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a high level of microbial diversity was maintained (Fig. 6.2b). This type of
community distribution where no one member is dominant suggests
that competition must be nearly absent, leading us to term this a non-
competitive diversity pattern. In contrast, the saturated samples exhibited
much less diversity of restriction types, and dominance of one or a few
community members leading to a competitive diversity pattern (Fig. 6.2b).
In these samples, it appears that a few community members were able
to out-compete the rest of the community for nutrients. The vadose zone
soils showed community patterns intermediate between the surface and
saturated samples; not as diverse as surface communities, but lacking the
strong appearance of dominance observed in the saturated zone.
Can spatial isolation and resource heterogeneity explain these
community patterns?
Two hypotheses could explain how the non-competitive and competitive
diversity patterns are formed. First, spatial isolation (because of low
moisture) in the surface samples could allow for the maintenance of diverse
types of microbes and lead to a high level of diversity. At the surface, water
films are transient, existing only after a rainfall. As gravity removes this
moisture, there will be a low level of connectiveness (high spatial isolation)
of soil particles, and microbial species that would normally be lost by
competitive exclusion are able to persist. In saturated soils, excess water
allows for a high level of connectiveness (low spatial isolation) which offers
ample opportunity for the transfer of nutrients and microbes. Under these
conditions, the organism best able to scavenge nutrients or migrate to a
nutrient source will outgrow less fit types and become dominant.
While the spatial isolation hypothesis fits well with the varied moisture
content of our soil samples, an alternative hypothesis is that greater
resource heterogeneity at the surface allows for the maintenance of high
microbial diversity. The merit of this proposal is that indeed total organic
carbon, and probably the variety of carbon types, decreases with increasing
soil depth. Thus, multiple resources at the surface could create a variety of
microhabitats that support a diverse collection of species. In the saturated
zone, the lack of diverse carbon sources means that fewer species will
dominate the community.
Do the non-competitive and competitive diversity patterns appear
as a general theme in soils?
If spatial isolation is an important determinant of the diversity pattern, then
one would predict that smaller particles, e.g. clays, would contribute more
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