Terrestrial ecosystems are driven by microbial guildes. We are, however, severely hampered by our limited understanding of the diversity and function of such mic- bial ecosystems. Growing on mineral particles and decaying organic matter, and l- ing in the vicinity of or within plant roots, are a cast of hundreds of competing versatile and diverse fungal species, including soil decomposers, pathogens, en- phytes and obligate symbionts (Frankland 1998). Amongst the extensive interm- gling webs of hyphae permeating the soil, those of mycorrhizal fungi play a crucial role in ecosystem sustainability through their role in biogeochemical cycles. These mycorrhizal species are no marginal oddity, having been shown to account for half of the fungal species in most temperate, montane and boreal forests. The hyphae of mycorrhizal fungi and plant short roots form a novel composite organ, the so-called mycorrhiza, which is the site of nutrient and carbon transfer between the two sym- otic partners. This mutualistic interactions allow terrestrial plants to grow efficiently in suboptimal environments (Read and Perez-Moreno 2003).
They are key drivers of ecosystem function, receiving 15-25% of carbon net productivity and providing most of the host plant nitrogen and phosphorus. The symbiotic relationship between roots and these fungi is undoubtly one of the most prevalent associations in all terrestrial ecosystems.