Non-trophic effects of earthworms on enchytraeids: An experimental investigation

• Non-trophic earthworm effects on enchytraeids were firstly experimentally estimated.
• In general, enchytraeids positively responded to earthworm non-trophic activities.
• Earthworm burrowing and mucus stronger affected enchytraeids than cast addition.
• Earthworm species differed by relative significance of trophic/non-trophic effects.

Lumbricids and enchytraeids are two keystone fauna groups strongly responsible for the functioning of detrital food webs in temperate soils but acting at contrasting spatial scales. The relationships between both groups are poorly understood: considering their close kinship and trophic habits as generally microbi-/saprovores, competitive interactions are expected. However, due to engineering activities, various non-trophic earthworm impacts on enchytraeids also play a role, which has never been evaluated. We experimentally simulated and quantitatively estimated the effects of major non-trophic earthworm activities (burrowing, cast deposition, mucus release) and, in addition, of earthworm mortmass decay on the abundance of enchytraeids. Effects of two earthworm species belonging to epigeic (Lumbricus rubellus) and endogeic (Aporrectodea caliginosa) ecological groups were tested.Non-trophic earthworm effects strongly varied (a) between the types of activity (burrowing, casts, mucus), (b) across the soil profile, (c) between the earthworm species. In general, enchytraeids responded positively to earthworm non-trophic activities. Mucus release and burrowing showed usually stronger impacts than cast deposition. The participation of different non-trophic effects in the resulting joint effect (burrowing + casts + mucus) varied with earthworm species and soil horizon. Decomposition of dead earthworms stimulated enchytraeid abundance at lower, but inhibited it at higher mortmass amount.A comparison between enchytraeid responses to live earthworm (integrated trophic and non-trophic activities) and joint non-trophic (burrowing + casts + mucus) impacts allowed an approximate separation of both activity types. Rates of trophic and non-trophic impacts were quantitatively comparable and earthworm species-specific. In particular, L. rubellus and A. caliginosa earthworms rendered adverse (negative and positive, respectively) influence on enchytraeid abundance. It was concluded that the negative effects of live L. rubellus on enchytraeids were primarily related to its trophic activity, whereas the positive effects of live A. caliginosa were strongly connected with its non-trophic activities.