Interactive effects of nitrogen fertilizer sources and soil acidity on ammonia volatilization

Introduction: While more than 2 million tonnes of nitrogen (N) fertilizers are annually used in Iran, it is documented that only 20% of applied nitrogen N are uptaken in some wheat farms of Iran. In other words, around 80% of N fertilizers are lost through leaching and volatilization, which increases the potential of environmental contamination with the increased N input expenditure of the farmer, or stored in the soil. So, the present study was aimed to quantify the role of ammonia volatilization from Iranian soils and to introduce simple and helpful techniques for ammonia volatilization reduction in Iranian wheat farms.
Material and methods: Firstly, a modified closed dynamic airflow system was provided. Soil materials were provided from the Tea Research Institute located in Lahijan, Iran as well as the National Salinity Research station located in Ashkezar, Yazd, Iran. The soils with three levels of acidity (7.88, 6.5, and 7.88) were incubated with five sources of nitrogenous fertilizers including ammonium sulfate, ammonium nitrate, urea, sulfur coated urea, and potassium nitrate for 22 days. Daily and cumulative volatilized ammonium was collected in 20 ml of 2% boric acid indicator solution and it was titrated with 0.01 N HCl. The analysis of variance for different parameters was done following the ANOVA technique. When F was significant at p ≤ 0.05 level, treatment means were separated using DMRT.
Results and discussion: Results showed that soil acidity, fertilizer source, and their interactions had significant effects on total volatilized nitrogen, maximum volatilized rate, and day of highest volatilization rate. Total volatilized nitrogen depends on nitrogen fertilizer source and soil acidity. The results showed that more than 59% of applied ammonium sulfate at the soil with the pH value of 7.88 was lost through ammonia volatilization and resulted in air pollution. The volatilized nitrogen for sulfur-coated urea and potassium nitrate equaled 7.6 and 0.018% while that of ammonium nitrate and urea equaled 49%. The results showed that potassium nitrate had the minimum cumulative ammonium volatilization of less than 0.07 mg N and it was not affected by soil acidity. In addition, our results proved that soil pH reduction from 7.88 to 6.5, reduced total ammonia volatilization for ammonium sulfate and nitrate from 227.15 and 189.82 to 2.39 and 0.99 mg N, respectively. Total volatilized nitrogen from ammonium sulfate and nitrate from soils with pH of 4.5 were 2.51 and 1.33 mg N, respectively. While soil pH reduction from7.88 to 4.5 from soils treated with urea reduced ammonia volatilization from 188 to 157, this increased ammonia volatilization from sulfur coated urea from 29.2 to 87.05 mg N. In other words, our results proved that increasing soil pH resulted in a significant decrease in total volatilized ammonia from sulfur coated urea. The total volatilized nitrogen from soils with the pH values of 4.5, 6.55, and 7.88 was equal to 22, 20 and 6% of applied sulfur coated urea. The maximum volatilized rate of ammonium was affected by soil pH and fertilizer sources. With decreasing soil pH the maximum volatilization rate from urea fertilizer increased. A similar trend was found for sulfur-coated urea. However, the maximum volatilization rate from ammonium nitrate and sulfate increased with soil pH increase. Interestingly, soil pH had no significant effect on the maximum volatilized rate from potassium nitrate fertilizer and it was equal to 0.04 mg N per day.
Conclusion: As ammonia volatilization depends on N sources, it is possible to decrease ammonia loss by selecting the proper nitrogen fertilizer source. The results demonstrated that potassium nitrate had minimum ammonia loss and can be introduced as the optimum source of nitrogen fertilizer for a wide range of soil pH from 4.5 to 7.88. Ammonium nitrate and ammonium sulfate fertilizers had the least ammonia loss at soils with a pH of 6.55 and less. However, sulfur-coated urea had the least ammonia loss at soil pH of 7.88 and more.