Future Directions For Research On Roots And Mycorrhizal Function And Biodiversity

As researchers and government agencies become ever more interested in and concerned about "sustainability" and long-term management of ecosystems, they will require much more information on system-level carbon allocation and energetics of these ecosystems. A recent example is related to concerns about carbon sequestration and ecosystem carbon cycling. Root-mycorrhizal interactions have been found to be diagnostic of significant differences in potential seedling relative growth rate (RGR) (Cornelissen et al, 2001). Plant species with ericoid mycorrhiza showed consistently low RGR, low foliar nitrogen and phosphorus concentrations, and poor litter decomposability. Species with ectomycorrhiza had an intermediate RGR, higher foliar nitrogen and phosphorus, and intermediate to poor litter decomposabil-ity. Plant species with AM showed comparatively high RGR, high foliar nitrogen and phosphorus, and fast litter decomposition. The incorporation of mycorrhizal associations into functional type classifications should prove useful in assessing plant-mediated controls on carbon and nutrient cycling.

Several recent studies of mycorrhizal function have noted the ways in which mycorrhiza facilitate nutrient uptake from a wide range of sources. For example, Perez-Moreno and Read (2001) measured an enhanced recycling of nitrogen and phosphorus from the necromass of nematodes in cultures with Betula pendula. In nonmycorrhizal treatments, the uptake of nitrogen and phosphorus was slightly greater, but with mycorrhiza of Paxillus involutus (an ECM) present, the nutrient uptake was significantly higher still. Ectomycorrhizal plants (e.g., lodgepole pine [Pinus contorta]) and mycorrhizal fungi such as Amanita muscaria have been shown to utilize organic forms of nitrogen directly (Abuzinadah and Read, 1989; Finlay et al., 1992). In addition, ericoid mycorrhiza can produce extracellular enzymes that mineralize nitrogen from protein-tannin complexes (Leake and Read, 1989; Bending and Read, 1996). In a number of ecosystems worldwide that have ericaceous or heath vegetation, this direct pathway, by "short circuiting" the microbial mineralization pathway, would enable the ericaceous plants to survive, indeed thrive, in the absence of adequate nitrogen from usual mineralization rates (Northup et al., 1995). Note that newly available analytical techniques such as accelerator mass spectrometry (see Staddon et al., 2003, on previous page) have allowed detection of small quantities of carbon other than those from atmospheric sources, such as those from the direct uptake of amino acids, as noted above, and also those from anaplerotic (dark fixation) pathways, the latter providing as much as 3% of total carbon in ECM or saprophytic tissues (Hobbie et al., 2002).

As noted above, the vast majority of plants have mycorrhizal associates. Little is known yet of the species richness of mycorrhiza, particularly of the arbuscular, or AM, type. Until recently, AM mycorrhiza were assumed to have little host-specificity and to generally colonize a wide range of possible host species. In an intensive study of the plant-AM fungal interactions within a 1-hectare old field in North Carolina, Bever et al. (2001) found that, rather than the initial estimate (in 1992) of 11 AM species in this field, they now have isolated at least 37 species, with one-third of them previously unrecorded. The ecological preference ranges of each species are quite different, reflecting significantly different optima for temperature, moisture content, host, and phenological phases of the plants in the field. The implications of these varied interactions for plant diversity are very large, and the subject is one of increasing interest among plant community ecologists; the extent of mycorrhizal growth and uptake of labile carbon may affect the overall plant community makeup in interesting, hitherto unthought-of ways (Bever, 1999).

The molecular identification of AM has increased greatly in the last decade. By taking samples from within growing roots, amplifying, and producing 18S rDNA sequences, Redeker et al. (2000) and Redeker (2002) found a phylogenetically deep divergence of lineages within the Glomales, one of the principal groups within the AM fungi. In addition, two or more species were found to be co-occurring within the same root, indicating the probable existence of complex interactions of the fungi involved. The possibility for various species of AM fungi to become active at various times of the year is an intriguing one for future research on root-mycorrhizal fungal ecology (Redeker, 2002).

In addition to plant-fungi interactions, there are additional interactions with underground fauna to be considered. Klironomos and Kendrick (1995) found that the hyphae of AM are generally less palatable to fungivorous fauna than the hyphae of soil-borne conidial fungi. Such species-specific differences in palatability have been observed in ectomycorrhizal fungi as well (Schultz, 1991). We consider feeding behaviors in Chapter 4 and system-level impacts in Chapters 5 and 6.

The tools and analytical skills are at hand; it is now necessary to proceed with as much care in the assessment and measurement of below-ground processes as has heretofore been given to aboveground processes.

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