Interactive effects of irrigation water salinity and urea fertilizer on wheat (Triticum aestivum L.) yield and yield components

Introduction Soil salinity adversely affects crop productivity and agricultural sustainability in many areas of the world, especially in arid and semi-arid regions. It is estimated that global lost crop production due to salt induced land degradation equals to US$ 27.3 billion annually (Qadir et al., 2014). Social and economic dimentions of salinity stress include employment losses as well as environmental degradation (Butcher et al., 2016). In addition, it is well documented that application of chemical fertilizers usually improve plant performance under saline conditons but results on plant fertilizer requirement under salt affected soils are contrary. While there is little evidence of yield benefits due to fertilizers addition of salinized fields at rates beyond optimal in non-saline conditions, there is enough evidence indicating that soil salinity does not affect or decrease plant fertilizer needs (Hanson, 2006). It is known that the growth inhibition and the adverse effects induced by salinity can be alleviated by proper use of fertilizer and water management, depending on plant species, salinity level, and environmental conditions. However, over nitrogen fertilization may result in soil salinization and negatively affect plant performance. Moreover, the potential for nitrate leaching may increase where moderate to high amounts of salts are present in the soils because plants under salt stress can not absorb and utilize the applied nitrogen as efficiently as the plants not subjected to salinity stress. Thus nitrogen fertilizer management may need to be modified under arid and semiarid conditions of Yazd peovince with wide range of irrigation water qualities. Accordingly, the objectives of this field study were to (a) elucidate the interactions between nitrogen nutrition and irrigation water salinity and their effects on wheat growth and (b) test the possibility of wheat improvement at saline conditions by applying higher levels of nitrogen fertilizer. Materials and methods A field experiment was conducted on wheat at Sadooq Salinity Research Station, Ashkezar, Yazd, Iran. The soil at the experimental site was calcareous with 30.92% total nutrient value, sandy loam texture, pH 8.06 and 0.22 % organic carbon. Mean annual temperatue is 18 °C and precipitation is 70 mm. The treatments, five urea application rates (0, 100, 200, 300 and 400 kg ha-1) and three irrigation water qualities (2, 7 , 14 ds/m)), arranged in a randomized complete block design in the form of split plot with three repelications. Consisting 12 rows of wheat, each field plot measured 3* 5 m. All plots received common cultural practices including tillage and fertilizer application. Rgarding typical recommendations and guidelines for this region and soil type (Balali et al., 2000: Moshiri et al., 2015), all fertilizers, except urea that applied in 4 splits, were soil-applied before plnating and included 100 kg ha-1 triple superphosphate, 40 kg ha-1 FeSO4, 40 kg ha-1 ZnSO4, 40 kg ha-1 Mn SO4 and 20 kg ha-1 CuSO4. To model the relationship between plant properties and irrigation water salinity, the data were subjected to different regression models at the probability level of 0.01 and 0.05 with the help of the Sigmaplot software. The analysis of variance for different parameters was done following ANOVA technique. When F was significant at p≤0.05 level, treatment means were separated using DMRT. Results and discussion The results showed that ura application is necessary for improving wheat yield at all irrigation water salinity levels. While wheat response to nitrogen fertilizer was similar at different salinity levels and followed second order equation, with increasing salinity levels the wheat responded weakly. In addition, the results showed 20 percent increase in wheat nitrogen requirement with increasing salinity leves from 1.88 to 7.22 dS/m while no decrease in wheat graine yiled was found. However, with increasing salinity levels to 16.14dS/m, wheat grain yiled and urea requirement decreased by 20 percent Conclusion In conclusion, application of 300kg ha-1 urea for production of 4.5t ha-1 wheat grain yield using irrigatin waters with electrical conductivities of 1.88 to 7.22 dS/m is needed. However, 240kg ha-1 urea is enough for production of 3.6t ha-1 wheat grain yield using irrigation water salinity of 16.14 dS/m.