into the soil inorganic nutrient pool. Nutrients are immobilized into recalcitrant organic resources (such as the log on the right) and into fungal biomass. Here there can be both temporal and spatial redistribution of nutrients within the fungus and among resources utilized by the fungi. Mycorrhizae on plant roots aid the uptake of mineral nutrients into the plant, help to defined the plant against root pathogens, and may effect decomposition of organic residues. Both fungal hyphae and fruiting structures (represented by mushrooms) are a food source for grazing animals, thus influencing the fitness of individuals and regulating animal populations. Source: Adapted from Dighton and Boddy (1989).

As with other micro-organisms, fungi obtain their energy and nutrients by the secretion of extracellular enzymes into the environment. Degradation products, containing carbohydrates and/or mineral nutrients, are then absorbed by the hyphae and distributed within the organism by translocation. This process is termed saprotrophy and has important consequences outside the mere nutrition of the fungus. The process of extracellular degradation of resources is less efficient than ingestion of food into a gut, resulting in a fraction of the useful resources being released into the environment around the fungus. This inefficiency results in one of the key functions of saprotrophic soil fungi, the mineralization of nutrients into the soil nutrient pool (Dighton, 1997). This attribute will be discussed in more detail as it pertains to the supply of nutrients for primary productivity. In addition to the usage of organic components of the soil, leaf litter, woody debris, and animal remains, there are a number of ways in which fungi can interact with the mineral component of the soil. The secretion of oxalic acids can assist in the weathering of calcareous bedrock. In the mutualistic symbiotic association with algae, lichens produce lichenic acid, which facilitates rock solubilization and the formation of

protosoils. Once a true soil has been formed, fungi are important contributors to the formation and stability of soil aggregates. These combinations of mineral soil particles and organic matter are refugia for microscopic organisms in soil, such as bacteria, protozoa, rotifera, and nematodes. Their activity at the micro scale results in the mineralization of inorganic nutrients for plant growth, and hence, soil fertility. The combination of polysaccharide secretions from bacteria and the binding property of threads of fungi assist in maintaining the structure of soil aggregates (Wright and Updahaya, 1998). Recent debates regarding the efficiency of highly intensive and mechanical agricultural practices have shown that minimal-or no-till agricultural practices reduce the disruption of hyphal networks in soil, and together with other factors improve soil aggregate stability and fertility. The pivotal role of fungi in the formation and maintenance of fertile soils for plant primary productivity will be discussed in Chap. 2.