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Doctoral thesis2021Open access

Comparing effects of endogenous and anthropogenic nitrogen supply on ectomycorrhizal fungi

Jörgensen, Karolina


Boreal forests are characterized by strong nutrient limitation and nitrogen is partic-ularly scarce. Ectomycorrhizal fungi acquire nitrogen from the soil in return for car-bon from their tree host and are, thus, pivotal mediators of carbon and nitrogen cy-cling. Anthropogenic activities during the past century have increased the amounts of reactive nitrogen supplied to boreal forest, through atmospheric deposition and fertilization. In this thesis, high-throughput sequencing of fungal communities and measurements of enzymatic activities and fungal biomass were used to assess if and how ectomycorrhizal fungi respond differently to nitrogen added by human activities than to natural variation in ecosystem fertility. The relationship between morpholog-ical characteristics of the ectomycorrhizal fungal mycelium and patterns of soil col-onization was also investigated.

Ectomycorrhizal fungal community composition changed after amendments with anthropogenic nitrogen and along a gradient in natural nitrogen availability, but the effects were small and different in the natural gradient compared to the shifts induced by external N additions. The relative abundance of ectomycorrhizal fungi declined in forests influenced by anthropogenic nitrogen addition, and Cortinarius and Pilo-derma were particularly sensitive. In fertilized Pinus sylvestris forests, the loss of Cortinarius – presumed to be efficient decomposers – was associated with increased organic matter accumulation in the organic topsoil. Further, there were weak con-nections between mycelial morphology and soil foraging patterns. Instead, variation in the proliferation of extraradical mycelium was proposed to be related to differ-ences in carbon use efficiency of ectomycorrhizal fungi.

Anthropogenic nitrogen additions are likely to induce an ectomycorrhizal tipping point, which is likely to depend on reduced host C investment. At this point, ecto-mycorrhizal fungal biomass rapidly decreases and trees may shift towards higher reliance on inorganic nitrogen. Possibly, the ectomycorrhizal community can ini-tially mitigate negative effects of increased anthropogenic nitrogen supply, and maintain nitrogen limitation in the forest, by shifting towards taxa with low carbon demand and high nitrogen retention capacity.


boreal forest; carbon cycling; ectomycorrhiza; ecosystem fertility; ferti-lization; forest management; fungal community; nitrogen cycling; nitrogen deposi-tion; nutrient availability

Published in

Acta Universitatis Agriculturae Sueciae
2021, number: 2021:69
ISBN: 978-91-7760-816-5, eISBN: 978-91-7760-815-8
Publisher: Swedish University of Agricultural Sciences