Marie Merrild:
Responses of mycorrhizal fungi and other rootassociated fungi to climate change

Date: 04-10-2013    Supervisor: Anders Michelsen and Rasmus Kjøller

Climate change is expected to affect many terrestrial ecosystem processes. Mycorrhizal fungi are important to soil carbon (C) and nutrient cycling thus changes in abundance of mycorrhizal fungi could alter ecosystem functioning. The aim of the present thesis was therefore to investigate responses of mycorrhizal fungi to climate change in a seasonal and long-term perspective.

Effects of elevated CO2 (510 ppm), night-time warming and extended summer drought were investigated in the long-term field experiment CLIMAITE located in a Danish semi-natural heathland. Mycorrhizal colonization was investigated by microscopy of roots from four sampling events during a year and from ingrowth cores. Ingrowth cores were also used for investigation of root production and nutrient uptake. Treatment effects on development of external mycelium and soil structure were investigated after four series of six months incubations of mesh bags at CLIMAITE. I analysed soil structure as dry aggregate size distributions. To further characterize the ecological role of less described fungal isolates I also carried out a mesocosm experiment in which plant 15N uptake was compared in different plant-fungus combinations at two temperature levels.

Colonization by arbuscular mycorrhizal (AM) fungi increased under elevated CO2 and warming in spring while ericoid mycorrhiza (ErM) colonisation decreased in response to drought and warming. Increased AM colonization correlated with higher phosphorus and nitrogen root pools. Dark septate endophyte (DSE) colonization did not change in the full treatment combination, mimicking the future climate scenario because a positive effect of CO2 was counteracted by negative effects of drought and warming. External AM mycelium did not respond to elevated CO2 and AM abundance did not correlate with the proportion of macro aggregates, instead soil aggregation correlated with carbon concentrations and soil water content.

Mycorrhizal and DSE colonization responded strongly to the climate change treatments. Higher AM colonization under elevated CO2 could support increased root growth, thereby contributing to increased C sequestration in the ecosystem. Lower ErM colonization did not seem to affect root production or root nutrient uptake, possibly due to the nitrogen saturated status of the ecosystem. In the mesocosm experiment under nutrient limitation, DSE fungi had similar effects as some of the ErM on plant growth and nitrogen uptake. In the field experiment, treatment responses of DSE colonization reflected plant responses suggesting that