While soil fungi include single-celled yeasts that live principally in anaerobic soils, most soil fungi are filamentous, producing masses of hyphae called mycelia. Some produce macroscopic fruiting bodies we call mushrooms. Fungi are efficient decomposers of a wide range of organic compounds. Fungi are even more important than bacteria in the production of humus and stabilization of soil aggregates. Some also produce antibiotics such as penicillin. A few, such as Aspergillus flavus, produce compounds that are highly toxic to humans. Some play important roles in the control of plant pathogens. Some fungi are, themselves, pathogenic to higher plants.
Closely related to certain pathogens are the fungi that form symbiotic mycorrhizae with plant roots. Both ectomycorrhizae (forming mainly on certain ericaceous and coniferous trees) and endomycorrhizae (forming on a wide range of plants) assist plants in obtaining nutrients (particularly phosphorus) from soil. Ectomycorrhizae are used routinely in forestry to assure vigorous growth of tree seedlings. Endomycorrhizae, which actually invade and form vesicles and arbuscules in plant root cells, are often called AM for short. Research continues to uncover the widespread influence of mycorrhizae on plant growth and ecosystems, including beneficial interaction with nitrogen-fixing bacteria in legume roots.
Several species of fungi prey on soil nematodes—often on those nematodes that parasitize higher plants. Some species of nematode-killing fungi attach themselves to, and slowly digest, the nematodes. Others, like this Arthrobotrys anchonia, make loops with their hyphae and wait for a nematode to swim through these lassolike structures. The loop is then constricted and the nematode is trapped. The nematode shown here is being crushed by two such fungal loops. Additional loops can be seen in their nonconstricted configuration.
Diagram of ectomycorrhiza and arbuscular mycorrhiza (AM) associations with plant roots. (a) The ectomycorrhiza association produces short branched rootlets that are covered with a fungal mantle, the hyphae of which extend out into the soil and between the plant cells but do not penetrate the cells. (b) In contrast, the AM fungi penetrate not only between cells but into certain cells as well. Within these cells, the fungi form structures known as arbuscules and vesicles. The former transfer nutrients to the plant, and the latter store these nutrients. In both types of association, the host plant provides sugars and other food for the fungi and receives in return essential mineral nutrients that the fungi absorb from the soil.
Mycorrhizal fungi, rhizobia bacteria, legumes, and nonlegume plants can all interact in a four-way, mutually beneficial relationship. Both the fungi and the bacteria obtain their energy from sugars supplied through photosynthesis by the plants. The rhizobia form nodules on the legume roots and enzymatically capture atmospheric nitrogen, providing the legume with nitrogen to make amino acids and proteins. The mycorrhizal fungi infect both types of plants and form hyphal interconnections between them. The mycorrhizae then not only assist in the uptake of phosphorus from the soil, but can also directly transfer nutrients from one plant to the other. Isotope tracer studies have shown that, by this mechanism, nitrogen is transferred from the nitrogen-fixing legume to the nonlegume (e.g., grass) plant, and phosphorus is mostly transferred to the legume from the nonlegume. The nonlegume grass plant has a fibrous root system and an extensive mycorrhizal network, which is relatively more efficient in extracting P from soils than the root system of the legume. Research indicates that some direct transfer of nutrients via mycorrhizal connections occurs in many mixed plant communities, such as in forest understories, grass–legume pastures, and mixed cropping systems.