Ectomycorrhizal Fungal Mycelial Dynamics and its Role in Forest Soil Carbon Cycling

Sammanfattning: Most boreal tree species rely on root-associated ectomycorrhizal fungi for nutrient acquisition, in exchange the trees allocate part of their photosynthetically fixed carbon (C) to these fungi. This has a feedback on soil C dynamics as mycorrhizal fungi are important in regulating soil C cycling and storage. However, mycorrhizal fungi are often not included in C dynamics models, as mechanistic understanding of their contribution to mycorrhiza-mediated processes are largely lacking. The aim of the work described in this thesis was to address this knowledge gap by studying the contribution of mycorrhizal fungi in regulating soil C fluxes. This was done by quantifying C fluxes associated with the extraradical mycelium (ERM) of mycorrhizal fungi, and examining how the ERM contributes to variations in soil C cycling along a nemoboreal chronosequence of managed Pinus sylvestris forests. Production and turnover of ERM was quantified by determining ERM biomass in sequentially harvested ingrowth mesh bags and by mathematical modelling. Respiration of ERM was measured as CO2 efflux from mesh bags, and carbon use efficiency (CUE) was calculated from ERM production and respiration rates. We assessed soil fungal communities along the chronosequence and investigated correlations between taxonomic composition and enzyme activities. The ERM standing biomass increased despite decreased production along the chronosequence. This contradiction was explained by a drastic decline in biomass turnover, from seven times to one time per year. The CUE decreased with forest age, but increased tenfold from summer (0.019) to autumn (0.200). This seasonal increase in CUE was associated with a decline in gross photosynthetic production, suggesting that variation in photosynthetic C supply regulates seasonal variations in CUE. Relative abundance of ectomycorrhizal taxa increased with forest age, and was dominated by Atheliaceae species in young forests, and by Cortinarius and Russula species in mature forests. Enzyme activities were related to community composition, and seem to be important for maintaining forest productivity, by facilitating organic nutrient mobilisation. This thesis is a first step in parameterizing mycorrhizal mycelial C fluxes to enable explicit inclusion of ERM parameters in forest ecosystem C models.