Functional substrate biodiversity of cultivated and uncultivated A horizons of vertisols in NW New South Wales

Concern over the effects of anthropogenic activities on soil ‘quality’ has fuelled efforts to identify and measure those factors that affect soil quality. Soil microbial diversity is one of many possible factors. Our objective was to compare the functional diversity of microbial communities in the A horizons of cultivated and uncultivated vertisols in NW New South Wales. Samples from two cultivated and two uncultivated sites were tested using the community level physiological profiles (CLPP) method. Substrate richness, the rate of substrate use and the diversity of substrate use, as measured by the Shannon index, were significantly larger in the uncultivated sites than in the cultivated sites. The CLPP also indicated a higher rate of substrate use in the uncultivated sites, although this may have been due to greater initial inoculum densities. When diversity values for each site were compared with several soil physical and chemical properties, a relationship between organic carbon and functional diversity was apparent. The fit to a broken-stick model showed that diversity increased up to 1.76% organic carbon and remained constant above that value. The implications of these results for soil quality will depend upon future investigations on the significance of soil microbial diversity as a component of soil quality.