NITROGEN TRANSFORMATIONS ASSOCIATED WITH EARTHWORM CASTS

Earthworms are intimately involved in the cycling of C and N in soil. Earthworm casts are enriched in mineral N; however, there have been few studies of the dynamics of microbial N transformations associated with earthworm casts. We evaluated the N-transformations in earthworm casts as affected by organic residues used as a food source by earthworms. Denitrification rate, nitrification potential and mineral N content of the casts of two earthworm species (Octolasian tyrtaeum Savigny and Aporrectodea tuberculata Eisen) were assessed in laboratory trials. Trials were made in plastic chambers (600 g soil) with three organic-C treatments: 20 g fresh hairy vetch (Vicia villosa Roth), 5.5 g air-dried hairy vetch or 5.5 g air-dried horse (Equus caballus) manure. Earthworm casts were enriched in mineral N, relative to surrounding soil, and that the amount of N accumulated in earthworm casts was a reflection of the N content of the organic matter used as a food source by the earthworms. Casts had elevated denitrification rates, compared to soil, however, rates were low relative to the elevated NO3- concentrations in the casts (80-100 mug NO3--N g-1 dry wt). Observed denitrification rates appeared to be related to the quality of organic matter available to the earthworms, but were not significantly affected by species of worm.