Assessment of the response of calcareous soils to 27 years vs. 16 years of irrigation with treated wastewater: a case study in Eastern Tunisia

The duration of irrigation with treated wastewater emerges as a key factor influencing the impact of this practice on the properties of calcareous soils in semi-arid regions. This study targets three selected cropland plots of calcisol: two plots are irrigated with treated wastewater for different periods (27 years, 16 years) and the third one is non irrigated (control). A systematic vertical sampling was performed, based on a triangular grid in 8 layers every 10 cm. All soil samples are characterized by their texture, alkalinity, organic matter, carbonates, CEC, salinity, sodicity and total contents of heavy metals such as Cu, Zn, Ni, Mn and Fe. Statistical analyses were performed to compare the variability patterns of physicochemical characteristics between layers, including: Wilcoxon-Mann-Whitney test and PCA analysis. The study revealed different effects of the irrigation durations on soil properties. The soils irrigated since 27 years showed an increase in pH and total CaCO₃ content and became saline sodic soils. These effects are attributed to the sodification process after long-term of irrigation. Conversely, after 16 years of irrigation, soils showed a decrease in pH and CaCO₃ content due to salt inputs from treated wastewater, which substitute Ca in the argilo-humic complex of the soil and become saline-non sodic. Moreover, the both irrigated soils revealed an increase of total heavy metal (Cu, Zn, Mn and Fe). These changes highlighted distinct correlation patterns between parameters for each irrigation period, emphasizing the importance of native calcite alteration.