SOIL STRUCTURAL FORM AND STABILITY, AND ORGANIC-MATTER UNDER COOL-SEASON PERENNIAL GRASSES

Temperate grasses conserve soil by providing vegetative cover and by favoring soil aggregation. A field study was conducted on a Charlottetown fine sandy loam (a coarse-loamy, mixed, frigid Typic Haplorthod) to determine the effect of cultivars of orchardgrass (Dactylis glomerata L.) and tall fescue (Festuca arundinacea Schreb.), in comparison to timothy (Phleum pratense L.), on soil structure and organic matter fractions after 4 yr of growth. Timothy is one of the main grasses grown in the cool humid climate of eastern Canada. Bulk density (1.27-1.32 Mg m-3), pore-size distribution, and shear strength (12.5-15.6 kPa), as a measure of soil structural form, were within the optimum range for this soil type under each grass species. Dry-aggregate mean weight diameter (MWD) was similar (3.66-4.30 mm) among grass species. Wet-aggregate MWD was greater under tall fescue cultivars and 'Farol' timothy (2.81-3.22 mm) compared with the orchardgrass and 'Champ' timothy (2.08-2.36 mm). These differences were also reflected by the difference or ratio of MWD between dry and wet sieving. Aggregate-size distribution indicated that wet-aggregate MWD differences were associated with greater levels of macroaggregates (> 2 mm). Differences in water-stable aggregate stability were not related to differences in plant parameters, organic matter, or organic matter fractions (i.e., microbial biomass C, carbohydrates, and particulate organic matter). These data suggest that organic matter parameters, commonly used to characterize soil stability in cropping systems, are less useful for soils under perennial grass with stable microaggregate structures.