Arbuscular mycorrhizas influence plant diversity, productivity, and nutrients in boreal grasslands

The effect of reduced arbuscular mycorrhiza (AM) colonization on nitrogen and phosphorus concentration in grass tissue, species diversity, cover, and productivity was investigated after 4 years of benomyl application in two boreal grasslands, Storvordlia (a high plant diversity field) and Kalvsvangen (a low plant diversity field), in Eastern Norway. In addition, AM colonization, spore numbers, and mycorrhizal inoculum potential was studied during one season. A split-plot design was used, with grazing as the main treatment and the application of benomyl as a subtreatment. AM colonization was significantly reduced because of benomyl application, whereas spore numbers were not significantly affected by the treatment. Mycorrhizal inoculum potential was reduced, but not consistently. In both sites nitrogen and phosphorus concentration of grass tissue was significantly higher in grazed plots than in ungrazed ones, and significantly lower in benomyl subplots than in controls. Plant diversity, composition, and cover had changed significantly after 4 years in grazed controls in Storvordlia, but this was not seen in Kalvsvangen. In both sites, total productivity increased significantly after 4 years of benomyl application. This experiment indicates that AM associations can influence grass nutrient contents and species responses, plant diversity, and productivity in boreal grasslands. These differences were larger in the higher plant diversity site, which is situated at a higher elevation and is under more marginal growth conditions.