Effects of lumbricids and enchytraeids on nematodes in limed and unlimed coniferous mor humus

Abstract

In a factorial laboratory experiment, specimens of Dendrobaena octaedra (Lumbricidae) and Cognettia sphagnetorum (Enchytraeidae) were added to microcosms with unlimed (pH 4.5) and limed (pH 5.5) coniferous mor humus containing bacteria, fungi, protozoans, and nematodes. Effects on the nematodes were assessed after an incubation period of 207 days at 15°C and a soil moisture content of 60% water-holding capacity. When D. octaedra was absent, nematodes were significantly more abundant in the limed humus than in the unlimed humus. The presence of D. octaedra markedly reduced the number of nematodes in the limed humus but not in the unlimed one, where D. octaedra lost weight and probably did not feed. Most nematodes (92–97%) were bacterial-feeders. The presence of D. octaedra did not decrease the number or biomass of bacteria, indicating that the reduction in nematode numbers was not the result of competition for bacteria between D. octaedra and the nematodes. The presence of C. sphagnetorum had no effect on the nematodes in either of the treatments. We suggest that the reason why D. octaedra, but not C. sphagnetorum, reduced nematode numbers is that the former was more likely to inadvertently ingest the nematodes because of its much greater size. The results provide a possible explanation for the observation that liming sometimes enhances nematode populations, when lumbricids do not respond to the treatment, and sometimes causes decreases, when lumbricids increase in number.