Ability of seedling roots of Lolium perenne L. to penetrate soil from artificial biopores is modified by soil bulk density, biopore angle and biopore relief

To predict root growth in heterogeneous soil, we need to understand when roots will grow into, along and out from continuous biopores. In this study we determined the effect of biopore angle and wall roughness on root penetration into surrounding soil of different bulk densities. Straight 'artificial biopores', 3-mm diameter and 34-mm long, were formed without radial compression in soil compacted to bulk densities of 1.25, 1.38 or 1.50 Mg m^-3. Biopores were inclined at 40° or 90° from the horizontal and their walls scarified or left smooth. Seminal roots of ryegrass (Lolium perenne) were grown for 7 days in the biopores and their penetration of the surrounding soil examined. Few roots (13%) penetrated soil from vertical biopores while most roots (78%) in 40°-from-horizontal biopores grew for only short distances (4-24 mm) before penetrating soil. As soil bulk density increased, the length of roots remaining in 40°-from-horizontal biopores increased, although scarification modified this effect by decreasing the distances roots grew before penetrating soil. Compared with smooth artificial biopores, micrographs of soil surfaces excavated by earthworms (Aporrectodea rosea) showed they were relatively uneven. This study provides a basis for developing simulation models of root growth in heterogeneous soil containing biopores.