Agriculture Reference
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maize and wheat monocultures as well as two-crop, three-crop, and four-crop rotations
(maize, wheat, in rotation with spring barley, alfalfa, or peas). Grain yields for both maize
and wheat were always lower in a monoculture than in a crop rotation, and the four-
crop rotation had the highest yields. Yield reductions in monocultured corn seemed to be
mostly caused by lack of moisture, whereas the reduction in wheat yield in a monoculture
could be mainly attributed to pathogenic factors stimulated by the weather (Berzsenyi et
al., 2000). Many other studies have tested the effect of rotations on diseases (e.g., soilborne
pathogens) (Abawi, 1989; Glynne, 1965; Trivedi and Barker, 1986).
Two or more crops in a rotation can reduce weed populations compared to monocrop-
ping, mainly because crops with different growth habits disrupt the life cycle of different
weed species (Benoit et al., 2003; Kegode et al., 1999; Liebman and Dyck, 1993; Schreiber,
1992; Teasdale et al., 2004). This is especially critical in organic cropping systems, in which
weeds cannot be controlled by herbicides. Porter et al. (2003) showed that weed control,
yields, and soil quality in an organic 4-year rotation with hay were better than in an organic
2-year maize-soybean rotation. Diverse crop rotations can also increase soil carbon (West
and Post, 2002) and decrease nutrient leaching (Sanchez et al., 2004).
7.2.2.2 Role of AM fungi
The interaction of crop rotation and the indigenous AM fungus community is important
from an agronomic viewpoint in several ways: the quantitative impact that the mycotro-
phic status of one crop has on the AM fungus population available for the next crop, the
change each crop imparts on the relative abundance of members of the AM fungus com-
munity, and the impact of that new community on the growth of the proceeding crop.
From a mycorrhizocentric viewpoint, crop plants can be categorized as mycotrophic
(hosts) or nonmycotrophic (nonhosts). Among angiosperms, the primary nonmycotrophic
species are members of the Brassicaceae and Chenopodiaceae. Therefore, many common
vegetable crops and weedy plants are not hosts for AM fungi and therefore will not sup-
port their growth and sporulation. As expected, when part of a crop rotation, these plants
depress populations of AM fungi and have been shown to have negative impacts on the
growth of proceeding mycotrophic crops (Arihara and Karasawa, 2000; Karasawa et al.,
2001; Panja and Chaudhuri, 2004). As with the impacts of tillage on mycorrhiza-mediated
crop response, the degree of this response is dependent on the available P level of the soil
(Ryan et al., 2002).
Among host plants, there are differences in the level of colonization by and sporu-
lation of specific species of AM fungi that they will support. Work on this topic began
with experiments to find good general hosts for inoculum production in the greenhouse
(Hetrick and Bloom, 1986; Struble and Skipper, 1985). These experiments showed that
certain AM fungi sporulated better in the presence of some plants than others, and this
phenomenon has been seen in the field (Johnson et al., 1991). Spores of the AM fungus
Gigaspora gigantea
were much more numerous after maize than small grains, soybean, or
bell peppers (Douds et al., 1993, 1997;
Figure 7.3
)
. This represents a qualitative effect on the
relative abundance of AM fungus species within the community of AM fungi to which
the next crop is exposed. Coupled with the functional diversity of AM fungi, these fluctua-
tions in abundance within the community can have impacts, subtle or otherwise (Hendrix
et al., 1992), on growth of the proceeding crop. These impacts are difficult to predict when
an AM fungus community may have as many as 26 members (Ellis et al., 1992), and little
if anything is known about their individual functional diversities.
One agricultural phenomenon in which the influence of a crop on its attendant AM
fungus community has a significant role is the yield decline noted for crops when grown
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