Soil fungi and fauna have key roles in many ecological processes, including decomposition and cycling of organic matter, and acquisition of mineral nutrients from both organic and inorganic forms. Fungi are heterotrophs, requiring external sources of carbon for metabolic energy, and use three different strategies to acquire carbon: saprotrophic, necrotrophic, and biotrophic. Saprotrophic fungi (Chapter 6) utilise carbon from dead organic matter, and are the fungi found on deadwood and soil organic matter; these include the white rot and brown rot fungi such as Pleurotus and Hypholoma. Necrotrophic fungi obtain carbon from dead or dying organic matter, but are disease fungi and are often responsible for the death of the organism; these include the classic tree disease fungi such as Armillaria. The final group is the biotrophic fungi, which obtain their carbon from other living organisms, often in the form of mutualistic or symbiotic association; this group includes the mycorrhizas. The term ‘mykorrhiza’ was first used by Frank in 1885 to describe the modified root structures of forest trees. Seven different categories of mycorrhizal symbiosis have been distinguished on the basis of their morphological characteristics and the fungal and plant species involved. Three of these types are of major significance in northern forests; these are arbuscular mycorrhizas (AM), ericoid mycorrhizas (EcM) and ectomycorrhizas (EM).
Arbuscular mycorrhiza (AM) is the most ancient and widespread form of mycorrhiza, forming symbiosis with both woody plants and herbaceous plants of the upper and mid-story, as well as the ground vegetation of forests.