Effects of nitrogen fertilizer and plant density on yield and nitrogen efficiency indices of sesame using a central composite design

Sesame (Sesamum indicum L.) is known as the king of oil seeds due to the high oil content (50-60%) of its seed.
Nitrogen (N) is one of the most important nutrients in crop production agroecosystems. Excessive application of N in crop production causes a reducing trend in nitrogen use efficiency (NUE).
Optimization of nitrogen and plant density is a management approach to conserve resources and decline environmental pollutions.
Response surface methodology (RSM) is defined as a set of mathematical and statistical techniques that are used to develop, to improve or to optimize a product. RSM is a statistical method for optimization of multiple factors which determine optimum process conditions by combining experimental designs. In this study, optimization of nitrogen fertilizer and plant density on yield and nitrogen efficiency indicators of sesame using central composite design (CCD) for RSM was done. An experiment was conducted using CCD with 13 treatments and two replications at the Agricultural Research Station, Ferdowsi University of Mashhad during the growing season of 2015-2016. The treatments were allocated based on low and high levels of plant density (10 and 40 plants.m-2, respectively) and nitrogen (0 and 100 kg Urea ha-1, respectively). Seed yield, biological yield, nitrogen uptake efficiency, nitrogen utilization efficiency and nitrogen use efficiency were measured and calculated as dependent variables and changes of these variables were evaluated by a regression model. Lack-of-fit test was used to evaluate the quality of the fitted model. The adequacy of the model was tested by analysis of variance. In general, the full quadratic polynomial equation was tested to determine the significance of the model and the component of the model (linear, squared and first-order interaction terms). The quality of the fitted model was judged using the determination coefficient (R2). The results showed that effect of linear component was significant on all studied characteristics except for nitrogen utilization efficiency. Effect of square component was significant on all studied criteria. Interaction effect of full quadratic was significant on nitrogen use efficiency. Lack of fit test had no significant effect on the studied traits. The full square model for the response variables gave insignificant lack-of-fit indicating that the data of experimental were satisfactorily explained. The highest estimated and observed values of seed yield were obtained for 25 plants.m-2 and 500 kg Urea ha-1 (1320.52 kg.ha-1) and 25 plants.m-2 and 100 kg Urea ha-1 (1272.45 kg.ha-1), respectively. The maximum estimated and observed values of nitrogen use efficiency were calculated for 10 plants.m-2 and without N fertilizer with 11.41 and 11.28 kg seed/kg N of soil, respectively. The slope of seed yield increased by an increase in density up to 25 plants.m-2 was higher under high levels of N fertilizer than under low levels, because the plant growth was improved in high amount of N fertilizer and resulted in high seed yield. By increasing nitrogen fertilizer declined nitrogen use efficiency. In general, it seems that resource use optimization such as nitrogen fertilizer and plant density based on the central composite design may be suitable cropping approach for sustainable production of sesame.