Vermicomposting is the digestion of organic materials by earthworms which produce excreta known as casts. Edwards (1995) reported that in a Rothamsted study with 25 types of vegetables, fruits or ornamentals, earthworm casts (EW) performed better than compost or commercial potting mixture amendments. It was suggested that the higher crop performance of the cast treatment was due to: better soil physical structure; presence of plant growth hormones; higher levels of soil enzymes; and greater microbial populations. The beneficial effects of earthworm cast utilization in other horticulture settings have also been reported (Tomati et al., 1987, Hidalgo, 1999, Saciragic and Dzelilovic, 1986).

EW typically have high N contents which suggests that they would be good sources of plant N (Parmelee and Crossley, 1988, Ruz-Jerez et al., 1992). Fresh casts often contain high ammonium levels, but rapid nitrification results in stable levels of both nitrogen forms due to organic matter protection in dry casts (Decaens et al., 1999). Nutrients in casts are initially physically protected, but this is reduced as the aggregate structure weakens over time (McInerney and Bolger, 2000). In addition to increased N availability, C, P, K, Ca and Mg availability in the casts is also greater than in the starting feed material (Orozco et al., 1996, Daniel and Anderson, 1992, Lavelle et al., 1992, Basker et al., 1993).

2.1. Soils and materials characterization

The surface horizon of a Nicholville (course-silty, mixed, frigid, Aquic Haplorthod) soil was obtained from the University of Maine Sustainable Agriculture Research Farm in Stillwater, Maine, USA. The soil contained 78% sand, 12% silt and 10% clay fractions as determined by the hydrometer method (Gee and Bauder, 1986). EW of Lumbricus rubellus were obtained from the Cape Cod Worm Farm (Buzzards Bay, Massachusetts, USA). Compost produced from cattle manure, leaves, and food scraps were obtained from the University of Maine Witter Research Farm. The particular feedstock utilized for cast production and composting will influence the specific chemical characteristics of the end products. However, we believe that the materials used in this study are representative of typical EW and compost available to growers. The compost and casts were stored at their native moisture state. The extractable NH4+-N and NO3−-N in the soil and amendment materials were determined by KCl extraction. Five g samples were extracted in 50 ml of 1 M KCl, placed on a reciprocal shaker for 15 min at 200 oscillations min−1. The suspensions were filtered and analyzed for NH4+-N and NO3−-N using an autoanalyzer. Nutrient contents of the amended soil mixtures were determined by extracting 5 g soil with 20 ml of modified-Morgan extract (1.25 M ammoniu…

3.1. Amendment materials

Although the compost material has a higher absolute N and C content than the casts, the C/N ratios were very similar (Table 1). The compost material had a higher level of extractable NH4+ than the casts, but both contained comparable amounts of NO3− (Table 1). The lower levels of NH4+ found in the EW are probably due to the high nitrification rates associated with cast stabilization (Decaens et al., 1999). The compost contained a greater amount of extractable K and lower amount of extractable P than the EW casts.

3.2. Microbial respiration and biomass

The average daily CO2 production is shown in Table 2. The elevated respiration across all treatments at day 3 is most likely due to the stimulation of the soil microbial activity by the greater oxygen availability attributable to physical mixing of the soil and amendments at the start of the experiment. Respiration in the control was relatively stable from day 7 to the end of the experiment. As shown in Table 2, respiration levels for the CP treatment were significantly higher than in the EW treatment for the initial 35 days and they then became statistically equivalent until the end of the incubation. In general, microbial bio…