الفهرس 
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Abstract
Two experiments were carried out on greenhouse and field scales using sugar beet (pot) and potato (field) crops. In pot experiment, sugar beet crop was cultivated on sandy clay loam soil, fertilized with three N rates (N100, N80 and N50) in addition to unfertilized (N0) control and irrigated with saline water (8 and 16 dS m-1) applied at three regimes (W100, W80 and W60). Field experiment with potato crop was consists of two different fertilizers (urea and ammonium nitrate) applied at different rates (N100 and N60) and two different irrigation water regimes (W100 and W75 ETc). 15N labeled N-fertilizers were applied using isotope dilution concept that assessed in recognizing the most proper management practices that improved crop growth and yield as well as enhanced nitrogen fertili*zer use efficiency with special emphasis on rational use of water requirements and nitrogen fertilizer demand. In this respect, the obtained results could be summarized as following:I.Sugar Beet •Shoot dry weight was not significantly affected by experimental factors while root dry weight significantly but negatively affected by reduction in water quantities and increases of water salinity. •Dry weight of root of plants treated with N50 was superior over other N rats especially under W80 and W60 water regimes.•the highest N uptake by shoot was recorded with the full dose of 100% N recommended rate followed by those of 50% and then those of 80% N recommended rate. •With respect to irrigation with saline water at 8 dS m-1, data indicated there was no significant difference in N uptake comparable to those of FW under unfertilized control and those treated with 100% N fertilizer.•On the other hand, salinity levels 8 and 16 dS m-1 induced remarkable increase in N uptake over those recorded with fresh water especially when plants were treated with 80% and 50% N recommended rates.• The highest values of N uptake by shoots of plants irrigated with 8 and 16 dS m-1 salinity levels were recorded with application of 50% N recommended rate.•Water salinity has a negative effect on N uptake by root.•Effect of N rates was significantly correlated to water regime. • In this respect, N80 and N100 interacted with salinity levels were the best under W100, W80 and W60, respectively.• Nitrogen derived from fertilizer (Ndff) by shoot tended to reduce with irrigation water reduction up to W60 (water scarce). • On the other hand, in most cases, Ndff values were increased with irrigation water salinity. •It means that sugar beet as salinity tolerant plant acted well and able to gain more nitrogen from chemical fertilizer.•More Ndff by shoot was gained when plants fertilized with either N80 or N50 rates.• Nitrogen derived from fertilizer by root under W100 regime tended to declines with increasing water salinity up to S16.•Ndff by root was negatively affected by reduction of water requirement and declined with increasing water salinity but enhanced with low rate of chemical fertilizer added especially at W100 regime.• Interaction of Salinity x Nitrogen rate indicated the increase of %NUE of shoot with S8 and S16 salinity levels comparing to fresh water.N utilized by root was severely negatively affected by water regime and salinity levels while it enhanced with reduction in N fertilizer rates. II.Potato Experiment• Reduction in water quantities (W75) leads to increase shoot dry matter yield over those of W100 water regime.• Interaction of W x N resulted in enhancement of shoot dry weight with reduction in water requirement and in the same time, it slightly increased with high rate (N100) of fertilizer-N comparing to those recorded with N60 rate.•Tuber yield of plants treated with ammonium nitrate showed similar trend but to somewhat lower extent as compared to those treated with urea. •This phenomenon was in contrast to those recorded with shoot dry matter yield. •N uptake by shoot and tuber was significantly increased over the unfertilized treatment. Shoot accumulated more nitrogen than tuber.•Response of potato plants to the effect of water regimes, nitrogen forms and N-rates was significantly varied in relation to plant organs. •With respect to shoot-N, W75 was preferred than W100 while reversible trend was noticed with tuber-N. Surpass of one N rate over another was mainly related to N fertilizer form.•The preference of one fertilizer-N form over another one was related to plant parts whereas urea was preferred in tuber-N, ammonium nitrate was the best in case of shoot-N. •Ndff by tuber was significantly positively affected by fertilizer-N rate (N100) and water regimes (W100).•This holds true under both urea and ammonium nitrate fertilization.• In the same time, Ndff values were to somewhat extent, higher in case of ammonium nitrate than those of urea fertilizer. •This phenomenon was vigorous under W100 regime comparing to W75 one.• Ndff gained by shoot of potato crop fertilized with ammonium nitrate was, to somewhat extent, higher than those recorded with those fertilized with urea.Urea-N was efficiently used by shoot when added at rate of N60 and W75 regime while those of tuber hasn’t any remarkable differences between the studied factors (W regime x N rate). • Concerning ammonium nitrate, NUE% by shoot and tuber was more efficient at rate of N60 and W100. • But, those used by tuber were less than those recorded with shoot. •Iit could be concluded that 40% of nitrogen fertilizer, despite of form, could be saved and 25% of water requirement for potatoes may be reduced and this will be translated into money and environment reservation. •Nitrate content in tuber was increased with high N fertilizer rate.•It was higher in case of ammonium nitrate than urea fertilization.•Tuber accumulated more nitrate under low water regime (W75).• Nitrate content as affected by N fertilizer was higher than those of the unfertilized control.