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

Author
item SHUSHEN, Z - Yunnan Agricultural University
item VIVANCO, JORGE - Colorado State University
item Manter, Daniel

Submitted to: Applied Soil Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/16/2016
Publication Date: 7/25/2016
Citation: Shushen, Z., Vivanco, J., Manter, D.K. 2016. Nitrogen fertilizer rate affects root exudation, the rhizosphere microbiome and nitrogen-use-efficiency of maize. Applied Soil Ecology. 107:324-333. doi:10.1016/j.apsoil.2016.07.009.

Interpretive Summary: The structure of rhizosphere microbial communities is the result of complex interactions between maize and N fertilization. When maize was supplied with increasing amounts of N, roots secreted more sugars, sugar alcohols, and phenolics, which subsequently altered the soil microbial community structure and abundance. In particular, high N rates appear to increase the activity of ammonia-oxidizing and denitrification bacteria leading to a decrease in nitrogen-use-efficiency. In summary, our results suggest that N has the ability to modify the composition and abundance of root exudates and to subsequently affect the rhizosphere microbial communities. As a result, excessive N fertilizer does not appear to benefit maize through the recruitment of beneficial microbes or the deterrence of undesirable microbes, but instead appears to lead to an overall increase in microbial abundance. Based on the results observed in this study, we suggest that identifying maize cultivars that have reduced root exudation at high N rates may promote NUE and crop yields by (i) reducing C and N losses through root exudation and (ii) minimizing N immobilization and/or N losses (e.g., denitrification) by soil microbes.

Technical Abstract: The composition and function of microbial communities present in the rhizosphere of crops has been linked to edaphic factors and root exudate composition. In this paper, we examined the effect of N fertilizer rate on maize root exudation, the associated rhizosphere 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 obtained from a soil leachate increased with increasing 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) 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 and the amount of fertilizer N lost from the system increased significantly at the two highest N rates (p < 0.05). In summary, our results suggest that root exudation quantity and composition, particularly sugars and phenolics, is influenced by N fertilizer rates and is associated with an increased abundance of N-cycle bacteria, which may help explain the decline in fertilizer-use-efficiency and loss of N from the system at higher N rates.