Earthworm effects on respiratory activity in a dung-soil system

In many pastures earthworms are crucially important in the disappearance of dung pars, as they are the major agent affecting transport of organic material away from the pat. The aim of this study was to examine to what extent this earthworm mediated process could contribute to the microbial respiration in soil below dung. Plastic pots with soil and cattle dung, added as a small pat to the surface, received zero, one, two, four or eight individuals of detritivorous earthworms. The respiratory activity of soil samples, from the upper 2.5 cm and from 2.5-6.0 cm, were measured as CO2-evolution after 0, 13, 34 and 48 days. After 48 days water- and carbon-content of the soil were also determined. The earthworms had a decisive effect on the respiratory activity of soil from both horizons. The most notable diffferences were found in the upper 2.5 cm, where a gradual increase in respiratory activity was seen during the 48 days when earthworms were present. The effect was more pronounced with four than with two earthworms. After 48 days, the earthworm-biomass of the pot; was positively correlated to the respiratory activity of soil from the upper few cm, as well as to carbon- and water-content of this soil; these two factors were also positively correlated to the respiratory activity. A reasonable explanation for this is that the earthworms deposit consumed dung, as casts, in the soil, notably in the upper 2 cm, thereby increasing carbon- and water-content of the soil, which again affect respiratory activity. The results of path-analyses suggests that the earthworms also have a separate: effect on soil respiration, which is not related to their deposition of carbon and conservation of water in the soil. (C) 1997 Elsevier Science Ltd.