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.