Different nitrogen fertilizers and ratios effect on growth, safety and quality of spinach

【Objectives】 Aammonium fertilizer and nitrate fertilizer are two major types of fertilizers generally used for vegetables growth, and applying different types and ratios of nitrogen fertilizers have a significant impact on growth and nutrient value of vegetables. Spinach is an important leafy vegetable, which is rich in micronutrients, vitamin C and vitamin E. In this study, effects of ammonium fertilizer, nitrate fertilizer and NH+4-N/NO-3-N on the growth, safety and quality of spinach were investigated in Fengyang, China. 【Methods】Water culture experiment was adopted in this study to investigate how ammonium fertilizer, nitrate fertilizer and NH+4-N/NO-3-N affect on the growth, safety and quality of spinach. Based on the hydroponics experimental configuration designed by our research team, spinach was cultivated in nutrient solution containing different ratios (100∶0, 75∶25, 50∶50, 25∶75 and 0∶100) of NH+4-N/NO-3-N. Spinach samples were periodically collected and analysed for spinach biomass, shoot height, root length, nitrate and nitrite accumulation, organic acids and amino acid content. 【Results】It was found that spinach biomass, shoot height, root length, nitrate and nitrite accumulation amount, organic acids content increased, and the amino acid content declined with the variations of NH+4-N/NO-3-N from 100∶0 to 0∶100. Spinach biomass, shoot height and root length were 6.2 g/plant, 16.3 cm and 22.5 cm, respectively with the NH+4-N/NO-3-N of 100∶0, which were 6.0, 2.2 and 2.0 times higher than those in the NH+4-N/NO-3-N of 0∶100. These results suggested that spinach might be one of nitrate-philic plants. When NH+4-N/NO-3-N changed from 0∶100 to 25∶75, the nitrate and nitrite content decreased from 398.5 mg/kg, 16.42 mg/kg to 249.1 mg/kg, 14.9 mg/kg, correspondingly, which indicated that increasing the ammonium fertilizer might contribute to the decrease of accumulation of nitrate and nitrite into spinach during its growth. Organic acids including Pyruvate, Citrate, α-Oxoglutarate, Succinate, Fumarate and Malate in spinach were also monitored when the NH+4-N/NO-3-N varied from 100∶0 to 0∶100, where Fumarate showed the largest increase, up to 8.6 times, and Citrate showed the smallest increase with 2.5 times, and the maximum content of Malate reached to 985.3 mg/L with NH+4-N/NO-3-N of 25∶75. Compared with shoot height, root length, nitrate and nitrite and organic acids content in spinach, amino acid content were significantly decreased with the variation of the NH+4-N/NO-3-N from 100∶0 to 0∶100, showing 21.80 μmol/g, 12.92 μmol/g, 9.20 μmol/g, 8.30 μmol/g and 7.50 μmol/g, respectively, leading to an noticeable decrease in nutritional quality. 【Conclusions】Compared with ammonium, spinach may prefer to nitrate fertilizer, in which spinach biomass would increase with the increasing of nitrate fertilizer application. However, excessive application of nitrate fertilizer may lead to the decrease in safety and quality of spinach. Applying appropriate ratio of ammonium fertilizer into spinach cultivation can help reduce the accumulation of nitrate and nitrite into spinach, and also effectively regulate the metabolism of organic acids and amino acids. Therefore, it is rather important of applying an appropriate proportion of ammonium and nitrate fertilizer in growth of spinach to enhance its safety and quality based on the maximization of spinach production.