Agro-ecological aspects of the mycorrhizal, nitrogen-fixing legume symbiosis

Vesicular-arbuscular mycorrhizal (VAM) fungi have fundamental effects on the ecophysiology of nodulated legumes, on the biota of the surrounding soil and on associated non-legumes. The effects are seen to be mediated through the direct transfer of nutrients from plant to plant by live hyphae of the soil mycelium, and through improving the competitiveness of legumes in nutrient uptake. As envisioned, the main role of legumes, in the open source-sink system between associated plants and the soil biota, is that of a nitrogen source. The export of nitrogen may be balanced by flows of carbon and phosphorus from plant to plant along gradients of sink demand. Soil microbes are beneficiaries of inter-plant nutrient transport, utilizing exudates as well as the soil mycelium as substrates, while the products of microbial metabolism serve to aggregate the abiotic component of the soil. Thus, the tripartite legume association is seen to function not only as a self-contained system, but as one involved in nutrient fluxes between adjacent plants and the soil that surrounds its roots.