2075 Reports Ecology, 86(8), 2005, pp. 2075–2082  2005 by the Ecological Society of America APPLICATION OF LIPID ANALYSIS TO UNDERSTAND TROPHIC INTERACTIONS IN SOIL LILIANE RUESS, 1,4 KIRSTEN SCHU ¨ TZ, 1 DOMINIQUE HAUBERT, 1 MAX M. HA ¨ GGBLOM, 2 ELLEN KANDELER, 3 AND STEFAN SCHEU 1 1 Institute of Zoology, Technical University of Darmstadt, Schnittspahnstraße 3, 64287 Darmstadt, Germany 2 Department of Biochemistry and Microbiology, Cook College, Rutgers University, 76 Lipman Drive, New Brunswick, New Jersey 08901-8525 USA 3 Institute of Soil Science, University of Hohenheim, Emil-Wolff-Straße 27, 70599 Stuttgart, Germany Abstract. Trophic interactions in cryptic belowground systems are difficult to assess, either experimentally or by direct observation. We used lipid analysis to determine feeding strategies in Collembola raised on various bacterial, fungal, plant, or nematode diets. Dietary fatty acids (FAs) were conserved and transferred through the trophic cascade into the neutral lipids of consumers. The presence of vaccenic type FAs (7 family) was indicative of a bacterial diet in general. More specifically, methyl-branched (iso, anteiso) and cyclic forms of FAs were markers for consumption of gram-positive and gram-negative bacteria, re- spectively. Fungal-feeding Collembola comprised a higher proportion of linoleic acid, whereas the profile of plant feeders showed an increased amount of oleic acid. The FA 20:1 9 was only present in Collembola with nematodes as prey. Based on the assigned marker FAs, field populations of Collembola in three deciduous forest stands were ascribed to feeding guilds (i.e., fungivores, bacterivores, herbivores, predators). In conclusion, FA biomarkers provide a high-resolution method to define feeding strategies of decomposer invertebrates and to determine their diets in situ. Lipid analysis has considerable potential as a new tool in soil food web studies. Key words: biomarker; Collembola; fauna; feeding guild; food web; lipids; soil; trophic transfer. INTRODUCTION The soil micro- and mesofauna, in particular the mi- crobial grazers, are key components of the food web, and thus they are important determinants for the energy and carbon flow through terrestrial systems (e.g., Ver- hoef and Brussard 1990, Lee and Pankhurst 1992, Bengtsson et al. 1996, Scheu and Seta ¨la ¨ 2002). Re- gardless of their significance, little is known about the feeding strategies of soil animals. Due to their small size and the cryptic habitat, trophic interactions are difficult to address, either experimentally or through direct observation. Therefore, the assigned feeding guilds often reflect more taxonomic rather than func- tional relationships. Recently, stable isotope analyses ( 13 C/ 12 C, 15 N/ 14 N) have been used to determine diet and trophic levels of soil invertebrates (Ponsard and Arditi 2000, Scheu and Falca 2000, Schmidt et al. 2004). However, this technique has restrictions, as it cannot distinguish between food sources with similar isotopic ratio. This drawback does not apply to biochemical components specific to living organisms such as fatty acids (FAs). Manuscript received 8 September 2004; revised 13 January 2005; accepted 11 February 2005; final version received 22 Feb- ruary 2005. Corresponding Editor (ad hoc): K. M. Scow. 4 Present address: Institute of Soil Science, University of Hohenheim, Emil-Wolff-Straße 27, 70599 Stuttgart, Germa- ny. E-mail: ruess@uni-hohenheim.de Lipids play a vital role in animals both as a source of energy (neutral lipid fatty acids, NLFAs) and as important structural components of cell membranes (phospholipid fatty acids, PLFAs). Energetically, it is more efficient to incorporate dietary FAs without mod- ification into body tissue, which, in turn, leads to a lipid pattern in the consumer that reflects the compo- sition of its diet (Stott et al. 1997). For marine food chains, Lee et al. (1971) were the first to demonstrate the trophic transfer of ingested diatom fatty acids into the wax esters of boreal pelagic copepods. Thereafter, lipid analysis of zooplankton provided much insight into diet history and feedings strategies (Ederington et al. 1995, Kattner et al. 2003, Stevens et al. 2004a, b). In soil, biomarker PLFAs have been applied to assess community composition, physiological status, and bio- mass of microorganisms (Frostega ˚rd and Ba ˚a ˚th 1996, White et al. 1996, Olsson 1999, Zelles 1999). Trophic transfer of fungal FAs to their nematode or Collembola grazers was only recently reported (Ruess et al. 2002, 2004, 2005, Chamberlain et al. 2004). Dietary routing of FAs occurred predominantly into the NLFA fraction of the consumers and was apparent across three trophic levels (Ruess et al. 2004). The present work investigates the effect of different food sources on the lipid composition of Collembola, a major decomposer group in soil. FAs indicating spe- cific diets (i.e., bacteria, fungi, animals, plant litter) were determined, and the data used to infer ingestion