Improving the suitability of desalinated water for irrigated agriculture using adsorbents based on clay and ash

Water scarcity and high pressure over available water among productive sectors are one of the main limiting factors on the competitiveness of irrigated Mediterranean crop production. In addition to agronomic practices to improve water productivity, using non-conventional water sources, such as desalinated water (DW), could be an effective solution if its suitability for irrigation is improved, as it often presents constraints related to its mineral composition, particularly due to high concentrations of boron (B), which can reduce crop productivity. In this regard, treating DW with natural and cost-effective adsorbents, such as clay or carbon ash, offers a promising strategy to reduce water nutrient excess and enhance its suitability for irrigation. For this reason, the objective of our work was to evaluate the effectiveness of adsorbents on DW quality, and the response of winter lettuce grown in pots with coconut fiber irrigated with treated DW. Three randomized treatments (n = 5) were established according to the irrigation water source: (i) a control treatment (CTL), in which plants were irrigated with DW containing 1.2 mg L⁻¹ of B; (ii) T1, in which DW was treated by decantation with 20 g L⁻¹ of clay, mainly composed of kaolinite and sepiolite; and (iii) T2, in which DW was treated by filtration using 50 g L⁻¹ of carbon ash. In all treatments, irrigation was scheduled using a threshold corresponding to a maximum soil water depletion of 20% relative to field capacity. Crop physiological response was assessed as net photosynthesis (Pn) and leaf stomatal conductance (Lc), together with aerial fresh weight and the concentration of macro- and micro-nutrients in both leaves and roots. Prior to the ANOVA assumptions were tested and when differences were detected, means were separated by using LSD-Fisher test (p<0.05). Lettuce irrigated with treated water (T1 and T2) showed a significant increase of 4.13% in fresh weight compared with the DW treatment, whereas net Pn and Lc were not affected. Boron concentrations in leaves and roots were reduced by 37.0% and 25.9% for clay and ash treatments, respectively; while leaf Cu concentration decreased by 28.2% in T2. Leaf manganese also decreased by an average of 38.0% in T1 and T2, while no differences were detected for Na, K, Ca, Mg, and Cl. Finally, our results suggested that the use of natural adsorbents could effectively reduce the excess of some nutrients in desalinated water, reducing the negative effect on growth. Furthermore, the selection of an adsorbent should be based on the crop's tolerance to the most critical element, with clay being most effective for filtering B and ash for Cu. This approach offers a practical strategy for utilizing non-conventional water sources in agriculture and reducing pressure on water resources through cost-effective and scalable solutions.

Funding: Biodiversity Foundation: “Innovative agricultural practices to contribute to environmental improvement and biodiversity in the Mar Menor area (NEWAGROMARMENOR)”.