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

Reduced tillage of agricultural soils has been shown to result in greater m acroaggregation, microbial biomass and microbial diversity. While it has be en shown that macroaggregates contain more microbial biomass per unit soil mass than microaggregates, it is unclear how microbial diversity varies wit h soil aggregation. We investigated the functional diversity (catabolic pot ential) of bacteria, evaluated by calculating Shannon's diversity index (H' ), substrate richness (S) and substrate evenness (E) from potential substra te utilization patterns, in whole soil (i.e. not separated into different a ggregate 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 an d managed for 6 years under conventional tillage (CT) or zero tillage (ZT) systems in northern British Columbia. There were no significant tillage eff ects on bacterial diversity in whole soils. In soil aggregates, H' and E we re significantly higher under CT than under ZT on the loam at barley planti ng time, with no significant aggregate size effects. However, at barley-hea ding stage, all diversity indices in both soils were significantly higher u nder ZT than under CT, and they tended to increase with increasing aggregat e size. Cluster analysis and principal component analysis of substrate util ization patterns also revealed differences in bacterial community structure s between CT and ZT,but the substrates that were utilized differently betwe en the two tillage systems were not the same between soil types or sampling times. The results during the cropping cycle imply that deterioration of s oil structure is probably one factor that explains the adverse effects of s oil tillage on soil microbial biomass and diversity. (C) 2001 Elsevier Scie nce B.V. All rights reserved.