Changes in the Microbial Properties of Olive Cultivated Soils under Short, Medium and Long-term Irrigation with Treated Wastewater

Aims: In Tunisia, Climatic changes and water shortage has led to the reuse of treated municipal wastewater (TMWW) in the agricultural sector since the sixties. This work was intended to study the short, medium, and long-term impacts of this practice on soil microbial properties.

Study Design: Five different experimental fields were chosen which had been irrigated with TMWW for 10, 20, 25, and 28 years, respectively. A pluvial irrigated field was selected as a control.

Place and Duration of Study: The study was conducted in Zaouit Sousse (Tunisian Sahel region) located in the south of Sousse City (longitude: 35°47′, latitude: 10°38′ and of altitudes: 20 m N.G.T.). The soil sampling campaign was carried out at the end of the dry season (September 2014). This study was undertaken in a semi-arid area that is facing a water crisis (water shortage and irreversible seawater intrusion).

Methodology: Soil fecal pollution indicators were determined with the most probable number MPN method. Bacterial and fungal enumeration was done by the plate count agar method. Pathogenic bacteria was determined using the conventional bacteria identification methods.

Results: Irrigation with TMWW (for more than ten years) induced a significant increase in soil microbial biomass (heterotrophic bacteria and filamentous fungi). Soil microbial contamination was assessed by measuring Total and Fecal Coliforms, E. coli, and Faecal Streptococci at three studied soil layers (0-20; 20-40 and 40-60 cm) show’s a significant increase in TMWW irrigated plots compared to the control. Salmonella spp. and Shigella spp. screening revealed the absence of those pathogens in all studied soils. This result is true for the three soil horizons (0-20; 20-40 and 40-60 cm). This result seems to be due to the short survival period of these pathogens in the soil.

Conclusion: TMWW irrigation had positive effects on soil fertility. However, this practice has led to a deterioration of the soil sanitary quality. The quality of the wastewater treated in Sousse Sud plant must be improved to ensure the reduction of emerging bacterial pathogens to non-detectable levels or to levels that have not been associated with human health risk.