عنوان مقاله [English]
Introduction Annually around 2 million tons of nitrogenous fertilizers is used in Iran. It was reported that increasing trend of chemical fertilizers application over the last 30 years with some fluctuations. So, proper fertilizer management in Iran has great effect on soil and water quality. Salinity stress is known as a worldwide abiotic stress responsible for reduced crop production. It is estimated that annual losses of yield due to salt induced land degradation is US$ 27.3 billion globally. Social and economic dimensions of salinity stress can be employment losses as well as environmental degradation. In addition, it is well documented that application of chemical fertilizers usually improve plant performance under saline conditions but results in plant fertilizer requirement under salt affected soils are contrary. While there is little evidence of yield benefits due to application of fertilizers in 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. It is well documented that salinity stress negatively affects wheat growth rate and it postpone the ripening in wheat plants. While flowering happened at 59. 33 days after planting at non-saline conditions, it occurred 62.22 days after planting for salt affected plants. The negative effect of salinity on wheat nitrogen content is reported. This is due to the negative effect of salinity stress on root growth and the chloride on nitrogen uptake. A hypothesis that salinity stress can adversely affect nitrogen uptake pattern of wheat has been proposed, but, contradictory results have been reported. Thus nitrogen fertilizer management may need to be modified under arid and semiarid conditions of Yazd province with wide range of irrigation water qualities. Accordingly, the objectives of this field study were to elucidate the effect of salinity stress on nitrogen uptake pattern and nitrogen timing in wheat.
Materials and methods A field experiment was conducted on wheat at Sadooq Salinity Research Station, Ashkezar, Yazd, Iran. Mean annual temperature is 18°C and precipitation is 70 mm. The treatments, nitrogen fertilizer sources (ammonium sulphate and urea) and three irrigation water qualities (1.7, 7.22 and 12 dS m-1), arranged in a randomized block, split plot design with three replications. Consisting 20 rows of wheat, each field plot was 6*4 m. All plots received common agricultural practices including tillage and fertilizer application. Regarding typical recommendations and guidelines for this region and soil type, all fertilizers, except urea that applied in 2 splits, were soil-applied before planting. Nitrogen was applied at a rate of 105 kg ha-1 at two stages (90 and 120 days after planting). As soil phosphorous and potassium content was above threshold level, these elements were not applied for wheat production. Plant samples were provided at four growth stages including tillering, stem elongation, flowering and harvest. The samples were analyzed for nitrogen content. Plant nitrogen content was determined using kjldal method. 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 soil at the experimental site was calcareous with 31.5% total nutrient value, sandy loam texture, high pH (8.06) and low organic carbon (0.51 %). The results showed that wheat top yield depends on irrigation water salinity level as well as nitrogen management. While a sigmoidal trend in wheat top yield for all treatments observed over time, increasing irrigation water salinity from 1.7 to 7.22 and 12 dS m-1 decreased wheat yield at harvest from 11058 to 7183 and 7933 kg ha-1. In other words, salinity stress significantly decreased wheat performance by 35.04 and 28.26 percent. The results also showed that nitrogen content decreased over time. While nitrogen content was more than 5 percent at tillering it decreased to 1 percent at harvest. Nitrogen uptake was not affected by nitrogen source but it was affected negatively by irrigation water salinity. Depending on salinity levels and the application rate of nitrogen, nitrogen uptake by wheat ranged from 81 to 189 kg ha-1. However, nitrogen uptake was not affected significantly by nitrogen sources.
Conclusion Overall, it was concluded that salinity stress did not affect nitrogen uptake pattern of wheat under field conditions of the experiment. As more than 80 percent of nitrogen was uptaken from mid tillering onward, it is recommended that just only 15 percent of nitrogen fertilizer be applied at planting. This would increase nitrogen uptake efficiency and prevent soil, water and air pollution.