Grassland plant diversity as an indicator of beneficial arbuscular mycorrhizal fungi

Background/Question/Methods

Arbuscular mycorrhizal fungi (AMF) are important symbionts that receive carbon from plants in exchange for soil nutrients and other ancillary benefits. AMF benefits, however, are not universal and mycorrhizal responses (MRs) depend on the plant-fungal match as well as the environment. Diverse plant communities can support a greater abundance and diversity of AMF, which suggests that these AMF should generate more consistently positive MRs. When comparing among sites, however, it is unclear whether environmental variation will obscure any signal of plant diversity in observed MRs. To test whether more diverse sites had more broadly beneficial AMF communities, we sampled soils and plant communities from 18 different grassland sites covering an area of approximately 240,000 km. For each site, we collected plant cover data. We also characterized the physical and chemical properties of the soil samples. We used trap plants to culture the AMF from these soils using an annual C4 grass (Sorghum x drummondii), a perennial C4 grass (Andropogon gerardii), and a legume (Medicago sativa). We then tested the effect of these AMF inocula on the growth of a variety of plant species belonging to different functional groups.

Results/Conclusions

Grassland soils varied significantly in their effects on trap plant growth and in the effects of their cultivated inoculums. Unsurprising, the strongest predictor of trap plant growth was soil salinity, a common stress that drives plant community assembly in the grassland sites. Plant diversity, however, was the second strongest predictor. Relative growth rates were similar for all trap plants in soils from diverse plant communities, but soils from less diverse communities increased growth for Medicago and decreased growth for Sorghum, with Andropogon intermediate. Rather than the characteristics of the field sites, trap plant growth was the best indicator of MRs, although this differed among functional groups. For legumes and C4 grasses, MRs were most positive when inoculated with cultures where Sorghum or Andropogon were productive. For non-leguminous forbs, however, MRs were negative with these same inocula and C3 grass MRs were unrelated to trap plant productivity. Combined, this indicates that diverse sites do have AMF communities that are beneficial for some species, but that trap plant performance may be a better indicator than site characteristics. Further, these benefits are not universal, suggesting that AMF composition at a given site may influence competitive outcomes among plants and thus community assembly.