Evaluating soil stabilisation by biological processes using step-wise aggregate fractionation

Soil biological processes contribute stability against physical disruption. We present an approach of step-wise fragmentation to assess the role that microbes and organic matter have on soil aggregate stabilisation. Compared to slaking and ultrasound procedures, the approach has a low impact on the microbial biomass. It also does not impose a severe drying stress. Grassland soil was found to be more stable than arable soil. Further examination of the arable soil revealed that increased disruption by shaking caused unstable microaggregates 53-250 mu m in size to fragment, leaving a higher proportion of stable microaggregates in this size range. Carbohydrates, C:N, and basal respiration were found to be higher in the stable microaggregates than the other size fractions. Our results indicate that a distinct size range of soil aggregates exists in which microbial stabilisation dominates. This contradicts other research and questions the usefulness of measuring the biological properties of aggregate size fractions without understanding the physical effects of the fractionation procedure. (C) 2008 Elsevier B.V. All rights reserved.