Mycorrhizae differentially influence the transfer of nitrogen among associated plants and their competitive relationships

The formation of a common mycorrhizal network among roots of different plant species growing close to each other can influence plant community dynamics, regulating plant relationships through the differential transfer of nutrients from one plant to another. However, knowledge of the mechanisms that regulate this process is poor. Here we quantify the contribution of arbuscular mycorrhizae to the transfer of N among heterospecific plants growing adjacent to each other and examine whether the differential transfer of N within the plant community via mycorrhizae can alter competitive relationships among plant species. Plants of four species (wheat, pea, flax, and chicory) were grown in four-compartment pots (one species per compartment) under three conditions: no belowground interaction permitted among the compartments (Clo-sys); belowground interaction limited to soil microorganisms (including arbuscular mycorrhizal [AM] fungi) and soil solution (Res-sys); and belowground interaction permitted, so the crossing of roots, soil microorganisms, and soil solution was allowed (Ope-sys). Each condition was tested in both the absence ( myc) and presence (+myc) of AM symbiosis. The transfer of N from pea to the three non-legume companion species (assessed via direct 15N labelling) was greater in Ope-sys than Res-sys for wheat and chicory but not for flax. In general, N transfer was greater in +myc than –myc in all species. In wheat the positive effects of AM symbiosis on N transfer were pronounced in Ope-sys but not in Res-sys, whereas in flax and chicory mycorrhization had similar effects in both Ope-sys and Res-sys. In Res-sys and in the absence of AM symbiosis, wheat intercepted about 50% of the total N transferred from pea, chicory about 40%, and flax about 10%. Mycorrhization altered these ratios, reducing the proportion of N transferred to wheat while increasing the proportion transferred to the other two species (especially to chicory), thus favouring the weakest components of the mixture. Similar effects of AM symbiosis were observed in Ope-sys. Our study shows that AM symbiosis affects the distribution of N and as a consequence the competitive relationships among adjacent plants of different species.