Integrated assessment of castor (Ricinus communis L.) productivity and soil salinity under treated wastewater irrigation in an arid farming system

Freshwater scarcity in arid regions has increased interest in treated wastewater irrigation for salt-tolerant industrial crops such as castor ( Ricinus communis L.), but integrated information on crop productivity and soil responses remains limited. This two-year field study evaluated freshwater (FW, control) and municipal treated wastewater (TWW) irrigation on castor growth, seed yield, soil salinity, exchangeable cations, water-stable aggregates, carbon and nitrogen dynamics, and δ 13 C and δ 15 N isotope signatures in El Paso, Texas, USA. A randomized complete block design with six replications was used, and soil was sampled from four depths (0-15, 15-30, 30-45, and 45-60 cm). TWW did not significantly affect plant height and leaf number, but average seed yield declined from 1.63 Mg ha -1 under FW to 0.99 Mg ha -1 under TWW, a 39.3% reduction. TWW increased soil salinity and soluble ion concentrations, particularly SO 4 2- , HCO 3 - , and Cl - , with greater accumulation in deeper layers. Exchangeable sodium percentage showed no severe short-term deterioration, indicating that soluble salt accumulation preceded sodicity-related degradation. Soil organic carbon, total carbon, total nitrogen, and isotope values varied mainly with depth. Soil δ 15 N was generally higher under TWW, whereas castor showed a contrasting δ 15 N response and less negative δ 13 C under TWW. The δ 13 C–δ 15 N biplot showed qualitative associations among bulk soil, carbonate-rich material, local vegetation, fertilizers, and castor rather than quantitative source apportionment. Over the two years study, deeper root-zone salt accumulation was the main concern, while aggregate stability remained largely unaffected. Long-term monitoring under continued TWW irrigation should prioritize ECe, SAR, and seed-yield stability.