Environmental heterogeneity has been hypothesized to promote plant and animal diversity by increasing resources that can be partitioned and facilitating species coexistence. Microbial community response to environmental heterogeneity is less understood. As such, we used a split-split block design to determine the effects of heterogeneity in soil resources (3 levels of nitrogen availability and 2 levels of soil depth), plant presence (Andropogon gerardii rhizosphere, Salvia azurea rhizosphere, and bare soil) on microbial communities characterized by phospholipid fatty acid biomarkers (PLFA). Community responses were analyzed using non-metric multidimensional scaling (NMDS), permutational multivariate analysis of variance (PERMANOVA), and mixed model analysis of Shannon’s diversity of biomarkers. Biomarker concentrations were summed within each broad microbial group for further analyses. Total abundance and abundance of major microbial groups were analyzed using mixed models with all possible interaction of soil depth, soil nitrogen availability, and plant identity (Andropogon gerardii, Salvia azurea, bare soil).
Results/Conclusions
The soil treatments did not influence community dissimilarity of PLFA biomarkers, but Shannon’s diversity was affected by plant species. Diversity was higher in rhizosphere soil as compared to bare soil (F3, 36 = 4.34, P = 0.020). The diversity response was not due to richness or evenness alone and was associated with greater proportional abundance of arbuscular mycorrhizal fungi and Gram-negative bacteria in rhizosphere soil. Abundances of major microbial groups were affected by nitrogen availability. There was greater PLFA biomass in reduced nitrogen treatments [total PLFA (F2, 5.9 = 9.94, P = 0.013), non-specific bacteria (F2, 5.9 = 10.38, P = 0.012), Gram-positive bacteria (F2, 5.9 = 11.99, P = 0.008), Gram-negative bacteria (F2, 5.9 = 14.93, P = 0.005), saprophytic fungi (F2, 5.9 = 13.30, P = 0.007), arbuscular mycorrhizal fungi (F2, 5.9 = 6.22, P = 0.035), and actinomycetes (F2, 5.9 = 5.94, P = 0.038]; soil depth and plant identity had no effect on abundance within these groups. These results suggest that a wide range of microbial groups positively respond to labile carbon enrichment used to reduce nitrogen availability, whereas root systems with more heterogeneous labile carbon inputs increase microbial diversity.