Heavy metal accumulation and changes in soil enzymes activities and bacterial functional diversity under long-term treated wastewater irrigation in East Central region of Tunisia (Monastir governorate)

Climate change and population growth generates a decrease in water availability around the world which can compromise the maintenance of sustainable agriculture. Thus, treated wastewater (TWW) became an alternative to minimize water shortage. However, this may indirectly affect the soil's microbial properties. In this study different soils irrigated for 0, 1, 8 and 20 years with TWW were sampled and from the east central region of Tunisia. The objective was to investigate its effect on heavy metal accumulation and enzyme activities of β-glucosidase, arylsulfatase, acid phosphatase, alkaline phosphatase, urease, dehydrogenase and fluorescein diacetate (FDA). Moreover, carbon microbial biomass (Cmic) using Substrate induced respiration (SIR) technique and microbial functional diversity using Biolog EcoPlate™ were assessed. Our results had shown a crucial decrease of the enzymatic activities in soils that were newly irrigated with TWW, while a crucial increase was recorded in soils exposed to TWW for 20 years. In addition, chemical analysis revealed an important accumulation of heavy metals such as Cd, Cu, Zn and Ni along with the irrigation period, especially in 20 years-irrigated soils with TWW. The assessment of the bacterial functional diversity showed an important change in Average well color development (AWCD) and in diversity index values. Besides, substrate utilization profile revealed a significant variation along with the irrigation periods. In conclusion, long-term TWW irrigation has increased the metabolic activities of the soil's microorganisms. Meanwhile, traces elements accumulation could pose a serious threat for biological processes in soils and a limiting factor for the reuse of TWW in agriculture.