Influence of chlorine injection on soil enzyme activities and maize growth under drip irrigation with secondary sewage effluent

Chlorination is an effective method to prevent and reduce emitter clogging caused by algae and bacteria in drip irrigation system. The optimal chlorination schemes should be developed to alleviate the potential negative effect of chlorination on soil properties and crop growth. A 2-year experiment was conducted to evaluate the effects of chlorine injection concentrations and duration on soil urease and alkaline phosphatase activities and production of maize for drip irrigation while applying secondary sewage effluent. The experiments were designed with injection concentrations of free chlorine residual at the end of laterals ranging from 0 to 8 mg L−1 and injection duration ranging from 0.5 to 3 h for each irrigation event. The control experiments with groundwater were applied in both seasons. Sewage application increased the residual Cl in soil while chlorination did not cause a substantial chlorine accumulation in the root zone. Compared to enzyme activities measured prior to sowing, soil enzyme activities were generally enhanced by sewage application with or without chlorination. The effects of chlorine concentration and injection duration on plant biomass, nitrogen uptake, enzyme activities, and yield of maize were insignificant although chlorination weakened the increment of soil enzyme activities to some extent. The yield of maize ranged from 12.1 to 13.3 Mg ha−1 and from 10.1 to 10.7 Mg ha−1 in the 2015 and 2016 seasons, respectively. For the treatments with chlorine injected, the yield of maize peaked at 0 mg L−1 in 2015, while the yield of maize peaked at 1.3 mg L−1 of free chlorine residual at the end of laterals and the 3 h chlorine duration in 2016. The results suggested that chlorination is safe for field crops with precipitation leaching.