The effects of earthworm functional group diversity on nitrogen dynamics in soils

A novel experimental design, based on the simplex model, was used to study the effects of earthworm functional group diversity on nitrogen dynamics in soils. This mesocosm experiment was carried out at two densities of earthworm and at two levels of food supply. Leachate was collected and analysed over a 20-week period. Soil nitrogen content was measured when the soil columns were destructively sampled on week 30.Results showed the presence of variation among the functional groups in their effects on N dynamics and that both population densities and levels of resource availability were significant. Ammonium concentrations in leachate were generally higher with anéciques and endogées; the opposite was true for nitrate-N, which under certain circumstances was higher with epigées. A significant synergy between the endogées and anéciques was shown in terms of nitrate in soil. Earthworm biomass was important in some instances. For example, larger amounts of soil nitrate were present at higher densities of endogées. A varying effect of food supply was seen for the three functional groups. For example, there were reduced concentrations of nitrate-N in the leachate from the anecic monocultures at low levels of food supply, while increased amounts were leached from the epigées at low food supply. Greater concentrations of ammonium-N were leached from anéciques monocultures at low levels of food supply. Increased food supply resulted in increased amounts of soil nitrate-N in monocultures of both epigées and endogées. It was apparent that nitrogen transformations and amounts available in soil water are dependant on the composition of the earthworm community.