Is the microbial community in a soil with reduced catabolic diversity lessresistant 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 disturba nce. We tested the hypothesis that the microbial community in a soil where catabolic diversity has been reduced by cropping is less resistant to incre asing stress or disturbance compared with a matched soil under pasture, whe re 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 o f the soils under these regimes, catabolic evenness (a component of microbi al functional diversity defined as the uniformity of substrate use) was cal culated 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 cataboli c evenness). Increasing Cu or salt stress caused increases in catabolic eve nness at low intensities in both soils, but, in the crop soil, greater stre ss caused greater declines in catabolic evenness. Declines in pH also cause d much greater decreases in catabolic evenness in the crop than the pasture soil. Catabolic evenness initially increased with increasing numbers of we t-dry or freeze-thaw cycles, but after four cycles, evenness declined in bo th soils. These changes in evenness could be attributed to significant chan ges (P < 0.05) in most catabolic responses. In contrast, there were general ly 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) tha t subsequently diminished at the highest stress or disturbance levels. Micr obial catabolic diversity generally followed the classical 'hump-back' resp onses of diversity to increasing stress or disturbance. We concluded that r eduction in catabolic diversity and changes in soil properties due to land use could reduce the resistance of microbial communities to stress or distu rbance. (C) 2001 Elsevier Science Ltd. All rights reserved.