Nitrogen-fixing bacteria, arbuscular mycorrhizal fungi, and the productivity and structure of prairie grassland communities

Background/Question/Methods

Due to their complementary roles as plant nutritional symbionts, arbuscular mycorrhizal fungi (AMF) and nitrogen-fixing bacteria (N₂-fixers) may have additive or synergistic effects on plant communities.  Independently, both AMF and N₂-fixers are known to shift plant community composition in favor of species that benefit most from associations with these symbionts, and each generally increases total community productivity.  However, interactive effects of AMF and N₂-fixers in plant communities are relatively unexplored.  These interactions are of applied interest because of their potential to increase diversity of plant communities, promote establishment of desirable species, and increase productivity on infertile or disturbed soils.  In microcosms containing low fertility top soil collected from active strip mines we tested the effects of AMF, N₂-fixers (Rhizobia spp. and Azospirillum brasilense), and their potential interactions on the productivity, diversity and species composition of diverse tallgrass prairie communities and on the productivity of Panicum virgatum in monoculture.

Results/Conclusions

We found a positive effect of AMF on diversity and productivity, but a negative effect of N₂-fixers on productivity of experimental communities.  Both AMF and N₂-fixers affected relative abundances of plant species.  AMF shifted the communities from dominance by early successional Elmus canadensis to Sorghastrum nutans, and seven other species increased in abundance with AMF, accounting for the increased diversity.  N₂-fixers led to increases in Astragalus canadensis and Desmanthus illinoense, two legumes that likely benefited from the presence of the appropriate Rhizobia symbionts.  Sorghastrum nutans declined 44% in the presence of N₂-fixers, perhaps due to competition with legumes.  Panicum monocultures were more productive with AMF, but showed no response to N­­₂–fixers.  We did not find interactions between AMF and N₂-fixers in communities or Panicum monocultures, potentially because of the short time scale of this study.  Our results do support the importance of soil microbial communities as drivers of plant community structure and function.