Plant diversity but not productivity is associated with community mycorrhization in temperate grasslands

Aims Mycorrhizal symbiosis influences the performance of plant individuals. However, its impact on plant communities is less well understood. We used a database of plant mycorrhizal traits and investigated how community mycorrhization — the prevalence of mycorrhizal symbiosis in plant communities — is related to plant community productivity and diversity. We analysed: (a) how soil fertility affects community mycorrhization; (b) how plant productivity and diversity are related to community mycorrhization; and (c) whether community mycorrhization directly mediates the relationships between soil fertility and plant productivity and diversity. Location Twenty-nine semi-natural grasslands in the south of Estonia, representing a steep productivity gradient (80–500 g/m2). Methods We estimated the proportion of plant biomass associated with mycorrhizal symbiosis (arbuscular mycorrhization index, AMI). The relationships between soil fertility (soil nitrogen [N], phosphorus [P] and potassium [K], accounting for soil moisture and pH), AMI, plant diversity and productivity were analysed using model averaging and pairwise generalized linear models, and support for direct relationships was determined using path analysis. Results There was a positive linear relationship between soil nitrogen and AMI. In turn, plant biomass showed positive, and plant diversity negative linear relationships with AMI. Path analyses indicated that AMI mediates the relationship between soil nitrogen and plant diversity, but not the relationship between soil nitrogen and plant community productivity. Plant productivity and mycorrhization increased along the soil fertility gradient, but plant diversity decreased. Conclusion These results suggest that community mycorrhization, which is sensitive to soil fertility (N), can modulate community composition in temperate grasslands, probably by enhancing the most dominant plant species in the community.