Soil Teeming with Life: New Frontiers for Soil Science - Sustainable Management of Soil Organic Matter - page 392

strongly to a spatially isolated environment. Hence, we would predict that
moisture and clay content would shape community structure in a manner
such as that hypothesized in Fig. 6.3. We currently are testing whether this
hypothesis is supported by examining the existing microbial communities in
soils that vary in these two features.
Test of the hypotheses that spatial isolation and resource
heterogeneity act to structure soil communities
In addition to examining existing soil microbial communities, we are con-
ducting controlled laboratory studies with simple two-strain microcosms to
evaluate the spatial isolation and resource heterogeneity hypotheses. While
our examination of existing communities will reveal community diversity
patterns, it is with these simple microcosms that we can test what forces
impact on most microbial community structure. The advantage of using
this simple system is that many of the abiotic soil components can be held
constant while the impact of a single variable, such as moisture, undergoes
evaluation. The low complexity of the two-strain community ensures that
the dynamics of each population can be measured precisely.
Competition experiments performed by Gause (1934) with two
species of Paramecium demonstrated that the more competitive species
predominated in a uniform, single-nutrient environment. This pioneering
work led to the concept of competitive exclusion, the idea that competitors
cannot coexist on a single limiting resource (Hardin, 1960). In many ways,
a species pair that exhibits strong competitive exclusion, where one species
is rapidly out-competed, would be ideal for evaluating our spatial isolation
and resource heterogeneity hypotheses. Thus we chose pairs of species that
differed in their growth kinetics in liquid culture, when spatial isolation
is low. Under these conditions, the species with superior growth kinetics
was demonstrated to predominate in a predictable manner. Once these
competitive interactions are defined under highly connected conditions, the
impact of varied levels of isolation or resources can be tested.
With two-species competition experiments, it must be ensured that
positive or negative interactions between the species do not interfere
with the hypothesis being tested. For example, if strain A cross-feeds on
secondary metabolites produced by strain B, then coexistence will occur
even under conditions of low spatial isolation. One solution to this problem
is to use two variants of the same species that differ in their growth kinetics.
In this case, it may be necessary to distinguish the strains by introduction of
a marker, such as B-galactosidase (LacZ) or the green fluorescent protein
(Tombolini et al ., 1997). Ideal for this second strain pair would be a
collection of strains isolated from the same environment. For example,
we have evaluated a collection of closely-related 2,4-dichlorophenoxyacetic