Mycorrhizal fungi as drivers of ecosystem processes in heathland and boreal forest biomes

Abstract

The importance of mycorrhizas in heathland and boreal forest biomes, which together cover much of the landmass of the Northern Hemisphere and store most of the global stocks of carbon, is reviewed. The taxonomic affinities of the organisms forming these symbiotic partnerships are assessed, and the distinctive structural features of the ericoid mycorrhizas of heathland dwarf shrubs and the ectomycorrhizas of boreal forest trees are described. It is stressed that neither in terms of the geographical distribution of the plants nor in terms of the occurrence of their characteristic mycorrhizas in the soil profile should these biomes be considered to be mutually exclusive. What unites them is their apparent affinity for acidic organic soils of inherently low accessibility of the major nutrients nitrogen (N) and phosphorus (P). These properties relate directly to the nature of the nutrient-poor recalcitrant litter produced by their host plants and through positive-feedback mechanisms that are reinforced by selective removal of labile nutrients by the mycorrhizas. We suggest that coevolution of these plant litter traits with mycorrhizal associations that are adapted to them has been one of the defining features of these ecosystems. Ericoid and ectomycorrhizal fungi have biochemical and physiological attributes that make them highly efficient at scavenging for organic sources of N and P in surface soil horizons. In so doing, they restrict supplies of these elements to the decomposer communities. Case studies involving exploitation of N and P in defined organic substrates are described. In both biomes the dominant plants depend upon the abilities of their fungal partners to recover nutrients, so the symbioses control nutrient cycles, productivity, species composition, and functioning of these ecosystems. It is in this context that the fungal symbionts are here considered to be drivers of nutritional processes in their respective biomes. Through their influences upon the quality of carbon residues mycorrhizal fungi must also affect the sink-source balance for this key element in soil. There is an urgent need for the evaluation of the relative contributions of symbiotic and saprotrophic components of the microflora to the processes of carbon storage and cycling in these biomes, particularly in the context of global climate change and impacts of anthropogenic pollutant N deposition.Key words: carbon sequestration, peatlands, C/N ratios, carbon and nutrient cycles.