Agriculture Reference
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and ten fungal species, a mixture of three bacterivorous and three
fungivorous nematode species, and a single predatory nematode. The
'simple' systems had the same microbial communities and predator, but the
bacterivore and fungivore levels were restricted to one of the three species
present in the complex systems. This design specifically allows the testing
of the set of diversity-function hypotheses outlined in Table 5.1. Over 20
weeks of incubation, gross evolution of CO
2
from 8 weeks onward was the
same in all systems. During the first 8 weeks, there were sporadic periods
when the diverse system produced more CO
2
than the simple systems, and
one of the simple systems evolved less CO
2
for the first 3 weeks. Microbial
respiration in this particular simple system was also significantly lower than
in the others throughout the experiment. The conclusions from this
decisive study were that the relationships between OM decomposition and
food web diversity were idiosyncratic. This was due to unknown differences
in efficiency in resource utilization and vulnerability to predation,
unpredictable indirect pathways controlling microbivore biomass and
activity, and varying responses to competitive release. The effects of species
diversity
per se
were certainly idiosyncratic, and this was because of subtle
differences in the functional attributes of the species, rather than the basic
fact that they were taxonomically distinct. This suggests that functional
diversity may be a more relevant parameter in relation to soil processes such
as OM dynamics than 'raw' species diversity.
Degens (1998) manipulated the diversity of a grassland soil by fumigat-
ing it with chloroform and then incubating it for 5 weeks either unamended
or following reinoculation with untreated soil. Unfumigated soil provided a
'diverse' control. Species or trophic diversity was not assessed in these soils,
but the functional diversity was measured using an
in situ
catabolic potential
(ISCP) assay (Degens and Harris, 1997). This technique profiles the ability
of the soil community to metabolize 26 defined and contrasting C sub-
strates, and differs crucially from the conceptually similar community-level
physiological profiling (CLPP or Biolog®) technique (Garland and Mills,
1991), in that the substrates are added directly to the soil. The functional
diversity indices for the three treatments were significantly different and
thus provided a gradient of decreasing functional diversity. However, the
microbial biomass was significantly lower in fumigated soils and basal
respiration significantly lower in fumigated uninoculated soil, which may
have confounded diversity effects. There was no consistent relationship
between functional diversity and the decomposition of added wheat straw
over 100 days, and those effects that were detected were conditional on the
moisture potential of the soil, again showing contextual dependency.
Using a deconstructive approach based on partial sterilization, we
produced grassland soils containing different degrees of diversity (Griffiths
et al
., 2000). Soils were fumigated with chloroform vapour for four progres-
sively longer times; it was hypothesized that the longer the fumigation
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