The Effect of Nitrogen On Rhizobacteria Associated with Wheat Shoot Productivity.

Rhizobacteria are known to be closely associated with plant productivity. In this study, we phylogenetically identified specific rhizobacterial groups associated with wheat productivity as affected by nitrogen fertilization. We grew wheat plants singly in 96 microcosms containing homogenized field soil fertilized to 100 lbs/A urea nitrogen and 96 microcosms with a basal level of 35 lbs/A nitrogen. Wheat was grown in a controlled environmental greenhouse set at 74o / 65o C day/night temperatures. Soil moisture was controlled gravimetrically between 15% and 36% volumetric water content. To condition the soil wheat was grown for four cycles of planting and harvesting prior to rhizosphere extraction. At the fourth cycle, after eight weeks growth, rhizosphere soil plus roots were extracted and grouped into eight discrete categories based on the corresponding shoot weights from low to high. DNA was extracted from the rhizosphere soil and roots using MoBio UltraClean DNA extraction kit. Rhizosphere DNA from each category was subjected to 16s rRNA pyrosequencing in order to quantify the OTU community composition. OTU classification was by the RDP II pyrosequencing pipeline. Linear regression was used to correlate the numbers of OTU sequences with shoot biomass from each category. Shannon and Chao1 indicies were used to analyze the diversity and richness of the rhizobacteria community at varying levels of productivity and with and without nitrogen addition. The effect of nitrogen addition on rhizobacteria community composition and association with productivity was determined. Phylogenetic groups most closely correlated both positively and negatively with productivity were identified. This work forms the basis of future studies to functionally characterize the rhizobacterial community with respect to nitrogen addition and shoot biomass productivity.