Free-living nematodes present several characteristics that have led to their use as bioindicators of soil quality. Analyzing the structure of nematofauna is a pertinent way to understand soil biological processes. Earthworms play an important role in soil biological functioning and organic matter dynamics. Their effects on soil nematofauna have seldom been studied. We studied the effect of the tropical endogeic earthworm, Pontoscolex corethrurus, on nematode community structure in a 5-month field mesocosm experiment conducted in Madagascar. Ten different treatments with or without earthworms and with or without organic residues (rice, soybean) were compared. Organic residues were applied on the soil surface or mixed with the soil. The abundance of nematodes (bacterial and fungal feeders) was higher in presence of P. corethrurus than in their absence. The type of plant residues as well as their localisation had significant effects on the abundance and composition of soil nematodes. The analysis of nematode community structure showed that earthworm activity led to an overall activation of the microbial compartment without specific stimulation of the bacterial or fungal compartment. 1. Introduction Soil organisms play a leading role in decomposition and mineralization of organic matter (OM) [1]. They are involved in processes that affect carbon (C) sequestration as well as in the modification of soil physical structure and chemical properties. They also interact with other soil fauna and these interactions result in complex food webs [2]. Nematodes are small organisms (ca. 1？mm long at the adult stage) abundant in soil (several million m ？ 2 soil), they present a high species diversity (about 11,000 species have already been described). Nematodes live in the film of water between soil particles and present various feeding behaviours. During the last twenty years, many studies have been conducted on these microfaunal organisms because they can be an efficient tool to assess soil quality and soil biological functioning [2–4]. Because they are present at different levels of the soil food web and present variable tolerance toward stress, nematofauna provide information about OM decomposition pathways and soil pollution status [3, 5–7]. Nematodes interact with other soil organisms including earthworms, which also play an important role in soil biological functioning and OM dynamics [8, 9]. Until now, studies on interactions between nematodes and earthworms have focused on the contribution of earthworm burrowing and casting activity to nematofauna abundance,
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