Impact of NaCl and biodegradable surfactant on water and oxygen transmission under aerated irrigation

Aerated irrigation can effectively alleviate the oxygen stress in the plant root zone and improve the environment of the plant root zone. This study investigated the effect of the addition of surfactant BS1000 in saline irrigation water on water and oxygen transmission under pure oxygen aerated irrigation. The oxygen transmission characteristic was investigated by using a Mazzei air injector, pressure swing adsorption oxygen generator and Seair diffuser coupled system (AirSep-Seair- Mazzei). Micro-bubbles produced by the AirSep-Seair-Mazzei system provide a good opportunity for its application in the long-distance aerated irrigation because of slow buoyancy characteristics and high dissolved oxygen concentration. The dissolved oxygen (DO) and temperature of water tank and drip tap were monitored by a dissolved oxygen meter. For simulating saline irrigation water, 0.1 mol/L NaCl solution was prepared. Four BS1000 concentrations were also prepared (0, 1, 2 and 4 mg/L). Therefore, a total of 8 combinations of NaCl and BS1000 were designed. During the experiment, oxygen transfer coefficient, DO saturation, flow uniformity coefficient and DO uniformity coefficient were determined. The results showed that the addition of BS1000 had a positive effect of oxygen transfer process, and resulted in an increase of the DO saturation. The DO saturation of the irrigation water averaged 299.87% under pure oxygen aeration. Under the addition of BS1000 in the aerated irrigation, an increase of the BS1000 concentration caused an increase of the oxygen transfer coefficient, while a decrease of the DO saturation. The oxygen transfer coefficient of 2 and 4 mg/L BS1000 was 22.76% and 46.90% higher than that of 1 mg/L BS1000, respectively, under the condition without NaCl addition. Under the addition of NaCl in the aerated irrigation water, the high surfactant concentration (not less than 2 mg/L) would result in a large oxygen transfer coefficient. The addition of NaCl could inhibit the DO saturation. The DO saturation in treatments of surfactant BS1000 of 0, 2 and 4 mg/L with NaCl addition was decreased by 5.55%, 4.92% and 6.57%, respectively, compared to the treatment without NaCl. The average value of flow uniformity coefficient under the aerated irrigation was greater than 95% and the DO uniformity coefficient of each combination remained above 97%. The high DO had a positive effect of the growth of crop when the oxygen concentration in soil water was lower than the upper limit demanded by crop. The oxygen transfer coefficient was greater than 18.85% under the addition of surfactant BS1000. The DO saturation was the highest in the treatment with 1 mg/L BS1000 regardless of NaCl addition. And all the treatments had high uniformity. Therefore, the surfactant concentration of 1 mg/L was suggested. This research demonstrates the great importance of uniform transmission of super-oxygenated water in drip tap and provides a method support for a wide range of aerated irrigation application.