Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: April 22, 2008
Publication Date: September 1, 2008
Citation: Nichols, K.A. 2008. Indirect Contributions of AM Fungi and Soil Aggregation to Plant Growth and Protection. IN: Siddiqui, Z.A., Akhtar, M.S., Futai, K. (eds.) Mycorrhizae: Sustainable Agriculture and Forestry. Spinger Science. pp. 177-194. Interpretive Summary: Arbuscular mycorrhizal (AM) fungi are arguably one of the most dominate and important organisms in the soil. These fungi receive carbon from the plant host for fungal growth, biomolecule production, and carbon exudates to attract soil organisms, which use these exudates as energy to transform organic matter and soil minerals into plant-available nutrients. Thread-like fungal hyphae rapidly fan out into soil microsites containing high concentrations of nutrients such as P, N, Fe, Cu and Zn to scavenge even these nutrients and deliver them to the host. The fine threads of hyphae have a much larger surface area to volume ratio than roots, and the fungal cell membrane is capable of concentrating solutes against a gradient. Mycorrhizal fungi assisted in the formation of soil aggregates (pellets of different shapes and sizes which contain a conglomeration of soil minerals (sand, silt, and clay); organic matter, such as plant debris; inorganic compounds like iron and aluminum oxides; roots; fungal hyphae; and other microbes). Roots and AM fungal hyphae act as a ‘net’ collecting soil debris, while exudates, such as polysaccharides and glomalin (a glycoprotein produced on AM hyphae), provide the ‘glue’ to stick soil debris to the ‘net’. Glomalin also appears to form a hydrophobic lattice around aggregates to keep them water-stable. Modern agricultural practices have put new pressures on the plant-mycorrhizal symbiosis stimulating increases in ecological or biological engineering, such as aggregate formation and stabilization, to maintain and proliferate the symbiotic relationship. Tillage physically disrupts soil aggregates and AM hyphal networks resulting in declining in soil structure, fertility, and nutrient-cycling ability and forcing more C allocation by AM fungi to reestablishing these networks rather than to glomalin formation. No tillage practices along with continuous cropping systems, using mycorrhizal host crops, and reducing synthetic inputs, especially P, enhance the plant-mycorrhizal symbiotic relationship. Higher levels of C sequestration are possible in these systems, since not only is C being allocated belowground to hyphal networks and the formation of the highly stable glomalin molecule, but organic matter occluded within aggregates appears to have a slower turnover time than free organic matter. To maintain ecosystem function and a consistent food, feed, fiber, and energy supply to a growing global population, effective management tools for proliferation of soil organisms, especially AM fungi, and more sustainable agricultural systems need to be developed and implemented.
Technical Abstract: Soil ecological and biological engineering are processes by which soil organisms modify their physiochemical environment resulting indirectly to enhancements in plant growth. The formation and stabilization of soil aggregates is an example biologically-mediated engineering. For soil organisms, water-stable soil aggregates are a habitat containing a readily-available food supply. For the plants, soil aggregates form an environment with stable, continuous pores for improved water infiltration, water retention, aeration, and root growth, and the growth of microbes within aggregates provide plant-available nutrients. Arbuscular mycorrhizal (AM) fungi, under most conditions, improve plant growth directly by providing greater and more efficient access via fungal hyphae for nutrient acquisition and delivery to the plant. In addition, the growth of fungal hyphae provides the framework upon which soil aggregates are made, the carbon exudates to stimulate microbial growth and the production of polysaccharide ‘glues’ which hold aggregates together, and the production of glomalin to stabilize aggregates. Glomalin is a glycoprotein produced by AM fungi which probably forms a protective polymer-like lattice on the aggregate surface. This lattice gives aggregates a wax-like coating which makes water enter the aggregate at a slower rate and keeps air pressure within the aggregate from building and rupturing the aggregate.