Effects of Deficit Irrigation and Organic Fertilizer Substitution on Lettuce (Lactuca sativa L.): A Two-Season Field Experiment and AquaCrop Simulation in Songjiang, Shanghai

Water scarcity and environmental degradation have emerged as critical challenges threatening sustainable agricultural development worldwide. This study hypothesized that organic substitution buffers the physiological stress of deficit irrigation on lettuce (Lactuca sativa L.), and that this buffering effect of organic substitution can be quantified using AquaCrop to optimize irrigation schedules. Field experiments were conducted to evaluate the interactive effects of irrigation levels and organic/urea fertilizer rates on soil moisture, lettuce growth and yield during two growing seasons (April to May and September to October 2024) in Songjiang, Shanghai. Six treatments combining two irrigation levels (full irrigation at 100% of crop evapotranspiration (ETc) and deficit irrigation at 60% ETc) with three fertilization levels (300 kg/ha urea, 150 kg/ha urea + 15 t/ha organic fertilizer, and 150 kg/ha urea) were conducted. AquaCrop was calibrated and validated using field observations. The results demonstrated that organic substitution under deficit irrigation preserved soil moisture, canopy development, and fresh yield at levels comparable to full irrigation and nitrogen fertilization, whereas 50% nitrogen alone caused substantial yield reduction. AquaCrop showed satisfactory performance in simulating lettuce growth under organic substitution, with normalized root mean square error (NRMSE) values of 3.07%~21.42%, 5.02%~20.11%, and 8.22%~20.95% for soil water content, canopy cover, and fresh yield, respectively. Using the validated model, scenario analysis across the different precipitation years revealed that lettuce yield was most responsive to irrigation during the seedling stage, followed by the rosette stage and cupping stage. Specifically, 100% ETc during the seedling and rosette stages was suggested for dry years; 100% ETc at the seedling stage combined with 80% to 100% ETc at the rosette stage for normal years; and more flexible strategies (60% to 100% ETc at the seedling stage) for wet years based on model predictions. These irrigation strategies, combined with organic substitution, can achieve above 90% of the potential yield of the full irrigation and nitrogen treatment. These findings offer preliminary guidance for irrigation and fertilization management of lettuce under the experimental climate and soil conditions, while further validation across diverse cultivars and environments is needed before broader application.