Relationships between earthworm populations and management intensity in cattle-grazed pastures in Ireland

Earthworm (Lumbricidae) populations were assessed in three replicated, field experiments with different management intensities conducted on grassland research farms on medium-textured mineral soils, located at Solohead (Co. Tipperary), Grange (Co. Meath) and Johnstown Castle (Co. Wexford), Ireland. The experiment at Solohead involved four levels of mineral fertilizer application (80, 175, 225 and 350 kg N ha−1), and the plots were rotationally grazed by dairy cows. Three levels of mineral fertilizer application were compared at Johnstown Castle (0, 225 and 390 kg N ha−1) and two at Grange (100 and 225 kg N ha−1), and the plots at these sites were rotationally grazed by suckler cows and followers or dry cattle. Between 10 and 15 earthworm species were recorded per site, with Allolobophora chlorotica and Aporrectodea caliginosa being dominant. Other abundant species included Aporrectodea rosea, Aporrectodea longa, Lumbricus terrestris, Lumbricus friendi, Lumbricus festivus, Lumbricus rubellus and Satchellius mammalis. Mean earthworm population densities per treatment ranged from 203 to 324 individuals m−2 and biomass from 60 to 176 g m−2. ANCOVA revealed significant overall positive relationships between management intensity (as indicated by N application rate) and earthworm abundance (F = 4.47, p < 0.05) and biomass (F = 17.32, p < 0.001). While there was considerable variation in earthworm response to treatments both within and between sites, populations were not adversely affected by intensive management in any of the sites. It appears that any adverse effect of trampling at higher stocking rates was more than compensated for by increased food supply in the form of dung and plant litter. The presence of high earthworm populations is potentially an important factor in counteracting adverse impacts of intensive grazing on pasture soils, especially on heavy soils liable to poaching.