Soil carbon quality and nitrogen fertilization structure bacterial communities with predictable responses of major bacterial phyla

• We test the idea of indicator taxa using a long-term fertilization experiment.
• N-fertilization not organic amendment was main driver of bacterial community change.
• Relative abundance of major taxa decreased with N-fertilization.
• Specific bacterial taxa were indicative of soil nutrient status.

Agricultural practices affect the soil ecosystem in multiple ways and the soil microbial communities represent an integrated and dynamic measure of soil status. Our aim was to test whether the soil bacterial community and the relative abundance of major bacterial phyla responded predictably to long-term organic amendments representing different carbon qualities (peat and straw) in combination with nitrogen fertilization levels and if certain bacterial groups were indicative of specific treatments. We hypothesized that the long-term treatments had created distinctly different ecological niches for soil bacteria, suitable for either fast-growing copiotrophic bacteria, or slow-growing oligotrophic bacteria. Based on terminal-restriction fragment length polymorphism of the 16S rRNA genes from the total soil bacterial community and taxa-specific quantitative real-time PCR of seven different groups, all treatments significantly affected the community structure, but nitrogen fertilization was the most important driver for changes in the relative abundances of the studied taxa. According to an indicator species analysis, the changes were largely explained by the decline in the relative abundances of Acidobacteria, Gemmatimonadetes and Verrucomicrobia with nitrogen fertilization. Conditions more favourable for copiotrophic life strategies were indicated in these plots by the decreased metabolic quotient, i.e. the ratio between basal respiration rate and soil biomass. Apart from the Alphaproteobacteria that were significantly associated with peat, no taxa were indicative of organic amendment in general. However, several significant indicators of both peat and straw were identified among the terminal restriction fragments suggesting that changes induced by the organic amendments were mainly manifested at a lower taxonomical level. Our findings strengthen the proposition that certain higher bacterial taxa adapt in an ecologically coherent way in response to changes induced by fertilization.