Soil Teeming with Life: New Frontiers for Soil Science - Sustainable Management of Soil Organic Matter - page 390

soils can be composed of almost entirely unique members (Borneman et al .,
1996, 1997; B. Xia and J.M. Tiedje, unpublished). These studies agree
with earlier work where DNA reassociation rates were used to estimate that
4000 non-homologous genomes were present in a forest soil sample
(Torsvik et al ., 1990). One explanation for these high levels of microbial
diversity is that some quality of the soil matrix must promote the develop-
ment and maintenance of complex microbial communities. The aim of this
section is to identify the soil qualities that are most active in shaping micro-
bial community structure and to detail methods by which these qualities
can be better defined.
An understanding of the mechanisms that control the structure of
microbial communities would clearly benefit any strategy designed to
enhance the growth and dominance of a microbial community member,
whether that member is indigenous or introduced. Measures of species
diversity in plant and animal communities often show that the majority of
species are rare and a few species are abundant, suggesting that competitive
interaction is a key determinant of community structure (Fig. 6.2a). To
determine if competitive interactions also play a key role in structuring
soil microbial communities, a small subunit rRNA gene-based approach
(Zhou et al ., 1997) was carried out on surface, vadose and saturated zone
soils. In surface samples, this analysis yielded a nearly uniform distribution
of rDNA restriction patterns (operational taxonomic units) indicating that
Fig. 6.2. Community diversity profiles. (a) A common diversity pattern seen by ecologists for
macroorganisms where competitive interactions define community structure, compared with
(b) microbial community diversity patterns for bacteria (determined by ARDRA) in surface and
saturated zone soils.