Nitrogen fertilizer rate affects root exudation, the rhizosphere microbiome and nitrogen-use-efficiency of maize

Abstract

The composition and function of microbial communities in the rhizosphere of crops have been linked to edaphic factors and root exudate composition. We examined the effect of N fertilizer (urea) rate on maize root exudation, the associated rhizosphere microbial community, and nitrogen-use-efficiency. Increasing N rate had a significant effect on root exudate quantity and composition. Specifically, the total abundance of sugars, sugar alcohols, and phenolics was positively correlated with N rate (p<0.005). Similarly, the abundance of rhizosphere bacteria (16S rRNA copies g−1 soil FW) was enhanced with increasing N rate. Using PICRUSt, we also explored the metagenomic contribution of bacterial OTUs to the abundance of N cycle-related genes in the maize rhizosphere. On a relative abundance basis, the nitrifying-(pmoA-amoA) and denitrifying-genes (nirK and nosZ) were significantly influenced by N rate (p<0.05); whereas, the nitrogen fixing (nifD and nifH) and urease (ureC) genes were not influenced by N rate (p>0.05). However, on a total abundance basis (gene copies g−1 soil FW) all N-cycle genes increased significantly with increasing N rate (p<0.05). Percent N recovery from both soil and fertilizer sources showed a curvilinear response that was highest at intermediate N rates; whereas, fertilizer N lost from the system increased significantly at the two highest N rates (p<0.05). In summary, our results show high N rates increase both root exudation and the abundance of soil bacteria, which may help explain the decline in fertilizer-use-efficiency and loss of N from the system at higher N rates.