Fungal Networks Acting as Nitrogen Reservoirs and Distribution Systems might Control the Rates of Forest Floor Carbon Fluxes

Basidiomycetes are emerging as potentially of central importance in ecosystem, and even global, carbon cycling. As the main decomposers (Boddy and Watkinson, 1995) and mutualistic plant symbionts (Leake et al., 2004) in N-limited soils, where the largest proportion of terrestrial carbon is sequestered (Post et al., 1982), they may act as gatekeepers of carbon fluxes. Their monopoly of available nitrogen would put them in control of inputs of limiting nitrogen to plants, and to the decomposer subsystem. It is thus appropriate to consider how fungal processes might be incorporated into integrated models of terrestrial carbon cycling (Falkowski et al., 2000). Fungal mycelium has the physiological potential to act as both an expandable reservoir and a distribution system for elevated nitrogen inputs (Watkinson et al., 2006), and may account for the responsive nitrogen absorption of the forest floor (Currie, 1999). We need better understanding of the coordinate regulation of carbon and nitrogen metabolism in fungi, to predict the effects of anthropogenic nitrogen deposition on soil carbon content (Bardgett, 2005).

Theoretical models of responses to environmental change are being developed to cover the range of scales, from micrometres to metres, that basidiomycete ectomycorrhizal fungi occupy. A critique of some models that have been developed suggests that, at organism scale, functional equilibrium and stoichiometric models are relatively easy to parameterize with accessible realistic data (Collins Johnson et al., 2006). Such models might thus have good predictive value to inform management.

Below we describe some approaches that we are using to collect ecologically relevant data on the responses of saprotrophic cord-forming wood decay basi-diomycetes to their carbon and nitrogen resources, ranging from cell to ecosystem.

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