Bacterial community structure associated with the addition of nitrogen and the dynamics of soluble carbon in the rhizosphere of canola (Brassica napus) grown in a Podzol

Little is known about the community structure of bacteria in the rhizosphere of canola during growth. This study was conducted to characterize the community structure of soil bacteria in relation to soluble-C dynamics in a Podzol during the growth of canola. Next generation sequencing of 16S rDNA characterized the main bacterial communities in a Podzol that was unplanted (soil alone), and planted to canola (Brassica napus) with and without N-fertilizer addition. PERMANOVA and ANOVA analysis showed that the number of operational taxonomic units and the relative abundance of bacteria were higher in canola rhizosphere than in soil alone for phyla, classes, orders and families. In soil alone and at the family level, communities of Acidobacteriaceae, Micrococcaceae, Nocardioidaceae, Intrasporangiaceae and Nitrosomonadaceae had the highest relative abundances (sequence reads ranged from 1,000 to 3,000). In the rhizosphere, communities of Sphingomonadaceae, Bacillaceae, Gaiellaceae, Micrococcaceae and Intrasporangiaceae presented the highest relative abundance (sequence reads ranged from ~8,000 to 23,000). The addition of N-fertilizer increased the relative abundance of bacteria in 5 families, decreased those in 39 and did not affect the communities in 56 other families. A two-way ANOVA showed that management, inclusive of canola and N-fertilizer, had the largest (87%) effect on the total relative abundance of soil bacteria and soluble-C content. Polynomial regression models of second order showed a close relation between the relative abundance of bacteria and soluble-C content during crop growth. We suggest that diffusional constraints depleted soluble-C in the vicinity of microbial communities to a growth limiting concentration more frequently in the soil alone than in canola rhizosphere.