Drip irrigation with suitable saline water improves water use efficiency for cotton

The scarcity of fresh water in arid regions makes saline water a valuable alternative water source for irrigation. In this study, field experiments were conducted to investigate the effect of saline water irrigation on growth, yield and water use efficiency of cotton plants (Gossypium hirsutum L.), as well as plant and soil 15N recovery. Three irrigation water salinity levels were designed as 0.35, 4.61 and 8.04 dS/m, representing fresh water (FW), brackish water (BW), and saline water (SW), respectively. Two irrigation amounts were set as 405 mm (I405) and 540 mm (I540) under each irrigation water salinity treatment. Overall, the dry matter weight, N uptake, and yield of cotton increased with irrigation amount increasing from 405 mm to 540 mm, but there were differences under different water salinity treatments. The highest dry matter weight of cotton was found under the BW treatment, which was 9.6% and 20.2% higher than that under the FW and SW treatments, respectively. Plant N uptake and yield under SW treatment was significantly lower than that under FW treatments, but there were no significant difference between the BW and FW treatments. The ET of cotton field increased with irrigation amount increased, but decreased with water salinity increasing from 0.35 dS/m to 4.61 dS/m, and to 8.04 dS/m. BW had the similar ET with FW, but ET was significantly reduced under SW treatment. The water productivity of cotton and the water productivity of irrigation under different treatments ranged from 0.70 to 1.02 kg/m3 and from 0.79 to 1.16 kg/m3, respectively. The water productivity of cotton and the water productivity of irrigation were significantly higher in BW and FW than the SW treatment. The result of 15N Isotopes tracer experiment showed that the plant 15N recovery under the I540 treatments was on average 7.51% higher than that under I405 treatments. The soil 15N recovery was reduced with 13.20% and 15N leaching percentage was increased with 29.47% when irrigation amount increasing from 405 mm to 540 mm. The plant 15N recovery under various treatments ranged from 47.02% to 59.86% under different water salinity treatments. The plant 15N recovery under the SW treatment was 10.17% and 15.23% lower than that under the FW and BW treatments, respectively. The soil 15N recovery under various treatments ranged from 16.75% to 22.14% and was slightly affected by the water salinity. The total recovery of 15N in plants and soils under different treatments ranged from 68.9% to 76.6% with an average of 72.2%. The total recovery of 15N was higher in BW and FW than the SW treatment. The 15N leaching percentage ranged from 1.56% to 4.71% for different treatments and was significantly affected by the water salinity. The 15N leaching percentage was significantly higher in SW and BW than the FW treatment, representing 80.53% and 136.00% increase, respectively. Those results suggest that the yield, water use efficiency, and 15N recovery of cotton are not affected by brackish water with an optimum salinity level and irrigation amount. Saline water irrigation could cause cotton yield, water use efficiency, and nitrogen use efficiency decreased significantly. Nitrogen leaching is one of the major ways of N loss in cotton field under drip irrigation conditions. Saline water and brackish water irrigation may increase the risk of N leaching loss from the root zone. Therefore, minimizing the N leaching loss is very important for promoting N fertilizer use efficiency in cotton field under drip irrigation with saline water.