Abstract
The utilization of wastewater for agricultural irrigation has been widely accepted in China and other arid and semi-arid regions around the world. Effects of lateral depth and irrigation frequency on inorganic nitrogen yield and quality of Lactuca sativa var. augustana (asparagus lettuce) under drip irrigation with sewage effluent was examined in a solar-heated greenhouse in North China Plain amid two growing seasons of 2014 and 2015. Three pan coefficients of 0.6, 0.8, and 1.0 and three lateral depths of 0, 10, and 20 cm were tested during the 2014 season. In contrast, three irrigation intervals of 4, 8, and 12 days and three lateral depths similar to the 2014 season were utilized in the 2015 season. The groundwater control experiment was applied for treatments with a pan coefficient of 0.8 (2014) and irrigation intervals of 8 days (2015). A higher irrigation frequency increased the yield of asparagus lettuce, while a lower frequency was favorable to the quality parameters of vitamin C and soluble sugar. Generally, compared with surface drip irrigation, subsurface drip irrigation increased inorganic nitrogen distribution in the subsurface soil layer, and also increased the yield, vitamin C, and soluble sugar of asparagus lettuce. When asparagus lettuce was buried laterally 10 cm below the soil surface, highest yield was observed. Sewage irrigation increases the accumulation of nitrogen in the soil, and asparagus lettuce yield is higher compared to groundwater irrigation. Additionally, compared with groundwater irrigation, no destructive effect of sewage irrigation on the quality of asparagus lettuce was found.