Earthworm secondary production and N flux in agroecosystems: a comparison of two approaches

Production was estimated for Aporrectodea spp. and Lumbricus spp. populations in corn agroecosystems with a 5-year history of manure or inorganic fertilizer applications during 1994–1995 and 1995–1996. Earthworm biomass and production were greater in manure than inorganic fertilizer plots, although biomass and production declined by about 50% between 1994–1995 and 1995–1996 due to unfavorable climatic conditions. Production was highest during the spring and autumn when soil temperatures were between 4 and 22°C. Production was higher in Lumbricus spp. than Aporrectodea spp. populations due to greater Lumbricus spp. biomass. Aporrectodea spp. production was 3.47–16.14 g ash-free dry weight (AFDW) m^–2 year^–1, while Lumbricus spp. production was 6.09–18.11 g AFDW m^–2 year^–1, depending on the fertilizer treatment and the method used to estimate production. However, production estimates from the instantaneous growth rate method were within 27% of the values calculated using the size-frequency method. Nitrogen flux through earthworms was used to estimate efficiency quotients. Net production efficiency (P/A) ranged from 0.64 to 0.76, assimilation efficiency (A/C) ranged from 0.1 to 0.3, and gross production efficiency (P/C) ranged from 0.06 to 0.22. Annual N flux through earthworm populations was higher in manure than inorganic fertilizer plots, and ranged from 2.95 to 5.47 g N m^–2 year^–1 in 1994–1995 and 1.76 to 2.92 g N m^–2 year^–1 in 1995–1996. The N flux through earthworms represented an amount equivalent to 16–30% of crop N uptake during 1994–1995 and 11–18% of crop N uptake during 1995–1996. We concluded that the effects of earthworms on N cycling in corn agroecosystems were substantial, and that N flux through earthworms was influenced significantly by fertilizer amendments. Received: 20 September 1999 / Accepted: 24 March 2000