Assessment of Triple Super Phosphate Fertilizer and Irrigation Salinity Effects on Soil Cadmium Mobility and Absorption

Cadmium (Cd) mobility in soil is affected by various factors and its absorption from soil by tobacco is higher than other crops. Application of phosphate fertilizers in agricultural lands is an essential step to increase the yield of tobacco plants. Since most phosphate fertilizers contain small amounts of Cd, the uptake of Cd by tobacco plant in its cultivated areas due to the application of triple superphosphate fertilizer (TSP) is not unexpected. In many tobacco growing areas, the water or soil used is between low and medium salinity in terms of salinity, which can also influence the solubility of cadmium and, consequently, its uptake by tobacco plant. Cadmium can be absorbed through food, drink and respiration. This metal not only is absorbed by the digestive organs, but also is absorbed by the respiratory organs through airborne particles and cigarette smoke. Tobacco is resistant to high concentrations of Cd in soil and can absorb it from Cd-contaminated soil. The aim of this study is to investigate the effect of P fertilizer and salinity on Cd mobility in soil and tobacco plant.

This experiment was conducted with the aim of investigating the interaction of three factors of irrigation salinity (0, 20 and 40 mM NaCl), triple tuper phosphate fertilizer (TSP) (0 and 1.5 g kg-1 soil) and soil Cd contamination level (0 and 12 mg kg-1 soil) in a completely randomized design with four replications on shoot Cd concentration, smoke Cd concentration, extraction percentage of DTPA, tobacco ash Cd concentration, Cd mobility factor and Cd fractions in soil. To homogenize the samples, they were thoroughly mixed together and the resulting composite samples were passed through a 2 mm sieve to incubate the samples and then implant. Cadmium contamination levels (0 and 12 mg kg-1) were prepared from Cd(NO3)2.4H2O source. Prior to planting, the relevant levels of contamination were added by spraying on the entire soil surface and mixed thoroughly. Soil samples were transferred to plastic storage containers and incubated for four months in a controlled greenhouse within a temperature range of 25-30 °C and 70% water holding capacity of the soil measured by the weighing method. Cultivation was carried out under controlled conditions in a greenhouse environment located in Bardaskan city. Two 60-day-old tobacco seedlings (Nicotiana tabacum L.) of Cocker 347 cultivar, which were previously seeded in non-contaminated cadmium soil and grown with non-saline water, were transferred to each pot and planted. The cultivar used in this experiment was a greenhouse tobacco cultivar used in the cigarette industry. Immediately after transferring the seedlings to pots, irrigation was performed with saline-free water (distilled water), salinity of 20 or 40 mM NaCl salt for 75 days according to the required treatment. Up to the fourth week, the amount of 400 ml per pot in each irrigation cycle, and after that until the end of the experiment, the amount of 800 ml per pot in each irrigation cycle was applied.

The results showed that Cd mobility factor in Cd-contaminated soil increased on average by 25.6%, 32.4% and 36.2% compared to non-contaminated soil at 0, 20 and 40 mM salinity, respectively. Application of phosphate fertilizer significantly reduced the mobility factor of cadmium in non-cadmium-contaminated soils. In Cd-contaminated soil, the extraction percentage of DTPA increased 26.5% and 56.4% with increasing irrigation salinity levels from 0-20 and 0-40, respectively. In non-Cd contaminated soil, TSP application reduced extraction percentage of DTPA 20.2%, 28.4% and 24.6% in 0, 20 and 40 irrigation salinity levels, respectively in compared to non-TSP application. With increasing the levels of soil Cd contamination, the percentage Cd concentration in oxide fraction of soil decreased and the percentage of Cd concentration in carbonate, organic and residual fractions increased. Application of TSP increased the concentration of residual Cd fraction in the soil.

With increasing the level of Cd contamination in soil, the percentage of Cd in carbonate and organic fractions increased compared to non-Cd contaminated soil. The results showed that TSP application in Cd contaminated soil in salinitylevels of 0, 20, and 40 mM increased Cd concentration of tobacco ash by 1.47%, 15.89% and 29.80% and increased Cd concentration of tobacco smoke by 23.20%, 23.30% and 18%, respectively. Salinity factor and phosphate fertilizer showed the reverse effect on soluble + exchangeable cadmium and DTPA available Cd in soil, so with increasing salinity, these concentrations increased and with increasing triple superphosphate fertilizer decreased.