The role of earthworms in managed, permanent pastures in Devon, England

Studies were carried out on experimental pasture plots which are drained or undrained and receiving 0, 200 and 400 kg N ha-1 yr-1 as inorganic fertilizer. Earthworm populations (dominated by Lumbricus rubellus and Aporrectodea caliginosa) were lowest (71.0 +/- 11.4 g m-2) on the undrained 0N plots and highest (100.3 +/- 13.4 g m-2) on the undrained 200N plots. Deep burrowing species appear to be excluded from colonizing the drained plots by the high water-table during winter and spring. Aggregations of earthworms beneath dung pats were recruited laterally peaking at a population equivalent of 417.5 +/- 60 g m-2 after 63 days. Local burrowing activities resulted in increased macroporosity over a period of 105 days. Earthworm casts were found to have 3-5 times higher denitrification rates than surrounding soil with lower endogenous rates of N2O production. Denitrification was determined by cast NO3-N content and fertilizer applications. Earthworm casts may make a significant contribution to the heterogeneity associated with field measurements of denitrification. Lysimeter studies were also carried out using soil monoliths to investigate the effects of earthworms (including Lumbricus terrestris) on nutrient leaching from treatments receiving the equivalent of 200 g N ha-1 as cattle slurry or inorganic fertilizer. Nitrate concentrations in leachates were low in the unfertilized treatments but earthworms increased losses three times. Nitrate losses were higher in the slurry treatments, possibly because of damage to the sward, with proportionally smaller earthworm effects. It is concluded that the moderate population densities of surface active species still have detectable effects on the hydrology and mineral N fluxes in intensively managed pastures although their effects on sward production are negligible in relation to the main plot treatments. Earthworm activity may be an important determinant of heterogeneity in the rates of soil processes.