Cereals are the most important crops all around the world and among them, wheat has the first rank in terms of production and the cultivation area. This plant is one of the main agricultural products in Iran and provides 45% of calories and 70% of the protein consumed by the Iranian people. Irrigation and soil nutrient availability are considered as main factors affecting the wheat yield. Nowadays, intensive application of these inputs in the conventional agricultural systems is considered to achieve maximum yield, whereas surplus use of them does not only have any significant positive influence on the yield but also has been led to environmental problems for example by leaching the nitrogen losses to underground water and imposes extra costs to the agricultural ecosystems. Therefore, the present study was conducted to optimize the application amount of nitrogen fertilizer and irrigation in wheat by using central composite design technique.
In order to determine the optimal application rates of nitrogen and water in wheat cultivation, a field experiment was conducted based on central composite design in the research field of Ferdowsi University of Mashhad during the 2011 growing season. The treatments were designed based on low and high levels of nitrogen (0 and 400 kg urea.ha-1) and irrigation (2500 and 5000 m3). Central point in each treatment repeated five times and the number of treatments were calculated based on 2k + 2k + r, in which k is the number of evaluating factors (nitrogen and irrigation) and r is the replication number of the central point. Therefore, 13 combination treatments were designed. Several features including seed yield, biological yield, nitrogen losses, nitrogen use efficiency and water use efficiency were measured as dependent variables and response surface of these variables under each combination treatment was calculated by regression model. Finally, the optimum values of water and nitrogen consumption were determined according to three scenarios including economic, environmental and economic-environmental scenarios.
The results indicated a positive effect of increasing nitrogen fertilization and irrigation on seed yield, biological yield and water use efficiency. Whereas, increasing irrigation level led to decreasing nitrogen use efficiency and increasing nitrogen losses. Finally, the optimum levels of nitrogen and irrigation were estimated based on three scenarios including economic, environmental and economic- environmental. In economic scenario, the optimum levels of fertilizer and irrigation were estimated 274 kg urea.ha-1 and 3964 m3.ha-1, respectively to obtain 4045 kg.ha-1 seed yield and 9908 kg.ha-1 biological yield. In environmental scenario, the optimum levels of the treatments to obtain the minimum nitrogen losses amounts were 64.65 kg urea.ha-1 and 2651 m3.ha-1, respectively. In economic-ecological scenario, both seed yield and nitrogen losses were considered, so the estimated input rates were 153 kg urea.ha-1 and 3030 m3.ha-1 irrigation. Simultaneous consideration of the economic aspects and environmental issues to the production in economic-ecological scenario shows the excellence of this scenario than other the two scenarios.
In order to achieve sustainable production of crops, one of the basic principles is to improve resource efficiency and prevent agricultural input losses. Based on our results, it seems that the optimization values of nitrogen fertilization and irrigation calculated in the economic-environmental scenario which simultaneously considered both economic and environmental aspects of wheat production can be suggested as the most appropriate levels of these two factors according to the study conditions.