| Plants interact with diverse microbial communities in the rhizosphere that serve as a critical link mediating soil nutrient cycling and plant nutrient supply. As such, the controls on the composition and activity of this community and, in particular, the role of plant species and genotypes shaping community composition and activity is a subject of ongoing research with important implications for plant breeding and agricultural management. This dissertation seeks to understand the extent of plant genotype driven variation in rhizosphere bacterial community composition within maize (Zea mays subsp mays) and among species of summer annuals characteristic of agricultural fields. In three field experiments a common garden experimental design is combined with measures of plant growth and nitrogen acquisition, profiling of bacterial community composition (BCC) via 16S rRNA gene amplicon sequencing, and measures of potential extracellular enzyme activity to test hypotheses that: 1) plant variation in rhizosphere BCC is predicted by plant evolutionary history, 2) plant variation in growth and nitrogen economy influences rhizosphere BCC, and 3) historical selection for yield in fertilized production systems has altered maize rhizosphere bacterial community assembly and plant N acquisition efficiencies. We find that plant species differentially select rhizosphere bacterial communities and that the magnitude of variation is related to both plant phylogeny and variation in growth and nitrogen economy. Intraspecific variation in rhizosphere BCC between genotypes within a plant species is also observed, but these differences are of lower magnitude and not well described by either functional variation or overall genetic distance between genotypes. Secondly, we observe that temporal variation in rhizosphere assembly and activity closely parallels temporal variation in plant growth and nitrogen uptake, which further highlights the link between plant function and plant rhizosphere effects. Finally, our results indicate that breeding has improved nitrogen uptake efficiency of maize hybrids but has not resulted in a parallel change in rhizosphere BCC. The implications of these patterns of variation in rhizosphere BCC are discussed in respect to agroecosystem management and plant breeding. |
