Impact of treated wastewater on the performance of young plants of stone pine ( Pinus pinea L.) and sweet chestnut ( Castanea sativa Mill.) under semi-controlled Mediterranean conditions

Lebanon is experiencing increased water shortages with a sharp rise in water demand and recurrent hydrological droughts. Of all the national economic sectors, agriculture utilizes the largest share (70%) of the available water. Irrigation with treated wastewater (TWW) could be a sustainable option for additional, non-conventional water resources. In this context, juvenile plants of two agroforestry species ( Pinus pinea L. and Castanea sativa Mill.) were grown under a shaded frame. Primary (T1), secondary (T2) and tertiary (T3) TWWs were used for the irrigation of these nursery plants. Primary TWW is not recommended for reuse but was included for comparative purposes only. The control (C) consisted of irrigation with well water. After 14 months of treatment, the primary TWW caused an increase in height of 17% for pine and 22% for chestnut. Dry matter production increased significantly in T2 as compared to the control: by 81% in pine against 61% in chestnut. In addition, leaf area of sweet chestnut increased by 33% in T1 only. Overall, the response of chestnut was slower than that of stone pine. Among the three macronutrients, nitrogen was the most different between effluents, with the highest concentration found in primary TWW. By the end of the trial, the plants had been provided with 600 mm of water enriched with nutrients, essentially nitrogen. Pine plants treated with T2 had received 10 g of mineral N and accumulated 1.5 g N plant −1 . The build-up of N in plants treated with T3 was 1.4 times lower in pine and 2.7 times lower in chestnut compared to the corresponding plants treated with T1. Pine plants receiving TWW had twice as much potassium as their control, while it increased by 3.6 times for chestnut. Phosphorus build-up did not differ between the pine plants, whereas for chestnut, those grown in T1 and T2 accumulated more P than the control. By the end of the work, no increase in salinity, sodicity hazards, nor critical fecal coliform levels were detected in the soil. Controlled reuse of secondary TWW can be considered a safe practice, notably in East Mediterranean soils with poor N contents.