Effects of the salinity of irrigation water on soil ammonia volatilization in drip-irrigated cotton fields

【Objectives】 Ammonia volatilization is one of the main pathways of nitrogen loss from farmland soil. Saline water irrigation can directly or indirectly affect the physical and chemical properties of soil, which influence soil NH3 volatilization. The objective of this study was to investigate the effect of irrigation water salinity on soil NH3 volatilization in cotton fields after drip fertigation with urea. 【Methods】 The experiment consisted of two levels of irrigation water salinity: 0.35 dS/m (freshwater, CK) and 8.04 dS/m (saline water, SW), the N application rate was 240 kg/ha. Volatilized NH3 was collected using the closed chamber method, the NH3 was trapped using dilute sulfuric acid. The amount of NH3 in the traps was measured using theindophenol blue colorimetric method. 【Results】 1) The soil salinity, urease activity, andNH+4-N content in the saline water treatment were significantly higher than the freshwater treatment. The soil salinity treated with saline water was averaged 4.53 times higher than that with the freshwater. Soil urease activity increased rapidly in the saline water treatment after fertigation. Urease activities reached a maximum four days after fertigation and then declined. Urease activity was averaged 20.6% higher in the SW treatment than in the freshwater treatment. The NH+4-N content in the saline water treatment was significantly higher than in the freshwater treatment. The greatest difference between the two treatments was observed two days after fertigation with theNH+4-N content 66.1% greater in the saline water treatment than in the freshwater treatment. 2) Soil pH was lower in the saline water treatment than in the freshwater treatment. However, soil pH in the saline water treatment increased after fertigation and then decreased with a range from 7.6 to 8.0. 3)Saline water irrigation inhibited nitrification, soil NO-3-N concentrations were significantly lower by an average of 7.68% in the saline water treatment than in the fresh water treatment.4) The average temperatures during the three fertigation cycles were 24.6℃, 26.05℃, and 24.9℃, respectively. NH3 volatilization reached a higher level during the second and third cycles than the first one. The greatest amount of rainfall was received during the first fertigation cycle, which was 3.7 mm and 10.2 mm higher than the second and third fertigation cycles. However, rainfall had little effect on ammonia volatilization due to the rainfall amount was far less than the irrigation.5) Generally, soil NH3 volatilization reached a maximum amount one or two days after fertigation with urea and then declined. Soil NH3 volatilization accounted for 45.7%-79.3% of the total NH3 volatilization during the first two days after fertigation. In the first day of the three fertigations, soil NH3 volatilization was 70.7%, 69.3% and 60.8% higher in the saline water treatment than in the freshwater treatment correspondingly. Soil NH3 volatilization was significantly higher in the saline water treatment than in the fresh water irrigation. Total NH3 volatilization during three consecutive fertigation cycles was 10.98 kg/ha in the saline water treatment, and 7.57 kg/ha in the freshwater treatment. Total NH3 volatilization in the saline water treatment was 45.1% higher than in the freshwater treatment. 【Conclusions】 Saline water irrigation promoted urease activity. However, saline irrigation water significantly inhibited nitrification, depressed the transformation of NH+4-N to NO-3-N. This led to an increase in soil NH+4-N content and NH3 volatilization. High temperature increased NH3 volatilization. The small amount of rainfall during the experiment had little effect on NH3 volatilization. The results indicated that irrigation with saline water increased N loss due to NH3 volatilization after drip fertigation with urea.