Effect of Soil Salinity, Type and Amount of Nitrogen Fertilizer on Yield and Biochemical Properties of Mustard (Brassica rapa L.)

Soil salinity is a major limiting factor in agricultural development within Iran. Nitrogen is the most important nutrient that its uptake is limited over other elements under saline conditions due to decrease in the permeability of plant roots, soil microbial activity and mineralization of organic compounds and nitrate uptake by high concentrations of chloride anions in the root zone of the plant. Mustard plant has a good compatibility to weather conditions and since there is an extreme need of vegetable oilseed in our country and also wide extent of saline soils in Iran, this study was conducted to determine the best type and amount of nitrogen fertilizers between calcium nitrate and ammonium sulfate under saline conditions.
A greenhouse experiment was conducted in a completely randomized design (factorial) with three replications in February 2012 in the Research greenhouse of the Ferdowsi University of Mashhad. The treatments were consisted of two types of nitrogen fertilizer (calcium nitrate and ammonium sulfate), each with three levels of N (40, 80 and 120 mg per kg of soil) in three levels of soil salinity (C0= control, C1= 5 and C2= 10dS m-1). Experimental soil (control) collected from agricultural experimental station was leached by salt solutions containing salts of calcium chloride, magnesium chloride and sodium sulfate with specified concentrations and ratios during 50 days to reach the similar salt concentrations of leached water consisting the desired levels of salinity. The seeds of mustard were planted at a depth of one centimeter in soil of each pot and were irrigated with tap water to field capacity (by weight). Plants were harvested after 5 months and plant fresh and dry weights and nitrogen concentration and uptake of plant were measured by the Kjeldahl method. Irrigation water and physical and chemical properties of soil before and after harvest were determined. Data obtained were analyzed using statistical software MSTAT-C and the means were compared using Duncan's multiple range test at 5 % percent.
In this study, plant dry weight increased significantly with increasing levels of calcium nitrate fertilizer in all levels of soil salinity, due to increasing plant internal needs under saline soil. While the maximum dry weight was obtained with ammonium sulfate fertilizer in saline conditions and plant positive response to increasing fertilizer consumption increased with soil salinity. Positive or negative effect of ammonium or nitrate salt varied among plant varieties and generally it depended on the types of crop, soil conditions and crop density and rotation. In addition, the nature of the culture medium (soil, sand or liquid culture), salinity and sodium levels (salinity, sodium absorption ratio or percentage of exchangeable sodium and pH), salinity and sodium distribution under field conditions, the nature of the salts used to make salt and Sodium in growth medium (sodium chloride salt or mixture of salts), environmental conditions (temperature, light intensity, etc) and the duration of the experiment (from days to years), all were effective on experiments results and can cause differences in these results with each other. Therefore, in this study, ammonium sulfate fertilizer, resulted in more plant dry weight due to its acidic characteristic because of sulfur and more nitrogen and less leaching than calcium nitrate, despite its higher salinity index in a calcareous soil with less soil primary concentration of nitrogen. In addition, a significant increase in uptake of nitrogen by plant was observed by application of ammonium sulfate fertilizer over calcium nitrate that effectively increased plant resistance to soil salinity.
The results showed that plant dry weight and nitrogen uptake increased with increasing nitrogen fertilizer despite decreasing nitrogen concentrations in plant and the significant reduction in plant dry weight compared to control also increased due to increasing soil salinity. But the plant's response to the type and amount of nitrogen fertilizer was different at various levels of salinity in soil. In general, dry matter and nitrogen uptake efficiency increased by ammonium sulfate usage in all levels over calcium nitrate under soil salinity conditions, but its amounts were partly limited more than calcium nitrate amount due to increasing levels of salinity in soil.