Implications of Soil Biodiversity for Sustainable Organic Matter Management - Sustainable Management of Soil Organic Matter - page 340

of testing for a genuine AM species richness effect. The macrocosms were
then sown with a uniform mixture of 15 characteristic old-field plant spe-
cies and the resultant plant communities allowed to grow for one season.
There was a clear increase in both root and shoot biomass with increasing
AM species diversity, as well as an increase in the plant community diver-
sity. Since the diversity component in this experiment was defined precisely
and covered a fairly wide numeric range, it was possible to construct
well-defined diversity-function response curves, which is very rare in soil
diversity studies. The shapes of the curves suggest a rivet-style relationship
at low diversity with a tendency to redundancy-style at higher levels. There
was a suggestion that shoot and root biomass showed different degrees of
sensitivity to changing diversity, with the curves being less steep for roots.
Turnover and Losses: Diversity Effects Upon OM
Decomposition and C and N Mineralization
In an early study on the effects of microbial diversity on soil respiration,
Salonius (1981) established soil microcosms containing different levels
of bacterial and fungal diversity by reinoculating sterilized soil with
progressively diluted non-sterile soil suspensions. The concept here was
that more dilute suspensions would contain lower diversity since rare forms
would be diluted to extinction sooner. More dilute suspensions would also
contain fewer organisms but, by allowing recolonization for 5 months, the
resultant communities in the soils comprised a broadly similar number
of bacterial and fungal colony-forming units. Diversity was not measured
rigorously but indications were that the concept was applicable and
species richness declined with dilution. Communities derived from the
lower dilutions respired at similar rates and there was a distinctive drop in
respiration rate at and beyond certain higher dilutions. This was more
pronounced with soils taken from under feather moss than sphagnum
moss. Thus there is some evidence that there may be threshold levels of
diversity below which OM decomposition is impaired.
As with many of the primary productivity experiments alluded to
above, the impact of trophic group diversity upon OM decomposition and
C mineralization has been shown to have varying effects, and to be context
dependent.
Swift et al . (1998) reiterate that there is little unequivocal evidence
of a direct causal linkage between soil biodiversity and nutrient cycling
efficiency. They discuss a correlative approach where land use change in
Nigeria from bush to cultivation resulted in a decline in abundance and
diversity of soil fauna but this resulted in little change in overall decomposi-
tion rates of surface litter; here the microbial pool appeared to compensate
for the decline in faunal diversity, and is perhaps an unusual example of