Implications of Soil Biodiversity for Sustainable Organic Matter Management - Sustainable Management of Soil Organic Matter - page 341

redundancy at the inter-trophic level. Microbial communities in soils from
field trials where plots had been amended with substrates ranging from
manure, pea residues, inorganic N or no addition for 63 years were able to
decompose added manure and pea residues with equal efficiency regardless
of their OM input history (Fauci and Dick, 1994; Burket and Dick, 1996).
Similarly, arable and horticultural soils managed conventionally or organi-
cally containing significantly different levels of OM and having different
biotic properties decomposed Medicago residues equally well (Gunapala
et al ., 1998). However, such field studies are rather blunt tools for looking
at diversity-function relationships.
Using a model inclusive of trophic group diversity and interactions, de
Ruiter et al . (1993) demonstrated the importance of inter-trophic diversity
in relation to N mineralization. They showed that the impact of removal of
a trophic group upon N mineralization considerably exceeded the direct
contribution of that group. In a detailed comparison of four below-ground
food webs, they further demonstrated the importance of interaction
patterns and strengths to community stability (de Ruiter et al ., 1998).
Coûteaux et al . (1991) grew chestnut trees under ambient or elevated
CO 2 atmospheres and obtained litter of contrasting C : N ratios (40 and
75, respectively). They mixed the litter into sterile glass bead microcosms,
and inoculated the systems with organisms to provide a factorial series
of ascending trophic groups, i.e. native microflora community (< 2 m);

native protozoan community; native nematode community; a collembolan
species ( Folsomia candida ); and an isopod ( Oniscus asellus ). Total C miner-
alized from the 'ambient' litter was not affected by the community struc-
ture, but there was a trend of increasing mineralization of 'elevated' litter
with increasing diversity. There was also a general increase in the amount of
leached C where the number of trophic groups increased. The study shows
the context dependency of trophic diversity effects, here dependent upon
the quality of the substrate being decomposed. It also demonstrates that
diversity effects on C losses are not restricted to mineralization to CO 2 . The
variability between replicates was noted to be greater in the more diverse
systems, which leads to the question of whether there are diversity effects
on the consistency of community dynamics and processes. There is evidence
for this diversity-stability relationship in terrestrial ecosystems (King and
Pimm, 1983; Frank and McNaughton, 1991; Tilman and Downing, 1994)
as well as in aquatic microcosms (Naeem and Li, 1997). Even if high
species richness does not always play a significant role in maintaining
ecosystem processes under normal environmental conditions, it may be
important when conditions change (Yachi and Loreau, 1999).
Mikola and Setälä (1998) studied C mineralization from Pinus litter
supplemented with Betula residues. This experiment involved biotically
'complex' and 'simple' systems comprised of three precisely defined trophic
levels. The 'complex' system consisted of a mixture of ten bacterial species

µ