Impact of arbuscular mycorrhizal fungi on N and P cycling in the root zone

The influence of arbuscular mycorrhizal fungi (AMF) on soil processes is difficult to define, in part because it varies with the plant and fungal genotypes as well as with environmental conditions. The influence of AMF on N cycling is just being discovered. This short review examines areas of knowledge related to the nature of the mycorrhizosphere, AMF-microbe interactions, and N and P mobilization in the root zone. The role AMF could play in the root zone in N and P dynamics is presented. The spatial organization of roots and hyphae in soil suggests that AMF may enhance the reabsorption of nutrients lost through root exudation. These fungi influence soil microorganisms and, hence, they may influence biochemical reactions in soil including mineralization of organic matter and nitrification. AMF seem to improve the capacity of their host plant to use organic sources of P and N. AMF might improve the ability of plants to compete with saprotrophs for the uptake of P and N. The impact from this latter effect of AMF could exceed that stemming from their possible direct use of amino-acid-N, their production of phosphatase enzymes and possibly of organic anions. Better competitive ability of mycorrhizal plants under conditions of low soil P and N levels could reduce saprotrophic populations and, if this occurs, regulate soil organic matter turnover rate. The management of biological soil resources, in truly sustainable cropping systems, may be the largest challenge in crop production.