UNC Ecology Seminar: Nancy Collins Johnson

Although phosphorus (P) is traditionally the most common focus of studies about resource exchange among plants and arbuscular mycorrhizal (AM) fungi, there is growing evidence that nitrogen (N) availability is also an important determinant of mycorrhizal function. Furthermore, light availability must be considered because it controls the amount of photosynthetic carbon (C) that is available for symbiotic exchange. The trade balance model accounts for the interactive effects of P, N and C on AM function. This model predicts that, depending upon P-availability, N-enrichment may either increase mutualistic benefits or exacerbate plant growth depressions because N controls both the C-supply from plants and the C-demand from AM fungi. Light limitation caused by shading should diminish the pool of photosynthate available for symbiotic exchange and reduce mycorrhizal benefits by increasing the relative C-cost. Predictions of the trade balance model were tested in a series of fertilization and shading experiments using naturally occurring combinations of plant hosts, mycorrhizal fungi, and soils from three different grasslands that varied naturally in their soil fertility. Results of these experiments support the trade balance model and show that the relative availability of N, P, and light determines mycorrhizal function across a mutualism-parasitism continuum. This finding is important if mycorrhizal mutualisms are to be effectively managed for plant nutrition and ecosystem restoration.