missing in “modern” farming systems.
Without cover crops, farmers might encounter erosion problems (especially if they are till-
ing their soil) and nutrient management issues (lack of nutrients, especially nitrogen, as
well as leaching of nutrients).
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