Water-stable aggregation of Quebec silty clay soils: some factors controlling its dynamics

Despite a relatively high organic matter content, the heavy soils of the St-Lawrence lowlands of eastern Canada are susceptible to structural degradation in the form of compaction, surface crusting and sheet erosion. The resistance of these soil to degradation is largely controlled by the presence of stable macroaggregates. Several agronomic studies involving various combinations of management options such as organic additions, crop rotations and reduced tillage have allowed to intensively monitor the changes in aggregation of these soils through time. The proportion of water-stable macroaggregates (>1 mm) showed large temporal variation. Short-range variations (days or weeks) were attributable to climatic conditions and tillage operations as revealed by significant correlations between soil water content and aggregation within a growing season. Longer-term trends (>2 years) were a function mostly of carbon (C) management i.e. placement or location of the residues in the soil profile, and the amount and quality of the residues. The effect of C management was reflected by correlations between aggregation, C inputs, soil C content and organic matter (OM) fractions. Based on a study of the OM associated with the primary particles it is proposed that the clay and silt particles of these soils are relatively saturated in OM which leads to highly stable microaggregation. Any additional OM accumulation in the surface soil provided by reduced- or no-till conditions, perennial forage crops or manure additions would lead to increased macroaggregation and be related to the accumulation of particulate OM. This hypothesis helps to explain the relatively high responsiveness of soil structure, and in particular stable macroaggregation, to changes in cropping systems in these soils. (C) 1998 The Department of Agriculture and Agri-food. Published by Elsevier Science B.V. All rights reserved.