BACKGROUND: Ectomycorrhizal fungi (ECM) play a central role in nutrient cycling in boreal and temperate forests, but their role in the soil food web remains little understood. One of the groups assumed to live as specialised mycorrhizal feeders are Protura, but experimental and field evidence is lacking. We used a combination of three methods to test if Protura are specialized mycorrhizal feeders and compared their trophic niche with other soil invertebrates. Using pulse labelling of young beech and ash seedlings we analysed the incorporation of 13C and 15N into Acerentomon gallicum. In addition, individuals of Protura from temperate forests were collected for the analysis of neutral lipid fatty acids and natural variations in stable isotope ratios. RESULTS: Pulse labelling showed rapid incorporation of root-derived 13C, but no incorporation of root-derived 15N into A. gallicum. The transfer of 13C from lateral roots to ectomycorrhizal root tips was high, while it was low for 15N. Neutral lipid fatty acid (NLFA) analysis showed high amounts of bacterial marker (16:1ω7) and plant marker (16:0 and 18:1ω9) fatty acids but not of the fungal membrane lipid 18:2ω6,9 in A. gallicum. Natural variations in stable isotope ratios in Protura from a number of temperate forests were distinct from those of the great majority of other soil invertebrates, but remarkably similar to those of sporocarps of ECM fungi. CONCLUSIONS: Using three in situ methods, stable isotope labelling, neutral lipid fatty acid analysis and natural variations of stable isotope ratios, we showed that Protura predominantly feed on mycorrhizal hyphae via sucking up hyphal cytoplasm. Predominant feeding on ectomycorrhizal mycelia by Protura is an exception; the limited consumption of ECM by other soil invertebrates may contribute to carbon sequestration in temperate and boreal forests.

Büsgen Institute Forest Botany and Tree Physiology University of Göttingen Büsgenweg 2 37077 Göttingen Germany

Institute of Systematics and Evolution of Animals Polish Academy of Sciences ul Slawkowska 17 31 016 Krakow Poland State Museum of Natural History Ukrainian Academy of Sciences Teatral'na St 18 L'viv UA 79008 Ukraine Institute of Soil Biology Biology Centre of Czech Academy of Sciences Na Sádkách 7 370 05 České Budějovice Czech Republic

J F Blumenbach Institute of Zoology and Anthropology Animal Ecology University of Göttingen Untere Karspüle 2 37073 Göttingen Germany

J F Blumenbach Institute of Zoology and Anthropology Animal Ecology University of Göttingen Untere Karspüle 2 37073 Göttingen Germany Centre of Biodiversity and Sustainable Land Use University of Göttingen Von Siebold Str 8 37075 Göttingen Germany

J F Blumenbach Institute of Zoology and Anthropology Animal Ecology University of Göttingen Untere Karspüle 2 37073 Göttingen Germany Russian Academy of Sciences A N Severtsov Institute of Ecology and Evolution Leninsky Prospect 33 Moscow 119071 Russia

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