Root microbiomes of Zygophyllaceae in the Kazakhstan desert: a weak evidence for rhizobial nitrogen fixation

The plants of the Zygophyllaceae family are often considered to be capable of symbiotic atmosphere nitrogen fixation, forming rhizobial nodules similar to those of Fabaceae and Urticaceae. In four species of Zygophyllaceae and the related Peganum harmala (hereafter called Z-plants), collected in Dzungarian deserts (Kazakhstan), we analyzed the prokaryotic component of the root microbiome and the ¹⁵N and ¹³C content in the leaves of these Z-plants compared with neighboring dicots belonging to other families (R-plants). Among all studied Zygophyllum fabago root samples only one was found to have the root nodule, however the microbiome of this sample was similar to that found in other Z. fabago root samples where nodu-lation was not observed. If compared to Z-plants, the R-plants had a significantly higher relative abundance of bacteria of genera Glycomyces, Massilia and Streptomyces, while the Z-plants had a higher relative abundance of the representatives of genera Actinomadura, Nocardia and Pseudonocardia. Unidentified representatives of the family Rhizobiaceae were present in some of R and Z plants, being most abundant in soil samples. On average, the Z- and the R-plants did not differ in their δ¹³С and δ¹⁵N. Overall, the results do not allow us to consider the studied Zygophyllaceae plants as species with effective atmospheric nitrogen fixation due to rhizobial symbiosis – a characteristic feature of the most legumes.