Implications of Soil Biodiversity for Sustainable Organic Matter Management - Sustainable Management of Soil Organic Matter - page 339

Inputs: Soil Biodiversity Effects Upon Primary
Productivity
There are generally no significant direct effects of the soil biota upon pri-
mary productivity since the majority of soil organisms are not carbon-fixing
autotrophs; an exception may be algal mats in some environments, but they
are beyond the scope of this discussion. However, there are many reported
effects of trophic diversity upon plant production that are mediated by
secondary interactions below ground. These predominantly take the form
of interactions between trophic groups in the decomposer food chain,
resulting in enhanced nutrient cycling and thus provision of plant-available
nutrients. Plant growth has been shown to be enhanced by the presence
of grazing nematodes or protozoa in sterile microcosms inoculated with
simple bacterial communities, as well as in soil microcosms (see Griffiths,
1994; Griffiths and Bardgett, 1997). Many experiments have been carried
out manipulating soil fauna using constructive or deconstructive
approaches, and diversity impact upon plant growth has been shown vari-
ously to be positive, negative or neutral, depending upon the circumstances
of the experiment. For example, Setälä (1995) grew birch and pine trees
with mycorrhiza and functionally complex (full set of microbiota and fauna)
or simple (microbes alone) detrital food webs. The biomass of both
seedlings was significantly higher in the complex system, despite faunal
grazing of the mycorrhiza. In a similar experiment, the mycophagous fauna
overgrazed the mycorrhiza and this resulted in decreased plant growth
(Setälä et al ., 1997). In this experiment, greater faunal diversity reduced the
growth of mycorrhizal Pinus seedlings only under N-limiting conditions; in
N-rich systems there was no effect of varying faunal diversity (Setälä et al .,
1997). In a third study, increasing trophic diversity from a single level
(nematodes) to two levels (plus enchytreids/dipteran larvae) or three
levels (plus mites) increased productivity, but increasing the intra-trophic
diversity from one to three species in the second trophic level resulted in
a decrease (Setälä et al ., 1998). These studies show that the status of a
functional group or the consequences of diversity are not pre-determined
but depend strongly upon the biotic and abiotic context in which they are
operating.
Mycorrhizas are clearly an important component of the soil fungal flora
in relation to plant growth, affecting nutrient uptake and competitive ability
of the plants with which they associate. In a particularly rigorous and effec-
tive constructive-type experimental design, van der Heijden et al . (1998)
demonstrated how species richness within this functional group affects the
diversity of plant communities and attendant productivity. They inoculated
macrocosms of sterilized soil with zero, one, two, four, eight or 14 species of
arbuscular mycorrhizal (AM) fungi randomly selected from a pool of 23
species. By using ten random iterations of such mixtures, they were certain