Ectomycorrhizal fungi and biogeochemical cycles of boreal forests

TitleEctomycorrhizal fungi and biogeochemical cycles of boreal forests
Publication TypeThesis
Year of Publication2005
AuthorsSmits MM
DegreeDoctor of Philosophy
Number of Pages121 pp.
UniversityWageningen University
CityWageningen, Netherlands

Ectomycorrhizal (EcM) fungi are soil-inhabiting fungi that live in symbiosis with a host tree. Most tree species in boreal forests live in symbiosis with EcM fungi. With their extensive network of mycelium EcM fungi extend the 'search' volume of the tree root. Much research on EcM fungi is focussed on their ability to release nutrients from organic matter in the soil. But there is increasing evidence that EcM fungi also are able to release nutrients from minerals. In podzols in Europe and America weatherable minerals contain microscopic tunnels, presumably created by EcM fungi. It is hypothesized the EcM hyphae living in close contact with the mineral surface dissolve the mineral by exuding organic anions like oxalate and subsequent removal of the weathering products. These tunnels were typically found in podzols, the main soil type in boreal forests. Podzols are characterized by distinct soil horizons with on top an organic O-horizon, underlain by a white/ash-coloured eluvial E-horizon, a usually dark coloured illuvial B-horizon on top of the unaltered parent material. Fungal tunnelling seems restricted to the E-horizon and is almost absent in the underlying B-horizon, suggesting also a link to the podzolization process. It is suggested the EcM fungi accelerate mineral weathering in the E-horizon, causing increased Al and Fe mobilization. In this research, which is part of a bigger program on the fungal tunnelling phenomenon, I focussed on the role of EcM fungi in the podzolization process, more specifically on mineral weathering and upward transport of Al. EcM weathering is difficult to quantify in the field, but fungal tunnelling gives us a tool to recognize and quantify a part of it. We studied fungal tunnelling in two soil chronosequences, one in Mid Sweden and the other in North Michigan. Tunnelling was faster in Michigan than in Sweden. We quantified the effect of tunnelling on total feldspar weathering in the North Michigan soil chronosequence. Tunnelling accounted for less than 0.5% of total feldspar weathering in the upper 2 cm of the mineral soil. There was a clear difference between the two feldspar species present, with a higher contribution of tunnelling to weathering in Na/Ca-feldspars than in K-feldspars. The fungal distribution over the different mineral species in one soil of the North Michigan chronosequence was determined using a fluorescent dye. Na/Ca-feldspar surfaces were more densely colonized than K-feldspar surfaces, strengthening the link between the tunnels found in the minerals and fungi colonized the minerals. Fungal tunnelling is only one aspect of EcM weathering and EcM weathering on the mineral surface may be quantitatively more important. I modelled the effect of EcM oxalate exudation on feldspar weathering in one soil of the North Michgian chronosequence. Oxalate is assumed to be a major fungal weathering agent due to its strong chelating capacities. The model calculated the maximum contribution of oxalate to total weathering with 14.5% for K-feldspar and 13.1% for Na/Ca-feldspar. I only considered the chelating effect of oxalate. EcM fungi also exude other strong chelating compounds like citrate. These anions are presumably exuded with protons. The combined effect could result in a much higher contribution to weathering. EcM weathering was studied via a third approach in a pot experiment. I performed a pot experiment with mycorrhizal and non-mycorrhizal tree seedlings with muscovite as only K source or hornblende as only Mg source. Of the three different EcM-species tested, only Paxillus involutus showed increased muscovite weathering compared to the non-mycorrhizal treatment. But the increased muscovite weathering did not result in increased K uptake into the shoot. None of the EcM fungi increased hornblende weathering compared to the non-mycorrhizal treatment. The second part of the research was about upward transport of aluminum in podzols. Aluminum budget studies in boreal forest indicate a considerable upward transport of Al from the mineral soil in the organic horizon. In this thesis I studied the possible role of EcM fungi in it. As a first step I tested if EcM fungi are able to transport Al. An in vitro test showed transport of Al in two of the five EcM isolates tested. To further test the role of EcM fungi in upward transport of Al we made an artificial podzol with soil material from South Sweden. In half of the pots the O-layer was separated from the mineral soil with a mesh, excluding growth of roots, but not of hyphae into the mineral soil. Because the Al-content is too high in such systems to observe hyphal transport of Al, I used gallium as a proxy for transport of Al. An in vitro test with Paxillus involutus and Suillus bovinus revealed similar trends for transport of Al and Ga, supporting the use of Ga as a proxy for Al. In the artificial podzol Ga was transported from the mineral soil to the organic soil, independent of the presence of a mesh indicating a major role for EcM fungi in upward transport of Ga and possibly, Al. Although we studied the impact of EcM fungi on weathering in three different ways, the question how much they contibute in the field still remains. In the General Conclusions I give some suggestions for further research to answer this question.