Soil properties and habitats determine the response of bacterial communities to agricultural waste water irrigation

The objective of this study was to characterize the importance of soil properties and habitat for the response of soil bacteria and archaea to the irrigation with secondary treated wastewater (TWW). Two agricultural soils with contrasting textures (loamy sand, silt loam), and for each, three variants differing in soil organic carbon and nitrogen, as generated by long-term fertilizations, were analyzed. For each of these six soils, prokaryotic communities from two habitats: i.e. root-free soil, and rhizosphere of developing cucumber plants in the greenhouse, were characterized. Communities were analyzed by quantity and diversity of their PCR-amplified 16S rRNA genes. To account for TWW associated nutrient effects, potable water (PW) served as a control treatment. Amplicon sequence analysis showed that prokaryotic communities manly consisted of bacteria (99.8 %). Upon irrigation, regardless of the water quality, prokaryotic diversity declined, pH increased, and no bacterial growth was detected in bulk soil. In contrast, growth of cucumbers was stimulated by TWW, indicating that plants were the main beneficiaries. Moreover, strong responses were seen in the rhizospheres, suggesting an indirect effect of TWW by altered rhizodepositions. Main bacterial responders to TWW were Proteobacteria, Bacteroidetes, Actinobacteria, and Planctomycetes. Changes of the bacterial community to TWW were less pronounced in all variants of the silt loam, indicating the importance of clay and soil organic carbon for buffering effects of TWW on soil bacterial communities. Hence, soil organic carbon and soil texture are important parameters that need to be considered when applying TWW in agriculture.