Ectomycorrhizal Fungi and Biogeochemical Cycles of Boreal Forests

Inpodzolsin Europe and North America tunnels in weatherable mineral grains were found, presumably created by ectomycorrhizal (EcM) fungi. This finding was the incentive for a research program on rock-eating mycorrhizas, of which this project is part of. The focus of this thesis is on the role of EcM fungi in mineral weathering and upward transport of aluminium in podzols of boreal forests. I studied fungal tunnelling in a soil chronosequence in Michigan because mineral weathering is a slow process. Tunnelling accounted for less than 0.5% of total feldspar weathering over 5000 yr of soil formation. Tunnelling is only one part of EcM weathering; processes on the surface were hypothesised to be more important. To study EcM weathering on the mineral surface I modelled the effect of oxalate exudation by EcM on feldspar weathering. The model estimated that the maximum contribution of oxalate produced by EcM fungi to total weathering is 14.5% for K-feldspar and 13.1% for Na/Ca-feldspar. Furthermore I did a pot experiment with Scots pine seedlings and three different EcM fungi and muscovite as only K-source or hornblende as only Mg-source. Only Paxillus involutus enhanced muscovite weathering compared to the non-mycorrhizal control. None of the EcM fungi increased hornblende weathering. The second part of the research was about upward aluminium transport in podzols. Budgets studies indicate a considerable unexplained upward transport of Al from the mineral horizons to the organic top layer. A possible mechanism could be transport through EcM hyphae, attached to root tips in the organic soil layer, and extending into the mineral soil. An in vitro test showed transport of Al by two of the five fungal isolates tested. Subsequently, tree seedlings were grown in an artificial podzol. I used gallium as a proxy for Al transport. A previous in vitro test with P. involutus and Suillus bovinus confirmed the use of Ga as proxy for Al. Gallium was transported from the mineral soil to the organic soil, also if onlyEcMfungi had access to it. This suggests a role forEcMfungi in upward Ga and, possibly, Al transport.