André Meyer & John Wooldridge
ARC Infruitec-Nietvoorbij, Stellenbosch
Joanna Dames
Department of Biochemistry and Microbiology, Rhodes University
A
s in all natural ecosystems,
soil
microbial
communities are active
in
agricultural
soils
where they perform such essential
ecosystem-level functions as organic residue decomposition and the
release of their contained nutrients
to the rhizosphere. Soil microbes
differ in terms of the types of carbonaceous substrate that they are
able to utilise.
This has led to the design of Biolog
Eco Plates™ as a tool for ecological
studies of microbial communities.
This approach quantifies the complexity of individual microbial communities by assessing how many
different carbonaceous substrates,
of which 31 are presented on a single microtitre plate, they are able to
utilise. These substrates not only
have high relevance to soil microbial communities, but are also known
to be contained within plant root
exudates. When inoculated into the
plate wells, each community gives a
characteristic
reaction
pattern
called a metabolic fingerprint. This
enables scientists to compare microbial communities in terms of
their metabolic diversity (known as
the community metabolic diversity).
Recently, scientists from the Agricultural Research Council (ARC) at
Infruitec-Nietvoorbij in Stellenbosch
in collaboration with Rhodes University, used the Biolog Eco Plate
approach as an assay to detect and
evaluate changes in the soil environment that stemmed from different orchard floor management
practices. Deciduous fruit producers
are under increasing pressure to
adopt environmentally friendly soil
surface management practices.
However, evidence that organic
practices result in commercial orchard soil ecosystems approximating more closely to natural ecosystems than conventional practices is
lacking. This study, which was carried out on a ‘Cripp’s Pink’/M7 apple orchard in the Elgin area aimed
to provide further information. Five
treatments (Table 1) were applied
to nine-tree plots in spring
(September) 2003 when the trees
were entering their third growth
season. Applications continued until January 2011. The community
metabolic diversity was determined
by the Biolog method in composite
soil samples from the 0-15 cm
depth interval on both sides of the
tree row, beneath the canopy drip
line, in spring (September) 2008
and 2009, and in summer (January)
2010.
Results showed that carbon substrate utilisation in the conventional
treatments where chemical weed
control and inorganic fertliser were
applied in the tree row (T1 and T2),
were low compared to the compost
and mulch (organic) treatments (T4
and T5) in 2008 (Table 2). In the
subsequent year (2009) the result
was similar. In 2010, utilisation was
lowest in T1. Averaged over all sampling dates, carbon substrate utilisation in T1 and T2 were lower than
in T4, and utilisation in T1 was lower than in T3, T4 and T5. Over all
treatments, utilisation increased in
the sequence: spring 2008 < spring
2009 < summer 2010.
The higher community metabolic
diversity in the organic compared to
the conventional treatments was
ascribed to respectively higher and
lower soil organic matter in these