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period, the greater and wider the kill of organisms. Biomass reductions were
compensated for by allowing recolonization for 5 months after fumigation.
Detailed analysis of nematode, protozoan, bacterial and fungal communi-
ties confirmed that diversity within and between these trophic groups had
been reduced progressively by increasing fumigation time. No larger fauna
survived even short fumigation. Microbial biomass and basal respiration in
the soils were broadly similar. The ability of these soils to decompose added
plant residues was assessed using isotopically labelled material. The two
least diverse soils decomposed added grass residue significantly faster than
the two most diverse (Fig. 5.1a): this effect was observed in a replication
of the experiment using soils sampled from the same field 1 year later
(Fig. 5.1b).
Diversity Effects on Resilience
Whilst it can be hypothesized that more diverse systems have greater resil-
ience (Giller et al ., 1997), we are unaware of any studies that have studied
the phenomenon explicitly. Using the soils described in the previous sec-
tion, the effects of diversity on the resilience of the decomposition process
were studied by subjecting them to additional stresses and measuring their
subsequent ability to decompose ryegrass residues (Griffiths et al ., 2000).
Addition of copper, a persistent stress, to the soils resulted in an immediate
decline in their ability to decompose the residue, and there was a distinct
trend of greater susceptibility (i.e. greater decline in decomposition relative
to the control) to copper with decreasing diversity (Fig. 5.2a). However,
after a further 16 days, the decomposition rate in the most diverse soil had
virtually reverted to the rate in the control soil, and after 63 days showed an
enhanced decomposition ability. The less diverse soils showed no such
recovery. Subjecting the soils to a heat shock (37
C for 18 h), a transient
stress, produced a different response. Here, the least diverse soils were less
susceptible to the stress, but the most diverse soils showed a faster recovery
rate and hence greater resilience (Fig. 5.2b). The experimental approach
suffers from the fact that fumigation itself represents a stress and may have
selected for particular attributes in the surviving and resultant community,
but nonetheless demonstrates that a general reduction in biodiversity may
have implications for the susceptibility and resilience of soils to stresses.
°
Conclusions
We consider that under most non-extreme conditions, the biodiversity of
soils is probably not the primary regulator of soil OM dynamics. This is
not because diversity has little impact on C dynamics, but rather that the
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