Resilience of soil microbiome to anthropogenic disturbance: the case of irrigation with water of different qualities

Limited availability of fresh water (FW) is driving agriculture to use water of lower quality such as treated wastewater (TWW), a practice that is on the rise in arid and semiarid regions. However, irrigation with TWW, which contains dissolved organic matter, salts and microorganisms, might alter the microbial populations in soils, and thus affect soil fertility. We characterized the effects of irrigation with TWW and FW on soil bacterial community composition for three consecutive years. Samples were collected and analyzed at the end of each rainy and dry (i.e. irrigation) season. Soil bacterial community composition, determined by 16S rDNA amplicon pyrosequencing, was highly similar in the different soil samples obtained at the end of three consecutive rainy seasons, regardless of dry season irrigation water quality. However, whereas community composition in FW-irrigated soils shifted slightly during the dry seasons, it was greatly influenced by irrigation with TWW. During the dry season (irrigation with either TWW or FW), a decrease in the relative abundance of Actinobacteria, the most dominant group in the soil, was observed; we also observed an increase in the relative abundance of Proteobacteria, particularly Alphaproteobacteria, with both water types, and an up to threefold increase in Gammaproteobacteria with TWW irrigation. These results and the return to the "baseline state" demonstrate that the soil bacterial community is not resistant to the anthropogenic impact imposed by irrigation water quality, but it is resilient in long term.