Influence of urban rainwater of city on yield, quality and heavy metal content of hydroponic lettuce

Abstract: Rainwater has become a big problem of the city, because rainwater led to urban waterlogging, affect urban transport. Urban waterlogging is also a kind of hidden danger for urban residents. At the other side, government spends a lot of money to meet municipal water demand, for example the South-to-North Water Transfer Project. Many cities land subsidence caused by groundwater exploitation. Urban agriculture which used rainwater as irrigation water is an effective way for urban rainwater as resources. The safety of urban agricultural products irrigated by rainwater is attention focus, because urban rainwater is polluted by automobile exhaust, asphalt, cement and so on. This paper study the food safety and nutrient quality of lettuce irrigated by urban rainwater aims to explore the feasibility of urban rainwater as urban agricultural irrigation water. A hydroponics experiment was carried out from October 2014 to January 2015 in Hainan University. Four kinds of treatments were designed using hydroponic lettuce design with four replicates, respectively: hydroponic lettuce using tap water (CK), hydroponic lettuce using rainwater (T1), hydroponic lettuce using rooftop rainwater (T2), and hydroponic lettuce using road rainwater (T3). 3 times rainwater from rainwater, rooftop rainwater of office and road rainwater were collected from October in 2014 to January in 2015. The rainwater was mixed respectively, allowed to stand for precipitating impurities, was used as nutrient solution water. Lettuce was cultured in tap plastic pipe. Plant height, leaf number, physiological indexes (chlorophyll content, net photosynthetic rate, root activity), nutritional indexes (soluble protein content, soluble sugar content, vitamin C content, nitrate content), and element (Ca, Fe, Mg, Zn, As, Cd, Pb) content in the lettuce mature stage under different rainwater nutrient solution and tap water nutrient solution were measured. The results show that heavy metal content of rainwater is lower than national agricultural irrigation water standards, urban rainwater is safety to be used as urban agriculture irrigation water. As content of lettuce was respectively 5.83 (CK), 4.10 (T1), 4.53 (T2), 4.60 μg/kg (T3), Cd content of lettuce was respectively 0.76 (CK), 0.78 (T1), 2.59(T2), 1.37 μg/kg (T3), Pb content of lettuce was respectively 102.37 (CK), 118.63 (T1), 151.53 (T2), 123.37 μg/kg (T3). They were less than the national limit values (As content 500 μg/kg, Cd content 200 μg/kg, Pb content 300 μg/kg), the lettuce meet the food safety requirements. Proteins content, sugars content and vitamin C content of lettuce cultivated in rainwater was not lower than that in rainwater; Ca, Mg content of lettuce cultivated in rainwater is higher than that in tap water, Fe content of lettuce cultivated in rainwater is less than that in tap water. The quantity of lettuce cultivated in rainwater is better than that in tap water. Chlorophyll content and net photosynthetic rate of lettuce cultivated in rainwater is lower than that cultivated in tap water. Root activity of lettuce cultivated in rainwater was increased. Root-shoot ratio of lettuce cultivated in rainwater was respectively 0.19(T1), 0.11(T2), 0.12 (T3), they were bigger than that cultivated in tap water(0.08). Lettuce cultivated in rainwater has worse growth power and lower biomass yield than lettuce cultivated in tap water. Lettuce biomass yield cultivated in road rainwater is lowest in three kind of lettuce cultivated in rainwater. In short, the urban rainwater and the agricultural produces cultivated by urban rainwater is safety, and the quality of agricultural produce cultivated in urban rainwater is better. At the same time, crop irrigated by urban rainwater grows weak and has lower yield. Experiments on lettuce demonstrate that urban rainwater is used as urban agriculture irrigation water is feasible.