In the humus layer, saprotrophic litter fungi seem to be out-competed by mycorrhizal fungi (Lindahl et al., 2007). Ectomycorrhizal symbiosis with tree roots has evolved on several independent occasions during evolution of basidiomycetes, and a major fraction of ground-living basidiomycetes are ectomycorrhizal. Phylogenetic relationships suggest that reversions from biotro-phic symbiosis back to saprotrophy have also occurred on several occasions during evolution, and most of the saprotrophic litter-degrading basidiomycetes of today, for example genera in the euagaric clade such as Mycena, Marasmius and Galerina, are likely to have evolved from mycorrhizal ancestors (Hibbett et al., 2000). The evolutionary instability of mycorrhizal symbiosis and the sometimes relatively close genetic relationship between ectomycorrhizal fungi and litter saprotrophs suggest that these two groups may not be as functionally distinct as previously thought. Most ectomycorrhizal fungi have some saprotrophic capabilities and produce enzymes such as proteases and polyphenol oxidases that enable mobilisation of nutrients in complex organic forms (Read and Perez-Moreno, 2003; Lindahl et al., 2005). Overall, most evidence suggests that free-living saprotrophic fungi have a much higher capacity to degrade fresh litter than ectomycorrhizal fungi (e.g. Colpaert and van Tichelen, 1996). However, as decomposition progresses, the competitive advantage endowed by labile carbon supplied by a plant host may compensate for the modest decomposition capacity.
The shift in community composition from saprotrophic taxa at initial stages of litter decomposition to mycorrhizal taxa at later stages coincides with a simultaneous shift in the carbon and nitrogen dynamics of the litter (Figure 2; Lindahl et al., 2007). During the saprotrophic phase, C:N ratios decreased with time, presumably due to consumption of litter carbon by fungal respiration, and retention of nitrogen in fungal biomass. In the mycorrhizal phase, C:N ratios increased slightly with time, presumably due to mobilisation and subsequent translocation of organic nitrogen to the plant roots in combination with respiration of host-derived carbohydrates rather than litter carbohydrates.
Throughout this discussion, nitrogen circulation within the forest floor has been described as a closed system. This is, of course, an over-simplification, as the nitrogen pools are continuously replenished by fixation and deposition of atmospheric nitrogen, and drained through loss of nitrogen-containing compounds to the ground water. In most coniferous forest ecosystems, throughput of nitrogen is, however, likely to be relatively small compared to the internal circulation.
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