Bacterial Diversity in Rhizosphere and Rhizoplane of Field Corn Grown with Different Fertilization Regimes.

Microbes mediate organic matter mineralization and nitrogen cycling in soil and thus make nutrients available to plants.  Most previous studies of soil bacteria have focused on communities found in bulk soil; few studies have examined controls of bacterial diversity in the rhizosphere, where the plants and soil microbes interact, and how this relates to plant growth.  In this study we compared the bacterial biomass and diversity in the rhizosphere, which we defined as soil loosely associated with the root ball, and the rhizoplane, which we defined as soil washed from root surfaces.  Rhizosphere and rhizoplane communities were compared between samples obtained from a field corn study (University of Arkansas, Marianna, AR), where plots received a range of fertilizer type and application rates.  Bacterial biomass, as assessed by qPCR with universal 16S primers, was significantly higher in the rhizoplane than the rhizosphere.  Bacterial diversity, as assessed by pryo-sequencing of amplicons generated with universal 16S primers, correlated negatively with bacterial biomass in the rhizoplane.  Bacterial diversity in the rhizoplane also correlated negatively with the nitrogen content of corn leafs—an indirect measure of productivity—sampled from the plots.  Preliminary sequence analysis suggested the relative abundance of Proteobacteria increased and the relative abundance of Actinobacteria decreased with increasing leaf nitrogen content. This suggests that increased exudates released from nutrient-rich plants selected for Proteobacteria that thrive on this resource.