Changes in chemical properties of semiarid soils under long-term secondary treated wastewater irrigation

Treated wastewater (TWW) is becoming a valuable resource for irrigation in semiarid regions, where freshwater is scarce; however, changes in soil chemical properties under TWW irrigation can have significant effects on agronomy, hydrology, and the environment. The effects of long-term irrigation with secondary TWW on chemical properties of two contrasting soils under semiarid Mediterranean conditions were determined in the present study. Experiments were conducted in two different grapefruit (Citrus paradise Macfad.) orchards, one with a noncalcareous sandy soil (a Typic Haploxeralf) and the other with a calcareous clayey soil (a Chromic Haploxerert). Two treatments were tested: (i) irrigation with fresh water and (ii) irrigation with domestic, secondary TTW. In both soils, a longterm irrigation (>7 yr) with TWW had insignificant effects on total organic matter content and concentrations of dissolved organic matter, Cu, Ni, and Zn in saturated paste extracts. In contrast, it increased salt accumulation in the upper ~1-m layer in both soils; however, this salt was leached during each rainy season below 1.5 m. Sodium adsorption ratio was higher under TWW than under freshwater irrigation down to at least the 4.0- and 0.7-m depths in the sandy and clayey soils, respectively. The high clay and CaCO ₃ contents in the latter limited deeper movement of Na ⁺ ions. The dominant N species in the soils under TWW irrigation was NO ₃ ^- due to nitrification, and its concentration increased in the whole sampling depth in both soils. Olsen P and K ⁺ accumulated in soil profiles above the 1.0-m depth. Boron concentration increased in the upper 1.75-m layer under TWW irrigation, where this concentration could be toxic to some crops.