Evaluating natural saline water and nitrogen interactions on yield, cumulative transpiration and water use efficiency in canola

Currently, development of sustainable agriculture is constrained by freshwater scarcity. The use of low quality water such as saline and brackish waters should be considered as alternative water resources for agricultural productions. Based on global experiences, management of low quality water application is more appropriate choice than development of new fresh water resources. On the other hand, the availability of some nutritional elements such as nitrogen for plant production in saline soils is detrimental due to various reasons. The consentration of some nutrients such as chloride are too high and toxic in these soils. Canola (Brassica napus L.) is one of the most important oilseeds worldwide. Some environmental stresses can impose severe limitation for canola production in widely different climates. In recent years, much attention has been paid to develop oilseeds cultivation in Iran including canola. However, only little information on adaptability of this plant in various environmental conditions of Iran has been published. Consequently, it is quite important to investigate the limiting environmental constrains such as soil salinity in arid and semi-arid regions of the country. The objective of this study was to investigate the interactions of soil salinity and different levels of nitrogen application on yield, transpiration and water use efficiency of canola seed production.
A factorial experiment in a randomized complete block design with salinity and nitrogen factors were carried out on canola plant. The salinity treatments were consisted of non-saline water (0.3 dS.m-1) and four natural saline waters (a water which is not artificially made saline and directly taken from available natural saline water resources) of 3, 6, 9 and 12 dS.m-1. The nitrogen levels were consisted of zero (N1), 75 (N2), 150 (N3) and 300 (N4) mg N per kg soil applied as ammonium nitrate. The statistical analyses were performed using the Mstat C software and the means were compared by Least Significant Difference test (LSD) at 5% probability level.
The results indicated that by increasing soil salinity, canola seed yield reduces but increases as result of nitrogen application in soil. Generally, by increasing nitrogen application, plant transpiration will increase due to the enlargement of plant transpiring area. Increase in transpiration by plant leaf would increase the nitrogen uptake by crops (nitrogen concentration × yield = N uptake). Application of N up to 75 mg per kg soil caused to increase the water use efficiency. Applying more nitrogen than 75 mg per kg soil reduced the water use efficiency. By increasing soil salinity to 6 dS.m-1, water use efficiency was first increased and reduced afterwards. Application of 150 and 300 mg N per kg soil when soil salinity was 12 dS.m-1, increased water use efficiency. This can be attributed to the sharp decrease of plant transpiring area. Application of nitrogen in lower salinity levels, however, decreased water use efficiency.
In general, using suitable amount of nitrogen under saline conditions not only would lead to increase canola yield and its oil content, but will improve its water use efficiency.