Earthworm effects on soil respiration in corn agroecosystems receiving different nutrient inputs

Available evidence suggests that earthworms enhance the mineralization of carbon in soil, but there are few data from field experiments that demonstrate that earthworms increase soil respiration under natural environmental conditions. We measured soil respiration (CO2 flux) during 1993–1994 in 20-m2 field enclosures in which earthworm populations had been decreased, increased, or left unmodified (the latter serving as a control). The enclosures were in corn agroecosystems receiving one of three different nutrients: legume cover crop, cow manure or inorganic fertilizer (NH4NO3). Soil respiration was measured in the enclosures by the static diffusion method. Earthworms had significant effects on soil respiration, but their effects varied seasonally and were influenced by environmental conditions. There were significant differences in respiration rates among earthworm treatments on seven of the 24 sampling dates, and where significant differences did occur, respiration rates were greates in plots with increased populations and lowest in plots with decreased populations. Most of the significant effects of earthworms on soil respiration were observed during the growing season (June–August) of 1994. A severe drought in the summer of 1993 decreased overall respiration rates relative to 1994, and also inhibited earthworm activity. Soil respiration was significantly greater, during the growing season, in the organically-amended plots than in plots treated with inorganic fertilizer; there were no differences in soil respiration among nutrient treatments in the autumn or in the spring before amendments were added. Our results show that earthworms had a significant influence on soil respiration in the field, but that their influence was seasonal, depended on environmental conditions, and was affected by temporal patterns in C supply.