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

Temperate grasses conserve soil by providing vegetative cover and by f avoring soil aggregation. A field study was conducted on a Charlotteto wn 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 t imothy (Phleum pratense L.), on soil structure and organic matter frac tions after 4 yr of growth. Timothy is one of the main grasses grown i n 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 t his soil type under each grass species. Dry-aggregate mean weight diam eter (MWD) was similar (3.66-4.30 mm) among grass species. Wet-aggrega te 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 ra tio of MWD between dry and wet sieving. Aggregate-size distribution in dicated that wet-aggregate MWD differences were associated with greate r levels of macroaggregates (> 2 mm). Differences in water-stable aggr egate stability were not related to differences in plant parameters, o rganic matter, or organic matter fractions (i.e., microbial biomass C, carbohydrates, and particulate organic matter). These data suggest th at organic matter parameters, commonly used to characterize soil stabi lity in cropping systems, are less useful for soils under perennial gr ass with stable microaggregate structures.