Nutrient Dynamics in Arbuscular Mycorrhizal Networks

Abstract

Transport and exchange of nutrients is a key feature to the function of arbuscular mycorrhiza (AM) and therefore also to the function of common mycorrhizal networks (CMNs). These networks establish between two or more plant individuals and one or more extraradical mycelia (EM). Complex networks with many nodes and linkages can be observed in sterile cultures and are probably common in nature. This chapter aims to describe how the nutrient dynamics of the CMNs influence plant competition. The discussion will concentrate on the rather variable access of the individual plants to the nutrient pool in the EM. Plant-to-plant transfer of nutrients via one or more connective EM does not appear to occur in significant quantities except in special cases. Competition between individual plants is in general asymmetric such that larger individuals will obtain a disproportionate share of a limited resource and suppress the growth of smaller individuals. Our major challenge is to unravel whether nutrient dynamics in CMNs will result in even stronger or in more relaxed competition. We investigated common outcomes in competition studies including: (a) adult plant-seedling combinations; (b) intraspecific competition of plant populations of similar age and (c) interspecific competition of plants of similar age. Results from root organ culture models indicate that AM fungi transfer phosphorus (P) to roots representing the strongest carbon (C) source strength. This is in accordance with the results of 32P-aided studies of P translocation in CMNs with large plants and seedlings. An evaluation of these results together with CMN studies not involving tracers leads us to suggest that nutrients in the EM are distributed to the plants in accordance with their size or C source strength. In consequence, the CMN may rather amplify than relax the competition towards establishing seedlings. Our model implies, however, that the suppression of the seedling is only temporary and is likely to shift to a typical mycorrhizal growth response when the large plant become senescent or is grazed. In conspecific plant populations we predict that AM fungal networks confer an advantage to plant individuals that are for various reasons slightly larger than their neighbors. Interspecific plant competition is more difficult to predict as different combinations of species-specific traits may either amplify or relax nutrient competition in a CMN in addition to effects of size differences. Future research needs are discussed including the need to investigate roles of CMNs in interplant transfer of plant signals and allelochemicals.