شبیه‌سازی عملکرد پتانسیل چغندرقند (Beta vulgaris L.) و خلاء عملکرد ناشی از محدودیت آب و نیتروژن در استان خراسان رضوی با مدل SUCROS

Introduction
Crop productivity is highly constrained by water and nitrogen limitations in many areas of the world (Kalra et al., 2007). Therefore, there is a need to investigate more on nitrogen and water management to achieve higher production as well as quality. Irrigated sugar beet in the cropping systems of Khorasan province in northeastern of Iran accounts for about 34% of the land area under sugar beet production (~115,000 ha) with an average yield of around 36 t.ha-1 (Anonymous, 2009). However, there is a huge yield gap (the difference between potential and water and nitrogen-limited yield) mainly due to biotic and abiotic factors causing major reduction in farmers’ yield. Accordingly, yield gap analysis should be carried out to reduce the yield reduction and reach the farmer’s yield to the potential yield. The current study aimed to simulate potential yield as well as yield gap related to water and nitrogen shortage in the major sugar beet-growing areas of Khorasan province of Iran.

Materials and methods
This study was carried out in 6 locations across Khorasan province, which is located in the northeast of Iran. Long term weather data for 1986 to 2009 were obtained from Iran Meteorological Organization for 6 selected locations. The weather data included daily sunshine hours (h), daily maximum and minimum temperatures (◦C), and daily rainfall (mm). Daily solar radiation was estimated using the Goudriaan (1993) method. The validated SUCROSBEET model (Deihimfard, 2011; Deihimfard et al., 2011) was then used to estimate potential, water and nitrogen-limited yield and yield gap of sugar beet for 6 selected locations across the Khorasan province in the northeast of Iran. This model simulates the impacts of weather, genotype and management factors on crop growth and development, soil water and nitrogen balance on a daily basis and finally it predicts crop yield. The model requires input data, including local weather and soil conditions, cultivar-specific parameters, and crop management information. Soil water module was used to determine soil water balance under water-limited conditions. Some questionnaires were then sent to extension agents to obtain information from the main sugar beet producing fields in each location.

Results and discussion
The SUCROSBEET model reasonably well predicted root yield across the study locations. The model could be used to simulate sugar beet yield under potential, water and nitrogen-limited situations. Simulation results of SUCROSBEET model showed that the lowest and highest sugar beet potential yield were obtained in Sabzevar (100 t.ha-1) and Neishaboor (137 t.ha-1), respectively. Total yield gap (the difference between potential and farmer’s yield) ranged from 74 to 109 t.ha-1, in Sabzevar and Neishaboor, respectively. Despite the fact that most of the farms had been irrigated up to 20 times over seasons, there were still yield gap of an average 42 t.ha-1 due to water shortage. To reach the potential yield in the study locations, more than 2000 mm water is required in Sabsevar and Torbat-Jam and 1400 to 1500 mm in Ghoochan and Neishaboor, respectively. On average, to fill nitrogen-limited yield gap, 440 to 530 kg.ha-1 of nitrogen for sugar beet uptake are also required. However, the farmers in various locations have been able to apply only 50% of the sugar beet nitrogen demands during the past decade. The results of the current study also suggested that the farmer yields of about 16-48 t.ha-1in the irrigated locations across Khorasan province, were not constrained by low genetic yield potential. It is also needed to irrigate more than two times in some locations for reaching water-limited yield to potential one. Although there is a high potential for production of sugar beet (more than 130 t.ha-1), the ratio of yield production to water consumption (known as water productivity) is not suitable in the study locations and production of sugar beet would not be cost-effective. Another issue which has not been considered in the simulations is qualitative traits of sugar beet (such as sugar content, Alkaloids, molasses sugar, sodium and potassium in storage organ, etc.) particularly under different levels of nitrogen applications. Although increasing nitrogen application would be resulted in higher yield and lower yield gaps, supplied nitrogen more than crop demand could be accumulated in storage organs and reduce white sugar yield. For instance, every 15 kg additional application of nitrogen reduced sugar content by 0.1 percent and reduced extraction coefficient of sugar. It is also worth noting that the current version of SUCROSBEET model is not capable to simulate qualitative traits of sugar beet and a few subroutines are needed to add to the model for future investigations.

Conclusion
The results indicated that although there is high yield potential for sugar beet in Khorasan province, water productivity would not be reasonable. In addition, yield gap in sugar beet cropping systems which reflects the actual yield gap in irrigated environments is essentially due to non-adoption of improved crop management practices and could be reduced if proper interventions are made.