Alternatively, higher nutrient availability in smaller size particles
may have caused higher bacterial diversities. According
to van Gestel et al. (58), the vicinity between microbes, organic
matter, and clay is required for the survival of microbes, in
which organic matter and clay particles provide substrates and
nutrients. The enrichment of organic matter and microbial
biomass in finer fractions seems to be a consequence of aggregate
formation along with the decomposition of particulate
organic matter (36). Therefore, T-RFs found in the sand fraction
may represent bacterial species being better adapted to
limited nutrient conditions or possessing the capacity to utilize
a wider range of substrates. They may be able to degrade
high-molecular-weight organic molecules derived from the initial
breakdown of plant material. Phylogenetic assignment of
sequenced 16S rRNA genes indicated a great abundance of
a-Proteobacteria, such as Sphingomonas (T-RF of 81 bp), and
bacteria belonging to the Rhizobium-Agrobacterium group
(T-RF of 195 bp). Some members of the a-Proteobacteria utilize
a wide range of substrates, and the genus Sphingomonas is
particularly known for its ability to degrade aromatic compounds
(16, 33). Because sand size particles seem to be preferentially
colonized by fungi (32), many bacteria were probably
outcompeted by eukaryotic organisms in this fraction. It is
further assumed that community composition was affected by
the oxygen concentration. Sequence analysis indicated the
presence of aerobic bacteria, as well as strict anaerobes such as
clostridia, in clay particles, suggesting that particles with sizes
smaller than 2 mm provided a niche for aerobes as well as for
anaerobes, whereas larger particle sizes were dominated by
aerobic microbes. Recently, the development of biofilms in
soils consisting of a dense lawn of clay aggregates containing
one or more bacteria, phyllosilicates, and grains of iron oxides
was observed (40). Clay particles were held together by an
extracellular polysaccharide matrix and were arranged as
hutches that served as housing for microbes. These “clay
hutches” have been proposed to represent a minimal nutritional
sphere for autochthonous bacteria and may at least
partly explain the higher microbial diversity in clay size particles.