EARLY CHANGES IN WATER-STABLE AGGREGATION INDUCED BY ROTATION AND TILLAGE IN A SOIL UNDER BARLEY PRODUCTION

Crop rotations and tillage practices can alter the structure of the soil through their effects on soil disturbance and mixing and on organic matter (OM) accumulation and mineralization. The objective of this study was to determine the changes in soil water-stable aggregation induced by 4 yr of different rotation and tillage practices on a Kamouraska clay at La Pocatiere, Quebec. The influence of soil water content (WC) at time of sampling and of different biological and biochemical properties on soil aggregation was also examined. Two rotations (continuous barley (Hordeum vulgare L.) vs. a 2-yr barley-red clover (Trifolium pratense L.) rotation) and three tillage treatments (moldboard plowing (MP) chisel plowing (CP) and no-tillage (NT)) were compared in a split-plot design. Measurements of the size distribution of water-stable aggregates were made on surface (0-7.5 cm) soil samples taken periodically during the 4-yr period. Rotations had no effect on soil aggregation. The mean-weight diameter (MWD) of water-stable aggregates did not vary significantly with time under the NT treatment during the study but decreased significantly under the MP and CP treatments. In the fourth growing season, MWD values in NT plots were on average 40% higher than in MP and CP plots, which were both similar. The larger values in MWD under NT were attributable to a larger proportion of aggregates > 2 mm at the expense of smaller ones (< 1 mm). The MWD data showed important temporal fluctuations, which could in part be explained by the effects of WC at time of sampling. This effect of WC on MWD was less apparent under NT, which suggested that these aggregates were less susceptible to slaking. Among different OM fractions, the proportion of organic C present as hot-water-soluble carbohydrates was best correlated with MWD.