Pedobiologia 57 (2014) 197–203 Contents lists available at ScienceDirect Pedobiologia - Journal of Soil Ecology j ourna l homepage: www.elsevier.de/pedobi Stable isotope labelling of earthworms can help deciphering belowground–aboveground interactions involving earthworms, mycorrhizal fungi, plants and aphids Andrea Grabmaier a , Florian Heigl a , Nico Eisenhauer b,c , Marcel G.A. van der Heijden d , Johann G. Zaller a,∗ a Institute of Zoology, University of Natural Resources and Life Sciences Vienna, Gregor Mendel Straße 33, A-1180 Vienna, Austria b German Centre for Integrative Biodiversity Research, University of Leipzig, Deutscher Platz 5e, 04103 Leipzig, Germany c Institute for Biology, University of Leipzig, Johannisallee 21, 04103 Leipzig, Germany d Agroscope, Reckenholzstrasse 191, CH-8049 Zurich, Switzerland a r t i c l e i n f o Article history: Received 27 May 2014 Received in revised form 20 October 2014 Accepted 20 October 2014 Keywords: Aboveground–belowground interactions Aphids Arbuscular mycorrhizal fungi Earthworms Multitrophic interactions Stable isotopes a b s t r a c t Functional relationships between belowground detritivores and/or symbionts and aboveground primary producers and their herbivores are not well studied. In a factorial greenhouse experiment we studied interactions between earthworms (addition/no addition of Lumbricus terrestris; Clitellata: Lumbricidae) and arbuscular-mycorrhizal fungi (AMF; with/without inoculation of Glomus mosseae; Glomerales: Glom- eraceae) on the leguminous herb Trifolium repens (Fabales: Fabaceae) and associated plant aphids (Aphis gossypii, A. craccivora; Hemiptera: Aphidoidea). In order to be able to trace organismic interactions, earth- worms were dual-labelled with stable isotopes ( 15 N-ammonium nitrate and 13 C-glucose). We specifically wanted to investigate whether (i) isotopic signals can be traced from the labelled earthworms via surface castings, plant roots and leaves to plant aphids and (ii) these compartments differ in their incorporation of stable isotopes. Our results show that the tested organismic compartments differed significantly in their 15 N isotope enrichments measured seven days after the introduction of earthworms. 15 N isotope incorporation was highest in casts followed by earthworm tissue, roots and leaves, with lowest 15 N sig- nature in aphids. The 13 C signal in roots, leaves and aphids was similar across all treatments and is for this reason not recommendable for tracing short-term interactions over multitrophic levels. AMF symbiosis affected stable isotope incorporation differently in different subsystems: the 15 N isotope signature was higher below ground (in roots) but lower above ground (leaves and aphids) in AMF-inoculated meso- cosms compared to AMF-free mesocosms (significant subsystem × AMF interaction). Aphid infestation was unaffected by AMF and/or earthworms. Generally, these results demonstrate that plants utilize nutri- ents excreted by earthworms and incorporate these nutrients into their roots, leaf tissue and phloem sap from where aphids suck. Hence, these results show that earthworms and plant aphids are functionally interlinked. Further, 15 N-labelling earthworms may represent a promising tool to investigate nutrient uptake by plants and consequences for belowground-aboveground multitrophic interactions. © 2014 Elsevier GmbH. All rights reserved. Introduction In the last decade it has increasingly been recognized that a combined aboveground–belowground approach is necessary to understand the functioning of terrestrial ecosystems (Wardle et al., 2004; van der Putten et al., 2009; Bardgett and Wardle, 2010; Eisenhauer, 2012). Plants thereby play an essential role as they interlink above- and belowground subsystems. Factors above the ∗ Corresponding author. Tel.: +43 1 47654 3205; fax: +43 1 47654 3203. E-mail address: johann.zaller@boku.ac.at (J.G. Zaller). soil surface can directly or indirectly influence the plant itself, but can also affect soil processes and soil organisms that can feed back to plants (Bardgett and Wardle, 2003; Porazinska et al., 2003; Schröter et al., 2004; Wardle et al., 2004; van der Putten et al., 2009). Several studies investigating the functional diversity and multitrophic interactions in terrestrial ecosystems have shown that aboveground–belowground interactions can have consequences at the ecosystem level (Scheu, 2001; Wardle et al., 2004; Megías and Müller, 2010; Eisenhauer and Schädler, 2011; Zaller et al., 2011b; Eisenhauer, 2012; Arnone et al., 2013). However, so far only a few studies have focussed on the effects of belowground detritivores and symbionts on aboveground herbivory (e.g. Poveda et al., 2003; http://dx.doi.org/10.1016/j.pedobi.2014.10.002 0031-4056/© 2014 Elsevier GmbH. All rights reserved.