The influence of plant species identity and seasonality on soil rhizosphere bacterial communities

p>Background/Question/Methods   Plant community composition and diversity can influence soil bacterial community composition, hypothetically, through differences in the quantity and quality of resources provided by plants. These resources include leaf and root litter, and root exudates into the rhizosphere. However, little is known how temporal variation of native tallgrass prairie species influences soil rhizosphere bacterial communities (SRBC). Our objectives were to examine: 1) if three native tallgrass prairie plant species have different SRBC, 2) if SRB diversity increases with increased plant diversity, and 3) if SRBC changes significantly over the growing season, possibly responding to plant phenology differences. In 2006, at the Nelson Environmental Studies Area (NE Kansas), three native perennial tallgrass prairie plant species were sown into bare ground: Koeleria macrantha (C3), Andropogon gerardii (C4), and Helianthus maximiliani (F). The experiment includes ten replicates of nine treatments. Two treatments are ‘controls': bare ground (BG; i.e. bulk soil) and plants germinating from the existing seed bank (SB). The remaining treatments are comprised of monocultures, three 2-species mixtures and a 3-species mixture. In 2007, rhizosphere soil was sampled twice: during K. macrantha anthesis in June and during A. gerardii and H. maximiliani anthesis in late August. Rhizosphere bacterial communities were assessed using T-RFLP.

Results/Conclusions Results of only the control and monoculture plots from June show that SRB richness and diversity were similar across treatments. However, in late August, SRB richness and diversity tended to be lower in these treatments except BG plots (no change observed). Rhizosphere bacterial communities associated with H. maximiliani in August were significantly lower in richness and diversity compared to both of the grasses and control plots. Non-Metric Multidimensional Scaling analyses of SRBC indicated no differences in SRBC composition in June. In contrast, SRBC diverged among treatments in August. Bacterial communities associated with H. maximiliani and BG plots were different from each other and the other treatments. In addition, only H. maximiliani had different bacterial communities in August compared to June. These results indicate that different rhizosphere bacterial communities can be associated with different native tallgrass prairie species. However, the degree to which these communities are different can depend upon when the communities are sampled, possibly reflecting community-level responses to plant phenology. Furthermore, distinct rhizosphere bacterial communities such as those seen in this study could allow different plant species access to different nutrient pools, potentially impacting ecosystem functions such as productivity and nutrient loss through the soil profile.