Influence of Lumbricus terrestris inoculation on green manure disappearance and the decomposer community in a walnut orchard

Nightcrawler (Lumbricus terrestris L. [Oligochaeta: Lumbricidae]) is an anecic earthworm typically lacking from Persian walnut (Juglans regia L.) orchards in northern California, yet this earthworm may be established by inoculative release. Our purpose of was to determine the effect of L. terrestris inoculation on green manure incorporation, in this case woolypod vetch (Vicia villosa Roth ssp. varia [Host] Cabriere), and on the decomposer community. An experiment using controlled-environment microcosms showed that disappearance of surface-applied vetch residue was increased by the addition of L. terrestris compared to a resident earthworm complex consisting of the endogeic Lumbricidae Aporrectodea sp. prob. trapezoides (Duges) and Allolobophora chlorotica (Savigny) as well as the epigeic Eisenia foetida (Savigny). An experiment conducted in an organic Persian walnut orchard during the spring of 1996 also showed that disappearance of surface-applied vetch residue was increased in plots that had been inoculated with L. terrestris. However, an additional study conducted in the same orchard during the summer of 1998 showed similar rates of vetch biomass disappearance in plots with or without inoculated L. terrestris. Colonization by L. terrestris did not lead to significant differences in densities or live biomass for resident earthworms in this orchard. However, night observations showed significantly greater densities of isopod Crustacea (Isopoda: Oniscidae) in plots without L. terrestris, suggesting possible interference between L. terrestris and the isopods.