Eight Years in the Soil: Temporal Dynamics of Wheat-Associated Bacterial Communities Under Dryland and Irrigated Conditions

Plants and their associated microbiomes are impacted by environmental factors that shape their interactions over time, but long-term trends in microbial community dynamics are not well characterized. We identified the seasonal and long-term population dynamics of root-associated bacterial communities over 8 years in monocropped wheat grown in adjacent dryland and irrigated plots in the low-precipitation region of Washington State. Spring wheat plants were sampled at various times during the growing season over 8 years, and rhizosphere and endosphere communities were characterized using 16S rRNA amplicon sequencing. Analyses throughout the first 3 years revealed genera with distinct annual periodicity in response to growing season conditions. Bacterial richness and diversity were significantly greater in irrigated than in dryland wheat and in the rhizosphere than in the endosphere, apparently driven by the physiological state of the root and root exudates and/or by soil water. Over 8 years, copiotrophic Proteobacteria and Bacteroidetes (Pseudomonas, Variovorax, Chryseobacterium) maintained stable populations, whereas Actinobacteria decreased in abundance independent of irrigation. Populations of some Bacteroidetes and Proteobacteria (Mucilaginibacter, Sphingomonas, Massilia, Burkholderia) persisted or increased in relative abundance in the dryland rhizosphere, whereas others (Rhizobium, Acidovorax, Terrimonas, Hyphomicrobium, Bdellovibrio) increased under irrigation. In contrast, endosphere taxa, including Actinobacteria, declined in relative abundance regardless of irrigation, indicating that the maturing host, and not water, is the main driver of these populations. Our results provide insight into the long-term dynamics of bacterial communities of wheat under contrasting soil water conditions and guide future efforts to improve crop performance under drought conditions. [Figure: see text] The author(s) have dedicated the work to the public domain under the Creative Commons CC0 “No Rights Reserved” license by waiving all of his or her rights to the work worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law, 2025.