Soil microbial communities are critical to mediating carbon (C) exchange between the land and the atmosphere, but their responses and contributions to C cycling feedbacks in response to climate change remain a significant gap in our ecological understanding. Previous research has suggested that plant community richness can influence soil microbial community composition, diversity, and functional characteristics and may have significant implications for how soil communities will respond to elevated temperatures under climate warming. In this study we asked: What are the interactive impacts of global warming and plant diversity on the quantity and chemical identity of soil C as mediated by plants and their associated soil microbial communities? We sampled soils from a nine-year factorial biodiversity and climate experiment located at the Cedar Creek Long-term Ecosystem Science Reserve in central Minnesota, USA. Rhizosphere soils of Andropogon gerardii (big bluestem) or Lespedeza capitata(bush clover) were collected from plots of single species monocultures and diverse 16-species polycultures that were warmed by ~3-5 °C relative to paired ambient control plots. We used amplicon sequencing of bacterial 16S rRNA genes and fungal ITS2 regions and conducted pyrolysis GCMS characterization of molecular soil C chemistry to develop a mechanistic understanding for predicting how microbial communities respond to global warming.
We hypothesized a significant role for soil C pools and molecular chemistry in mediating microbial community ecological and coevolutionary interactions in soil, and consequently, on microbial functional characteristics. Soil C content was significantly greater in polyculture soil relative to monocultures of either species (χ2 = 4.32, P = 0.038), but was not different among plant species. Moreover, our data suggest that soil C molecular diversity is significantly greater in polyculture assemblages relative to monocultures (P < 0.0001). Lignin and phenolic compounds, a major C class in soil from polycultures, were either present in low abundances or were absent in soil from monocultures of either species. Finally, preliminary findings show that fungal and bacterial community assemblages differ among monocultures and polycultures. Together our results suggest that plant diversity controls soil C pool size and molecular chemistry as well as the associated soil microbial community. We anticipate that such diversity effects will moderate warming effects on microbial community structure, which will also be reflected in differences in the metabolic potential among communities.