Sustaining citrus production with tertiary-treated wastewater in arid agriculture: Long-term soil, crop, and food-safety responses

Treated wastewater is increasingly promoted to reduce freshwater demand in arid agriculture. Yet long-term field evidence for perennial fruit systems remains limited, particularly when reclaimed water is compared with desalinated groundwater. This study evaluated the soil, crop, and food-safety implications of eight years of irrigation with tertiary-treated wastewater (TWW) in an Omani lime (Citrus aurantiifolia) orchard under hyper-arid conditions in northern Oman. A paired-field assessment compared a TWW-irrigated field with a desalinated groundwater (DGW)-irrigated field, using targeted monitoring campaigns conducted from 2021 to 2025. The assessment covered irrigation-water chemistry, soil salinity–sodicity, nutrients, water-extractable trace metals, leaf and fruit elemental composition, vegetative growth, yield, fruit quality, and microbiological safety. Compared with DGW, TWW had higher electrical conductivity and greater concentrations of major ions and nutrient-related elements, including sodium, calcium, magnesium, potassium, phosphorus, boron, and measurable nitrogen fractions. Irrigation-water SAR was 11.59 for DGW and 7.77 for TWW, indicating that sodium-related risk was not governed by sodium concentration alone, but by the balance among salinity, sodium, calcium, and magnesium and by the resulting soil response. Soil EC under TWW reached 4.10 dS m⁻¹ at 30–60 cm in 2024 and declined in 2025, indicating interannual variability in root-zone salinity rather than progressive salt accumulation during the monitoring period. In contrast, soil SAR remained consistently higher under TWW and approached or exceeded the conventional sodicity threshold in the upper and middle soil profile, identifying sodicity as the main soil-related constraint requiring continued monitoring. Water-extractable trace metals remained low, and fruit microbiological quality complied with the applied food-safety criteria. Trees in the TWW-irrigated field showed greater plant length and trunk diameter, whereas yield and most fruit-quality traits were statistically comparable to DGW. Overall, long-term TWW irrigation can support citrus production under arid conditions, highlighting its role in sustainable agriculture and informing stakeholders about potential risks and benefits.