Agriculture Reference
In-Depth Information
4.
F
UNGI IN
S
OIL
A
GGREGATION
Perusal of the literature on the microbial role in soil aggregation leaves the impression
that fungi have a prevalent role in soil aggregation compared to bacteria. While passing
mention is made in the literature of bacterial involvement, the emphasis has consistently been
on fungi. This is apparently based on such data as that indicating that fungal (biomass)-
dominated soil communities provide increased carbon storage and that the fungal biomass
consists of a higher C:N ratio (Six et al., 2006) than bacteria. This fungal dominance of
microbial biomass in natural or uncultivated settings is furthermore linked to the “alteration
of soil physical properties”, apparently meaning soil aggregation (Tisdall and Oades, 1982).
Fungi stabilize macroaggregates and bacteria help stabilize microaggregates (Tisdall, 1994).
Endomycorrhizae have been described as the apparent driving force in soil aggregation, with
their growth and production of the soil-binding glycoprotein glomalin (Wright and
Upadhyaya, 1996) being stimulated by sloughed off root material and furthermore this
combination stimulating increased bacterial populations (Dighton, 2003). These findings
could have important future implications in the use of mycorrhizal fungi to promote the
production of soil stable aggregates, improve water infiltration, and soil C sequestration in
agricultural systems. However, there are some points to consider regarding the supposed
primacy of such fungi in soil aggregation. A recent study has shown that bacteria have
growth-yield efficiencies in soil that are equivalent to fungi, contrary to long-held
assumptions (Thiet et al., 2006). This finding alone might properly call into question the
larger role fungi are presumed to play in soil aggregation compared to bacteria.
Bacteria because of their smaller size compared to fungal hyphae have access to a greater
variety of niches in the soil matrix than any other biotic contributor to microaggregation (Six
et al., 2006) and presumably thereby, more substrate carbon. In addition, microaggregates, the
formation of which are chiefly attributable to bacteria, have a greater capacity to protect
carbon against decomposition than macroaggregates, which are formed chiefly by fungi
(Tisdall, 1994; Oades andTisdall, 1982). A major caveat against microscopical methods of
determining whether a particular soil is fungal-dominated is that much of the apparent fungal
biomass may not be active (Foster, 1988, Six et al., 2006). Considering these findings
collectively, it is difficult to conceive how on balance any particular ecological setting could
be categorically “dominated” by either fungi or bacteria. Regarding the relatively more
prominent role generally attributed to fungi, in particular to endomycorrhizae through
production of glomalin, in soil aggregation, both in macroaggregates and microaggregates,
perhaps recognition of these data about bacteria and aggregation might be cause for revising
the perceived magnitude of that role to recognition of a greater role for bacteria. We agree
with the interpretation that the formation and degradation of microaggregates may be more
dynamic than generally predicted by carbon turnover times (Jastrow and Miller, 1998).
Furthermore, our findings cause us to postulate that the microaggregates are dynamic to such
an extent that bacterial species predominant in microaggregates differ in disturbed compared
to non-disturbed conditions; we found gram-positive species in microaggregates from tilled
soils with greater frequency than from no-tilled or uncultivated CRP settings (Caesar-
TonThat et al., unpublished). Presumably, this is a result of the generally lower level of labile
carbon and other nutrients in disturbed soil conditions, which would select for spore-forming
species such as
Bacillus
or for
Arthrobacter
spp. or similar coryneform species, which can
