Bacterial diversity in water-stable aggregates of soils under conventional and zero tillage management

Reduced tillage of agricultural soils has been shown to result in greater macroaggregation, microbial biomass and microbial diversity. While it has been shown that macroaggregates contain more microbial biomass per unit soil mass than microaggregates, it is unclear how microbial diversity varies with soil aggregation. We investigated the functional diversity (catabolic potential) of bacteria, evaluated by calculating Shannon's diversity index (H'), substrate richness (S) and substrate evenness (E) from potential substrate utilization patterns, in whole soil (i.e. not separated into different aggregate sizes) and aggregates of different sizes (2-4, 1-2, 0.5-1, 0.25-0.5, and 0.1-0.25 mm diameter) in loam and silt loam soils grown to barley and managed for 6 years under conventional tillage (CT) or zero tillage (ZT) systems in northern British Columbia. There were no significant tillage effects on bacterial diversity in whole soils. In soil aggregates, H' and E were significantly higher under CT than under ZT on the loam at barley planting time, with no significant aggregate size effects. However, at barley-heading stage, all diversity indices in both soils were significantly higher under ZT than under CT, and they tended to increase with increasing aggregate size. Cluster analysis and principal component analysis of substrate utilization patterns also revealed differences in bacterial community structures between CT and ZT,but the substrates that were utilized differently between the two tillage systems were not the same between soil types or sampling times. The results during the cropping cycle imply that deterioration of soil structure is probably one factor that explains the adverse effects of soil tillage on soil microbial biomass and diversity. (C) 2001 Elsevier Science B.V. All rights reserved.