Fungal stable isotope compositions reflect contrasting nutrient-cycling dynamics in ecto- vs. arbuscular mycorrhizal-associated forests

Miss Saskia Klink1, Prof. Johanna Pausch1

1University Of Bayreuth, Agroecology, Bayreuth, Germany

Introduction: Increasing evidence indicates that nitrogen dynamics in forest soils depend on the associated mycorrhizal type. For temperate forests, two main mycorrhizal types are distinguished: ectomycorrhiza (EcM) and arbuscular mycorrhiza (AM). Due to differing nutrient dynamics, higher quality of AM litter, faster N cycling rates and lower C stocks in AM-associated systems compared to EcM are predicted. However, the direct impact of mycorrhizal fungi on C and N cycling in forest ecosystems remains elusive.

While EcM fungi release exo-enzymes to actively decompose SOM, AM fungi may activate saprotrophic microorganisms and scavenge for nutrients released. We hypothesize that different N cycling rates and the accessibility of SOM pools with distinct turnover times lead to contrasting isotopic compositions of AM and EcM fungi in forest ecosystems.

Methods: Stable isotope natural abundance measurements were performed on fruiting bodies of EcM and saprotrophic fungi (SAP), AM hyphae, plant shoot and root material, mineral N and SOM fractions with varying turnover time. Stable hydrogen (²H), carbon (¹³C), nitrogen (¹⁵N) and oxygen (¹⁸O) isotopes, C/N ratios and C and N contents were determined. Fractionation factors between fungal tissue, autotrophic reference plants and various SOM fractions were calculated.

Outcome: The specialization of mycorrhizal fungi on specific nitrogen sources and their effect on SOM decomposition results in nutritional differences and distinct isotopic signatures of AM and EcM fungi. To recent knowledge, the production of exo-enzymes and assessment of more stable SOM fractions by many EcM and SAP fungi results in their 3-5 ‰ ¹⁵N-enrichment compared to reference plants. AM fungi lacking these enzymes show ¹⁵N values between plants and EcM/SAP fungi, indicating their utilization of more labile compounds.

Overall, our study indicates that the isotopic composition of mycorrhizal fungi reflects different nutrient-acquisition strategies, with a stronger ¹⁵N-enrichment for actively SOM-decomposing fungi and a ¹⁵N-depletion for passively SOM-decomposing fungi.

Biography: Saskia Klink is a graduate student at the department of Agroecology at the University of Bayreuth, Germany. After graduating as medical-technical assistant, she got a B.Sc. in Biology at the University of Duisburg-Essen and a M.Sc. in Biodiversity and Ecology at the University of Bayreuth.

Her research focusses on the impacts of mycorrhizal types on SOM decomposition and nutrient cycling. The use of stable isotopes under laboratory and field conditions is a main approach applied in her studies elucidating nutrient allocations. Her projects comprise a range from small microcosm studies up to field experiments, aiming to achieve deeper knowledge about priming mechanisms under different mycorrhizal types and the impact of mycorrhizas on nutrient and carbon ecosystem cycling.


7th International Symposium
Soil Organic Matter

6 – 11 October 2019

Hilton Adelaide

Adelaide, South Australia


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