Cornell University
Biological and Environmental Engineering
Nitrate and Oxalate
Author: Professor Louis Albright
Introduction
Every living thing has a need for nitrogen because it makes up part of the DNA backbone that is in every cell. Primary producers such as plants, algae and phytoplankton must take up and transform elemental nitrogen into organic compounds. Higher plants primarily take up nitrogen in the following forms: nitrate (NO3-), ammonium (NH4+), urea, and amino acids. Different plant types will preferentially incorporate the different nitrogenous forms but in general ammonium, followed by nitrate, are incorporated the fastest. Farmers worldwide apply more than 160 tons of fertilizer per year to optimize the growth of their crops. Not all of the applied fertilizer is used by the plants and some of that extra fertilizer ends up in both groundwater and surface water. Some processed meat has added nitrates to enhance color and increase shelf life. The food items that have received the most attention are bacon and hot dogs. Additionally, nitrate from the manure associated with livestock production consistently ends up in the environment. To summarize, nitrates are found in the ground and surface water, in processed meats, and accumulated in plant biomass.
Plant growth/Human health
Nitrate
Plants must take up nitrogen in some form to survive. The two main forms of nitrogen taken up by the plant are nitrate and ammonium. Ammonium must be incorporated into organic compounds in roots before it can be transported to other parts of the plant, but nitrate is mobile as soon as it enters the plant. The energy-consuming process where ammonium is taken up by the plant is controlled at the interface between the cell membrane of the root.
In the human body, nitrate is converted to nitrites in the gastrointestinal tract. The nitrites react with the oxygen carrying protein in the blood called hemoglobin. As a result, oxygen deprivation can occur. Nitrate toxicity is most well known (and perhaps the most striking) in infants and is known as blue baby syndrome or methemoglobinemia. Long-term exposure in adults can lead to increased urine production and hemorrhaging of the spleen. Nitrates can also react with other compounds in food to become a carcinogenic compounds called nitrosamines. The acceptable daily intake for adults is 3.7 mg/kg body weight per day. Some regions have placed restrictions on nitrate content in produce. In Europe, the maximum allowable nitrate concentration in greenhouse grown spinach must be lower than 4500 mg/kg in the winter and 3500 mg/kg in the summer
Oxalate
Oxalate is produced in both non-soluble crystal and soluble salt forms that plants synthesize for many reasons, the most common of these being calcium regulation, plant defense, and metal detoxification. Oxalate does not occur in every plant. The following is an incomplete list of plants where oxalate is produced in quantities that can harmful to animals: philodendron, dieffenbachia, calla lily, star fruit, spinach, broccoli, swiss chard, rhubarb, and oxalis. Ingestion of oxalate can produce a variety of symptoms but is particularly dangerous as a component of kidney stones (around 80%) and causes some of the symptoms of gout.
Cornell Nitrate Research
Researchers’ at Cornell University in the Controlled Environment Agriculture Program have been looking the relationship between nitrate available to plants and the accumulation or disappearance of nitrate in plant material for over a decade. In 1998, Wheeler et al. examined how light and the nitrogen concentration in hydroponic solution influenced the nitrate concentration in lettuce tissue. They found that nitrate uptake is a complicated phenomenon that can change based on growth stage, light quantity, and history of stress. In general, larger plant size and higher light intensities allowed for a greater nitrate uptake. A mathematical model was developed to relate light and nitrate nutrition to plant growth and nitrate uptake.
The next set of experiments was on spinach by Johnson et al. in 1999. They involved looking at the quality of light and how that influenced nitrate uptake since an essential enzyme for this process is regulated by phytochrome, the protein responsible for detecting day length and directing morphological changes associated with light. The addition of far-red (735 nm peak emission) light for 5 days increased nitrate concentration by 10%. Thus the influence of light quality on nitrate uptake was clearly demonstrated. Further detail including a protocol for producing low-nitrate spinach can be found in Johnson, C., 1999.
In 2004, Seginer et al. developed a mathematical model called ‘NICOLET’ short for Nitrate COntrol in LETtuce, to prevent excessively high nitrate concentrations, especially in winter. At Cornell, the final work with nitrate was a mathematical model adaptation of NICOLET for the prediction of lettuce growth and nitrate uptake was developed as a first step in the creation of a real-time fault detection system whereby the monitoring of nitrate concentration in hydroponic solution could provide information about the health of a crop. This could provide early notification that a less than optimal set of environmental or biological conditions was negatively affecting the growth of the plants (Mathieu et al., 2006).
Plant nitrate research by others
Scientists quantified the nitrate and oxalate content in 26 varieties of fresh vegetables and determined that nitrate concentrations are greater in leafy vegetables than in root vegetables. The highest nitrate values could be found in chard, arugula (rocket) and spinach in that order. An interesting finding was that as much as 50% of the nitrate in those greens was found in the. Therefore, by removing the petiole, one could significantly reduce the amount of nitrate consumed (Santamaria et al, 1999).
In 2009, Rico-Garcia et al. analyzed the difference in nitrate content in lettuce that was grown hydroponically using either a traditional salt-based nutrient solution or fish culture water. They concluded that there was no significant difference in the nitrate concentration in lettuce leaves grown in the two different nutrient sources. An interesting side note to this experiment was that low natural light levels caused the nitrate concentration to rise over time in the second repetition of the experiment. Low light levels have been often correlated to accumulation of pollutants in produce. (Link to air pollution page)
Oxalate Research by Others
It has been demonstrated many times that oxalate levels can be manipulated by varying the nutrients supplied to the plants. This is done almost exclusively in a hydroponic growing system. The ratio of nitrate to ammonium nitrogen has a dramatic effect on oxalate production. A reason for this phenomena is that when nitrogen is supplied to plants as nitrate, it must be reduced before it can be transported to the rest of the plant which results in the production and accumulation of organic acids such as oxalic acid. Plants where the manipulation of the nitrogen source ratio has shown a significant reduction (50% on average) in oxalate concentrations include: New Zealand spinach, purslane and spinach (Spinacia oleracea) (Ahmed and Johnson, 2000; Palaniswamy et al., 2002; Zhang et al., 2005)
In 2008, Mou screened the USDA spinach germplasm collection (338 accessions and 11 commercial cultivars) for their natural oxalate content. He found significant differences in oxalate concentration among genotypes evaluated with some cultivars exhibiting double the oxalate of others. Therefore, it is possible to reduce the oxalate content of the final product by selecting a low oxalate variety when choosing a commercial cultivar.
Conclusion
While low nitrate produce may no longer be necessary according to the most recent medical research, the public perception of nitrate is still negative and thus a market for low nitrate produce still exists. Oxalate remains a harmful substance and every effort should be made to reduce concentrations in produce. The general public is less aware of the dangers of oxalate, so the financial gain to the commercial grower that produces and markets ‘low oxalate’ produce is not likely to be as great as the grower that produces and sells ‘low nitrate’ produce.
References
Ahmed A. K., K. A. Johnson. (2000). The effect of the ammonium:nitrate nitrogen ratio, total nitrogen, salinity (NaCl) and calcium on the oxalate levels of Tetragonia tetragonioides Pallas. Kunz. J Hort Sci Biotec 75,533-538.
Hord N. G., Y. Tang, N. S. Bryan. (2009). Food sources of nitrates and nitrites: the physiologic context for potential health benefits. Am J Clin Nutr 90,1-10.
Johnson C. F., R. W. Langhans, L. D. Albright, G. F. Combs, R. M. Welch, L. Heller, R. P. Glahn. (1999). Spinach: Nitrate analysis of an Advanced Life Support (ALS) crop cultured under ALS candidate artificial light sources. Society of Automotive Engineers, Inc
Johnson, C.F. (2000). Genetic and environmental influences on the nutritive value of spinach, Spinacia oleracea, for humans. PhD Dissertation. Cornell University Libraries. Ithaca, NY 14850. 157 pp.
Johnson-Rutzke C. F., R. P. Glahn, M. Rutzke, R. Wheeler, R. M. Welch, R. W. Langhans, L. D. Albright, G. F. Combs. (2002). Light quality effects on the nutritional value of spinach plants. 2002-210268,1-27.
Mathieu J., R. Linker, L. Levine, L. D. Albright, A. J. Both, R. M. Spanswick, R. Wheeler, E. Wheeler, D. S. deVilliers, R. W. Langhans. (2006). Evaluation of the Nicolet model for simulation of short-term hydroponic lettuce growth and nitrate uptake. Biosys Eng 95,323-337.
Mou B. (2008). Evaluation of oxalate concentration in the U.S. spinach germplasm collection. Hort Science 43,1690-1693.
Palaniswamy UR, Bible BB, McAvoy RJ. 2002. Effect of nitrate:ammonium nitrogen ratio on oxalate levels of purslane. Alexandria, VA:ASHS Press. p 453-455.
Rico-Garcia E., V. E. Casanova-Villareal, A. Mercado-Luna, G. M. Soto-Zarazua, R. G. Guevara-Gonzalez, G. Herrera-Ruiz, I. Torres-Pacheco, R. V. Velazquez-Ocampo. (2009). Nitrate content on summer lettuce production using fish culture water. Trend Ag Econ 2,1-9.
Santamaria P., A. Elia, F. Serio, E. Todaro. (1999). A survey of nitrate and oxalate content in fresh vegetables. J Sci Food Agric 79,1882-1888.
Seginer I., R. Linker, G. van Straten, F. Buwalda, P. Bleyaert. (2004). The Nicolet lettuce model: A theme with variations. Acta Hort 654,71-78.
Wheeler E. F., L. D. Albright, R. M. Spanswick, L. P. Walker, R. W. Langhans. (1998). Nitrate uptake kinetics in lettuce as influenced by light and nitrate nutrition. Transactions of the ASAE 41,859-867.
Zhang Y., X. Lin, Y. Zhang, S. J. Zheng, S. Du. (2005). Effects of nitrogen levels and nitrate/ammonium ratios on oxalate concentrations of different forms in edible parts of spinach. J Plant Nutr 28,211-2025.