INFLUENCE OF THE EARTHWORM APORRECTODEA-TRAPEZOIDES (LUMBRICIDAE) ON THE COLONIZATION OF ALFALFA (MEDICAGO-SATIVA L) ROOTS BY RHIZOBIUM-MELILOTI L5-30R AND THE SURVIVAL OF RHIZOBIUM-MELILOTI L5-30R IN SOIL

A greenhouse study was performed to examine the ability of the earthworms Microscolex dubius and Aporrectodea trapezoides to transport Rhizobium meliloti L5-30 R through soil. When R. meliloti L5-30R was inoculated into either ezi-mulch (a pelleted formulation of cereal - pea straw), oat hay, pea hay, or sheep dung and placed on the soil surface together with either A. trapezoides or M. dubius, > 10(4) colony-forming units (CFU) of R. meliloti L5-30 Rg-1 soil were detected at 90 mm soil depth after 18 days. In the absence of earthworms, R. meliloti L5-30R was not detected at 90 mm soil depth after this time. In a second experiment using ezi-mulch as the inoculant material and in the presence of A. trapezoides (in a number equivalent to 471 or 785 m-2), ca. 10(3) CFU of R. meliloti L5-30R per 10 mm of alfalfa roof were detected at 0-30 , 30-60, and 60 - 90 mm soil depth after 18 days, while < 3 CFU were detected per 10 mm of root in the absence of A. trapezoides. In a third experiment in which R. meliloti L5-30R was distributed evenly through soil at the start of the experiment, A. trapezoides (in a number equivalent to 157, 471, or 785 m-2) significantly decreased the survival of L5-30R in soil after 40 days of incubation at 15-degrees-C, but not after 20 days. The decrease in survival of R. meliloti L5-30R was correlated with the density of A. trapezoides. These results demonstrate that A. trapezoides can increase root colonization of alfalfa by R. meliloti L5-30R, but may also reduce the ability of R. meliloti L5-30R to survive in soil.