Irrigation with treated wastewater in humid regions: Effects on Nitisols, sugarcane yield and quality

Treated wastewater (TWW) reuse for crop irrigation has been developing in recent decades in areas hampered by severe water shortages. However, in humid conditions, the agronomic impacts of supplementary irrigation are less and poorly documented. On the island of Réunion, we measured sugarcane yield and quality on a Nitisol over 2 years by providing 2600 mm of TWW or irrigation water (control) as a supplement to the 3500 mm rainfall. Meanwhile, with the same TWW, irrigation water (IW) and rainfall inputs, we monitored leachates and soil under controlled conditions without plants. Sugarcane yield, Brix degree, fiber and sucrose contents were similar between the TWW and control plots each year. The leachate pH and electrical conductivity at 60 cm depth showed marked differences between the TWW and control modalities. Similarly, after a constant increase in chloride, sulfate, potassium and sodium contents, or breakthrough curves for calcium and magnesium, the concentrations of these elements under the TWW modality were stable from 12 pore volumes (PV), except for calcium which was stable from 16 PV. At the end of the experiment, the hydraulic conductivity and organic carbon contents did not show any significant change, while the pH of soils that had received TWW and of the control soil was higher overall than the pH of the initial soil. The dynamics of the major elements in leachates at 65 cm depth were consistent with the input concentrations and the variations in exchangeable base contents in the soil at the end of the experiment. However, potassium and mineral nitrogen had accumulated in the soil profile while there were low concentrations in the leachate. The TWW quality and quantities provided, uptake by plants, competition for adsorption or leaching, and the pedoclimatic context, could therefore: (i) explain the absence of negative agronomic impact, and (ii) highlight the benefits of irrigation with TWW in humid environments to supplement water requirements and fertilize intensive crops.