The effect of Frankia and multiple ectomycorrhizal fungil species on Alnus growing in low fertility soil

Plant interactions with single symbionts of nitrogen fixing bacteria or mycorrhizal fungi are well studied. However, less is known about how plants interact with multiple symbiont species. The aim of this study was to assess whether multiple symbionts had a synergistic effect on host plant performance compared with a single symbiont and whether increasing the number of non-specific fungal species would increase plant performance due to niche complementarity. Green alder ( Alnus viridis ssp. crispa ) were inoculated with non-specific ectomycorrhizal fungi ( Lactarius torminosus , Lactarius theiogalus , Hebeloma crustuliniforme ) alone or in combination, with and without Frankia , and grown in nutrient-poor soil. Frankia significantly increased plant growth by 48.5%, doubled the proportion of plants forming ectomycorrhizae, as well as increased root extracellular phosphatase activity by 8.8% compared to non- Frankia treatments. However, increasing number of fungal species decreased nodulated plant biomass. Plants that formed a Hartig net had reduced nodule number and nodule biomass allocation, as well as total nodule activity compared with plants with no Hartig net. Our results indicated that Frankia inoculation provided benefit for both host plants and ectomycorrhizal fungi in the nutrient-poor soil, while ectomycorrhizal fungi did not. These negative effects of mycorrhizal fungi are likely dependent on soil nutrient P availability.