Earthworm coenoses in wet grassland of Northwest-Germany. Effects of restoration management on a Histosol and a Gleysol

The present study is part of a grassland research project on the effects of different extensive management practices and natural succession on vegetation and soil ecological processes at nine wet grassland sites in Northwest-Germany. This paper presents the results of investigations on earthworms at two of these sites, a Histosol and a Gleysol. The investigations on earthworms were carried out in 1993 and 1999. The effect of the management practice “mowing twice a year” is compared to the influence of “natural succession” without human interference. In 1993, the number of species varied between four on the mowed plot in the Gleysol and six on the succession plot in the Histosol. Six years later, the number of species had increased to seven on the mowed plot in the Gleysol and to nine on the succession plot in the Histosol due to the occurrence of small species typical for wet soils. This can be attributed to re-wetting since 1986. Ten years after extensive management practices, mature individuals of the epigeic species Lumbricus rubellus were not present in the Gleysol. Adults of the endogeic species Aporrectodea rosea were found only in the Histosol and those of Allolobophora oculata only in the Gleysol. In 1999, abundances of earthworms in the Histosol were two-fold higher compared to population numbers from other investigations on wet grassland sites, but worm biomass reached similar values. In the Gleysol, the range of earthworm abundance was similar to results reported for wet grassland sites by other authors. At present, there was no significant effect of the management practices on earthworm density and biomass, neither in the Histosol nor in the Gleysol. However, in case of mowing earthworm biomass is higher in the Histosol than in the Gleysol. These observations as well as the shift in species related to wet soil conditions provide evidence that for our study sites the soil moisture regime seems to be the key factor that regulates most soil processes and controls earthworm populations. Due to high groundwater tables and re-wetting effects anecic species such as Lumbricus terrestris, which play an important role in bioturbation in dry grassland sites, are lacking in the investigated wet grassland sites. Moreover, the species spectrum of wet grassland sites is characterised by a shift to species that are indicators of wet soil conditions such as Eiseniella tetraedra. Even though evidence for an environmental change is not yet visible in the humusprofile, for humus form descriptions, the earthworm coenoses provide a valuable early indicator for an environmental change, especially with respect to moisture regime.