Wastewater reuse for tree irrigation: Influence on rhizosphere microbial communities

Abstract

Irrigation water can increase tree biomass production, but it is often unavailable near grove locations. Alternative water resources, such as treated wastewater (TWW), offer a potential solution, while additionally contributing to a circular economy. To assess environmental sustainability, the impact of residual wastewater contaminants on soil microorganisms must be understood. This study compared soil microbial communities from purpose-grown tree groves irrigated with TWW for at least ten years, with communities from unirrigated control plots. Short-term batch testing was conducted to compare the degradation capabilities for five trace-level organic contaminants (TOrCs) associated with reclaimed wastewater. Microbial community composition was examined using 16S rRNA Illumina sequencing. Microbial biomass and abundance of a putative bisphenol A (BPA) degradation gene were quantified via droplet digital PCR. Principal component analysis of the sequencing results revealed distinct clustering of the irrigated and non-irrigated communities at two sites. Control soils from all sites exhibited intrinsic degradation of BPA and ibuprofen. Additionally, several OTUs of the Acidobacteria and Proteobacteria were correlated with soil organic carbon and TOrC degradation. Together, these results suggested that rhizosphere microbial communities were altered in long-term wastewater infiltration galleries, though the impacts of increased soil moisture due to irrigation and the effects of exposure to residual contaminants are difficult to separate. A greater understanding of TWW impacts on soil microbial communities will help expand the prevalence of wastewater reuse for biomass production, contributing to the growth of the circular economy. • Treated wastewater irrigation correlated with changes to soil bacterial communities. • Soil microbial communities intrinsically degraded bisphenol A and ibuprofen. • Moisture, carbon, and tree species may shape communities in infiltration galleries. • Droplet digital PCR was used to quantify microbial biomass in soil samples.