Published online by Cambridge University Press: 10 December 2009
Introduction
This chapter reviews the distribution, mechanism and impact of mineral tunnelling by soil ectomycorrhizal fungi (EMF). Most trees in boreal forests live in close relation with EMF (Smith & Read, 1997). These EMF mediate nutrient uptake; they form an extension of the tree roots. In turn they obtain carbohydrates from the tree. Over the years ectomycorrhizal (EM) research has a strong focus on nutrient acquisition by EMF from organic sources (Read, 1991). In boreal forest systems, however, minerals could also be an important nutrient source, especially for calcium, potassium and phosphorus (Likens et al., 1994, 1998; Blum et al., 2002). Recent developments in EM research suppose a role for EMF in mobilizing nutrients from minerals (see Wallander, Chapter 14, this volume).
In 1997, Jongmans et al. described small tunnel-like features in feldspar and hornblende grains from Swedish forest soils. These tunnels have the shape of fungal hyphae: a constant width between 3 and 10 μm, smooth borders and a rounded end. In that way they differ from other weathering phenomena such as etch pitches and cracks (Fig. 13.1). In some tunnels hyphae were found. Jongmans et al. (1997) postulated that EMF created these tunnels by mineral dissolution through the exudation of low-molecular-weight organic compounds and subsequent removal of the weathering products. The weathering products like calcium, magnesium and potassium are supposed to be transported to the tree roots. In this way the host tree has direct access to mineral-bound nutrients, bypassing the bulk soil solution (van Breemen et al., 2000a; Landeweert et al., 2001).
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