Effect of chlorination on yield and nitrogen uptake of tomato and emitter clogging in a drip irrigation system with sewage effluent

Chlorination is a common practice used in prevention and treatment of emitter clogging caused by algae and bacteria, but the effect of chlorination on crop growth has not been adequately addressed. Field experiments were conducted in a solar-heated greenhouse to investigate the effects of chlorine concentration and injection frequency on emitter clogging of drip system with sewage effluent and tomato yield, quality and nitrogen uptake. A multiple regression equation to predict nitrate content in soil from the measured soil bulk electrical conductivity, water content and temperature was derived. Then the equation was used to investigate the effects of chlorination on nitrate dynamics and nitrogen uptake. The results demonstrated that chlorination is an effective method for preventing the reduction of emitter discharge in drip systems applying sewage effluent. Drip irrigation with sewage effluent increased nitrate content in upper 15 cm layer of soil and nitrogen content of plant stem and produced a greater nitrogen uptake than irrigation with groundwater. Chlorination decreased the total nitrogen content of plant stem and nitrogen uptake, and increased the accumulation of nitrate in upper layer of soil. The increase of chlorine concentration and injection frequency led to a significant increase of nitrate accumulation. A slightly greater yield of tomato was observed for drip irrigation with sewage effluent than irrigation with groundwater, while chlorination resulted in a decreased yield. Irrigation with sewage effluent increased soluble sugar and water-soluble acid of tomato fruits, but decreased ascorbic acid and soluble solids. Chlorination might alleviate the decreasing trends of ascorbic acid and soluble solids resulted from sewage application. The results obtained from this study suggested that injection of chlorine with an input concentration of less than 50 mg/L and an interval of larger than two weeks do not impose a negative influence on tomato growth, although the injection of chlorine may restrain the nitrogen uptake of plant.