Is the microbial community in a soil with reduced catabolic diversity less resistant to stress or disturbance?

Microbial catabolic diversity can be reduced by intensive land-uses, which may have implications for the resistance of the soils to stress or disturbance. We tested the hypothesis that the microbial community in a soil where catabolic diversity has been reduced by cropping is less resistant to increasing stress or disturbance compared with a matched soil under pasture, where catabolic diversity was high. Increasing stress was imposed by reducing pH, increasing salinity (imposed by increasing soil electrical conductivity; EC) or increasing heavy metal contamination in the soils. Disturbance was simulated by a series of wet-dry or freeze-thaw cycles. After incubation of the soils under these regimes, catabolic evenness (a component of microbial functional diversity defined as the uniformity of substrate use) was calculated from catabolic response profiles. These profiles were determined by adding a range of simple C substrates to the soils and measuring shortterm respiration responses. Stress or disturbance caused much greater changes in catabolic evenness in the crop soil (low catabolic evenness) than the pasture soil (high catabolic evenness). Increasing Cu or salt stress caused increases in catabolic evenness at low intensities in both soils, but, in the crop soil, greater stress caused greater declines in catabolic evenness. Declines in pH also caused much greater decreases in catabolic evenness in the crop than the pasture soil. Catabolic evenness initially increased with increasing numbers of wet-dry or freeze-thaw cycles, but after four cycles, evenness declined in both soils. These changes in evenness could be attributed to significant changes (P < 0.05) in most catabolic responses. In contrast, there were generally few changes in microbial biomass C as a result of stress or disturbance treatments. Except for EC stress, all treatments caused slight increases in biomass C at low levels (only significant in the pH and Cu treatments) that subsequently diminished at the highest stress or disturbance levels. Microbial catabolic diversity generally followed the classical 'hump-back' responses of diversity to increasing stress or disturbance. We concluded that reduction in catabolic diversity and changes in soil properties due to land use could reduce the resistance of microbial communities to stress or disturbance. (C) 2001 Elsevier Science Ltd. All rights reserved.