Responses of soil characteristics and bacterial communities to agricultural irrigation with different treatment levels of wastewater

This study aimed to investigate the effects of domestic wastewater treated at different treatment levels on the biochemical characteristics of irrigated soils. Raw domestic wastewater was subjected to four treatment processes: (1) grit sedimentation; (2) grit sedimentation combined with ultrafiltration (UF); (3) grit sedimentation combined with anaerobic activated sludge treatment followed by UF; and (4) grit sedimentation combined with aerobic activated sludge treatment followed by UF. The resulting effluents (designated TWW1–TWW4) were used to irrigate soil cultivated with spinach, with tap water irrigation and non-irrigated soil serving as controls. After 50 days of irrigation, soil biochemical properties were determined. TWW1 and TWW2 exhibited COD and fecal coliform concentrations exceeding the Chinese irrigation water quality standards (GB5084-2021), whereas TWW3 and TWW4 met all stipulated requirements. TWW irrigation generally increased soil organic carbon content and electrical conductivity while decreasing soil pH, with the extent of pH reduction depending on the NH₄⁺-N concentration in the TWW. Microbial diversity was higher in TWW-irrigated soils than in controls. Specifically, genera involved in nitrogen cycling (e.g., Rhizobium , Nitrospira , and Nitrosospira ) and complex organic compound degradation (e.g., Vicinamibacteria and Blastocatellia ) were significantly enriched. This study provides valuable insights for optimizing wastewater treatment strategies to achieve safe and sustainable agricultural reuse.