Indirect Contributions of AM Fungi and Soil Aggregation to Plant Growth and Protection

Abstract

Ecological and biological engineering contribute indirectly to the fitness of the soil environment and promote plant growth and protection. This engineering modifies soil physical, chemical, and biological attributes to enhance nutrient cycling, increase soil organic matter, and improve soil quality. Arbuscular mycorrhizal (AM) fungi, under most conditions, improve plant growth directly by providing greater and more efficient access via fungal hyphae for absorption of nutrients, especially P, and delivery of these nutrients to the plant. The AM symbiosis also augments disease resistance in host plants and suppresses the growth of non-mycorrhizal weeds. When plants moved from an aquatic to a terrestrial environment, mycorrhizal fungi were an integral part of their success by providing efficient nutrient absorption from the low organic matter mineral soil. In addition, AM fungi stabilize soil aggregates and promote the growth of other soil organisms by exuding photosynthetically-derived carbon into the mycorrhizosphere. Glomalin is a glycoprotein produced by AM fungi which probably originated as a protective coating on fungal hyphae to keep water and nutrients from being lost prior to reaching the plant host and to protect hyphae from decomposition and microbial attack. This substance also helps in stabilizing soil aggregates by forming a protective polymer-like lattice on the aggregate surface. AM fungal growth and biomolecules engineer well-structured soil where the distribution of water-stable aggregates and pore spaces provides resistance to wind and water erosion, greater air and water infiltration rates favorable for plant and microbial growth, nutrients in protect micro-sites near the plant roots, and protection to aggregate-occluded organic matter.