Investigating the effect of irrigation method and nitrogen rate on yield and water productivity of rice (Oryza sativa L. cv. Anbarboo) in Ilam province

Introduction
Rice is a strategic cereal crop that secures food security for millions of people in Iran and worldwide. However, traditional flooding irrigation, which is still the dominant practice in Iranian rice paddies, consumes approximately 1.5 times more water than the global average – an unsustainable practice under the severe water scarcity conditions prevailing in most parts of the country. Dry seeding technology eliminates the nursery and transplanting stages, thereby saving significant amounts of irrigation water and labor. Nevertheless, this system increases crop sensitivity to drought stress and nitrogen deficiency during the early growth stages because the roots establish in non-saturated soil. Drip irrigation, especially when combined with plastic mulch, has demonstrated promising water savings in various field crops, while nitrogen remains the most limiting nutrient for rice yield. There is a strong interactive effect between water and nitrogen management on crop growth and productivity. However, to date, no comprehensive field study has investigated the interactive effects of irrigation method and nitrogen application rate on grain yield and water productivity of the local rice cv. Anbarboo under the specific agro-climatic conditions of Ilam province, western Iran. Therefore, this study was conducted to determine the optimal combination of irrigation method and nitrogen application rate for maximizing grain yield and water productivity under dry seeding conditions.
Materials and Methods
A two-year field experiment was conducted during the 2023 and 2024 growing seasons at the research farm of Ilam University, Ilam province, Iran. The experimental design was a split-plot arrangement based on randomized complete blocks with three replications. Four irrigation methods were assigned to main plots: (1) drip tape, (2) drip tape with plastic mulch, (3) furrow irrigation, and (4) furrow irrigation with plastic mulch. Three nitrogen application rates were assigned to sub-plots: 50%, 75%, and 100% of the recommended nitrogen rate (100 kg ha⁻¹ pure N based on soil test results). Rice (Oryza sativa L. cv. Anbarboo) was direct-seeded into dry soil. Measured traits included plant height, number of panicles per plant, number of grains per panicle, 1000-grain weight, above-ground biomass, grain yield, harvest index, and water productivity. Combined analysis of variance was performed across the two experimental years, and means were compared using Fisher's Least Significant Difference (LSD) test at the 5% probability level.
Results and Discussion
The combined ANOVA revealed that grain yield was significantly affected by year, irrigation method, and nitrogen rate (p < 0.01). Furthermore, all double and triple interactions among the experimental factors (year × irrigation method, year × nitrogen, irrigation method × nitrogen, and year × irrigation method × nitrogen) were also significant (p < 0.01), indicating the complex dependency of rice yield on the combined management of water and nitrogen. The highest grain yield (4724 kg ha⁻¹), harvest index (48%), and water productivity (0.74 kg m⁻³) were obtained in the second year of the experiment from the combination of drip tape with plastic mulch and 100% nitrogen application. In contrast, the lowest values for these traits (1347 kg ha⁻¹, 29%, and 0.14 kg m⁻³, respectively) were recorded under furrow irrigation with 50% nitrogen application. The superior performance of mulch treatments can be attributed to reduced evaporation from the soil surface, improved soil moisture retention, and enhanced water and nutrient uptake efficiency by the root system. Increasing nitrogen application up to the full recommended rate (100%) significantly improved yield components, including panicle number, grains per panicle, and 1000-grain weight. The significant triple interaction among year, irrigation method, and nitrogen rate indicated that the optimal treatment combination strongly depends on the prevailing climatic conditions of each growing season.
Conclusion
Based on the two-year experimental results, drip tape irrigation combined with plastic mulch and full (100%) nitrogen application was identified as the superior treatment, positively improving all measured growth and yield traits. The better performance of drip tape with plastic mulch in increasing grain yield and water productivity is due to the elimination of unproductive evaporation, maintenance of soil moisture in the root zone, and increased root length density and root activity. Plastic mulch effectively reduced unproductive evaporation, improved soil moisture retention, and enhanced both water and nutrient use efficiency. Full nitrogen application improved all yield components, leading to higher grain yield and harvest index. This integrated management system is strongly recommended for dry-seeded rice cultivation in arid and semi-arid regions to simultaneously improve grain yield and water productivity. A limitation of this study is that the results are based on only two years of data collected under specific climatic conditions of Ilam province; therefore, generalization to other regions and growing seasons requires further research under different climatic and soil conditions. Future research should evaluate the performance of this system across different rice varieties and diverse geographical regions. Additionally, a comprehensive economic feasibility analysis of plastic mulch and drip tape installation should be conducted to facilitate adoption by local farmers.