Implications of Soil Biodiversity for Sustainable Organic Matter Management - Sustainable Management of Soil Organic Matter - page 350

This chapter is a short review of how catabolic diversity measurements
can indicate changes in qualitative properties of soil microbial communities
that can signal adverse impacts of land uses on soils, particularly through
effects on organic C cycling. Throughout, I use examples from the litera-
ture and my own unpublished work.

What is Soil Catabolic Diversity and How can it be
Characterized?

The functional diversity of soil microbial communities is broadly consid-
ered to include the range and relative expression of activities involved
in decomposition, nutrient transformations, plant growth promotion/
suppression, plant nutrient acquisition and various soil physical processes
influenced by microbes (Giller et al ., 1997; Wardle et al ., 1999). While
these cannot yet be measured easily and completely in soils, it is possible to
assess the catabolic diversity of heterotrophic microbial communities
involved in decomposition processes. Most soil organisms are hetero-
trophic. Thus changes in the catabolic functioning (catabolic diversity)
of these organisms present a key property for detecting adverse impacts of
land uses on soil microbial communities, and possibly soil functioning. It
must be emphasized, however, that catabolic diversity is only one compo-
nent of the overall functional diversity of soil microbial communities.
Current methods to characterize microbial catabolic diversity are based
on assessing patterns of substrate utilization (functional profiles). These
are determined by adding a specified range of organic compounds to soil
in separate bottles and measuring the short-term respiration responses
(Degens and Harris, 1997; Degens and Vojvodic-Vukovic, 1999). This
approach differs from that used by the Biolog approach (Zak et al ., 1994) in
that it is not culture-based and relies on more direct measurement of
functional responses of the whole soil microbial community. The patterns
of substrate use (termed catabolic response profiles or CRPs) can be easily
compared by calculation of diversity indices, which can capture some of the
characteristics of the functional profiles to enable easy comparison of
this information. Catabolic diversity is comprised of catabolic richness, the
range of substrates metabolized, and catabolic evenness, the variation
between the use of different substrates within a soil. Catabolic evenness
(E) p i 2 ,

where p i = activity of individual substrates as a proportion of total activity
induced by all substrates (Degens et al ., 2000b). Using this formula, the
maximum value of catabolic evenness that can be achieved for a CRP
determined using 25 substrates is 25, and large values indicate high levels of
diversity.

can be described readily using the Simpson index: E = 1/ Σ