Dual amino acid stable isotope analyses (13C and 15N) as a novel method to unravel energy fluxes and trophic relationships in soil animal food webs
Final Report Abstract
Within this project, I successfully established the dual compound-specific isotope analysis of amino acids as a new tool to gain more detailed insights into structure and basal resources of soil animal food webs. The method comprises two approaches. The first, termed ‘δ13C fingerprinting’, uses δ13C signatures of essential amino acids to estimate relative proportions of bacteria, fungi and plants as basal resources in consumers. The second approach uses δ15N signatures of ‘trophic’ and ‘source’ amino acids to calculate precise trophic positions of consumers. In cooperation with Dr. Thomas Larsen, who coined the ‘δ13C fingerprinting’ approach, I established the extraction and derivatization process in our laboratory in Göttingen (Prof. Stefan Scheu, Animal ecology). With Dr. Jens Dyckmans, we established the compound-specific δ13C and δ15N measurements of amino acids on a coupled system with a gas chromatograph and an isotope ration mass spectrometer at the ‘Centre for Stable Isotope Research and Analysis’ in Göttingen. In a controlled feeding study I tested and refined both the δ13C fingerprinting approach and the calculation of trophic positions for soil food webs. By constructing food chains with bacteria, fungi and plants as basal resources, springtails and oribatid mites as primary consumers, and spiders and gamasid mites as predators, I could show that in the fingerprinting approach, consumers of the first and second trophic level grouped close to their basal resource, suggesting that the method is suitable to trace contributions of basal resources. When calculating trophic positions with δ15N signatures, the universally used trophic enrichment factor per trophic transfer was valid for primary consumers, but was considerably lower for arachnid predators, suggesting that for this group, lower trophic enrichment factors have to be used in field studies, presumably due to a different feeding mode (fluid-feeding) and type of excretion. Using the δ13C fingerprinting approach, I was able to separate ectomycorrhizal and saprotrophic fungi and their consumers. For the first time, this allowed to separate the relative contribution of these fungal functional groups to the diet of soil animals. In a subsequent field study I could show that abundant species of springtails and oribatid mites, important soil mesofauna decomposers, mainly rely on saprotrophic and not on ectomycorrhizal fungi as diet. This was the first direct evidence that ectomycorrhizal fungi do not play a major role for the nutrition of soil animals and this was published in addition to the original article also with a photograph in the photogallery of the Bulletin of the Ecological Society of America (https://esajournals.onlinelibrary.wiley.com/doi/10.1002/bes2.1869). The use of compound-specific amino acid analyses represents a great advantage for soil animal food webs that rely on detritus as a mixture of plant material, fungi and bacteria as basal resource and wherein trophic relationships are hidden from direct observation and hard to disentangle. The method is now being increasingly used to investigate trophic relationships in laboratory and field studies and across different systems such as temperate and tropical forests. It holds great promise to gain detailed insights into food web structure and energy fluxes and therefore enables to sensitively trace changes as e.g. caused by land-use/management.
Publications
- 2019. Combining bulk and amino acid stable isotope analyses to quantify trophic level and basal resources of detritivores: a case study on earthworms. Oecologia 189, 447–460
Potapov, A.M., Tiunov, A. V., Scheu, S., Larsen, T., Pollierer, M.M.
(See online at https://doi.org/10.1007/s00442-018-04335-3) - 2019. Compound-specific isotope analysis of amino acids as a new tool to uncover trophic chains in soil food webs. Ecological Monographs 89, e01384
Pollierer, M.M., Larsen, T., Potapov, A., Brückner, A., Heethoff, M., Dyckmans, J., Scheu, S.
(See online at https://doi.org/10.1002/ecm.1384) - 2020. Isotope analyses of amino acids in fungi and fungal feeding Diptera larvae allow differentiating ectomycorrhizal and saprotrophic fungi‐based food chains. Functional Ecology 34, 2375–2388
Pollierer, M.M., Scheu, S., Tiunov, A. V.
(See online at https://doi.org/10.1111/1365-2435.13654) - 2021. Soil decomposer microarthropods predominantly feed on saprotrophic rather than ectomycorrhizal fungi. Bulletin of the Ecological Society of America 102, e01869
Pollierer, M.M., Scheu, S.
(See online at https://doi.org/10.1002/bes2.1869) - 2021. Stable isotopes of amino acids indicate that soil decomposer microarthropods predominantly feed on saprotrophic fungi. Ecosphere 12
Pollierer, M.M., Scheu, S.
(See online at https://doi.org/10.1002/ecs2.3425)
