Nitrogen And Carbon Cycling By Litter Saprotrophs

Although litter is colonised by fungi when it reaches the forest floor, the phyllo-sphere fungi are soon replaced by other fungi colonising from adjacent, decaying litter. Dowson et al. (1989) described how the fairy-ring forming fungus Clitocybe nebularis in deciduous forest colonised fresh litter by horizontal growth-sweeps across the forest floor. Mature rings typically consisted of a 30-40 cm wide band with the mycelial front extending into fresh litter and the inner edge leaving bleached litter behind (Chapter 1). In contrast, Frankland et al. (1995) described a 'sit-and-wait' strategy, where Mycena galopus in a coniferous forest formed sporocarps at the same position year after year, presumably reflecting the location of the mycelium. In such cases, it appears that the fungi forage for resources mostly vertically, extending out from more decayed litter below into freshly fallen litter deposited on top. Both examples illustrate the polar growth of litter-degrading mycelia, and how they constantly advance into new resource-units from older, depleted ones. The dynamics of resource colonisation by saprotrophic mycelia has been thoroughly investigated in wood-decomposing fungi. A wide range of studies describe how saprotrophic basidiomycetes in heterogeneous environments optimise resource utilisation and maximise their competitive strength by translocating carbohydrates and nutrients throughout their mycelia (reviewed by Boddy, 1999; Chapter 1). Translocation implies that the whole mycelium functions as a single entity where the nutritional status of one part of the mycelium affects distant parts. Resources are translocated within the mycelium from areas with a surplus of a specific resource to areas with high demand, according to source-sink relationships (Lindahl and Olsson, 2004; Chapter 3). In coniferous ecosystems, the amount of nitrogen available to organisms is generally low, and decomposition rates are tightly linked to nitrogen availability. Therefore, in the following sections, we focus on the dynamics of this particular element within decomposing litter. During decomposition of litter, nitrogen is enzyma-tically released from decomposing plant tissues, but immediately absorbed by mycelium in the litter. The two pools are difficult to separate, and in the following text the nitrogen dynamics of the entire litter-mycelium complex is discussed.

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