Nitrate leaching and its hazardous impact on human health

Nitrogen, being the one of the most important macronutrient, taken up by the plants possesses a great impact on human life. Nitrogen is taken as nitrate and ammonium ions. Here, the nitrate ion is negatively charged and due to this, it does not attach to the negatively charged surface of soil particles. This is the main reason behind nitrate leaching. Along with rainfall and irrigation water the nitrate ions move downward through the soil profile. Due to flooding of fields during irrigation and in rainy seasons, nitrate leaching is more common. The role of fertilizer in crop production became gradually high after the green revolution. Nitrate consumption drastically increase from year 1980 to 2008. Nitrate is already present in drinking water but the amount present here is below hazardous level but due to application of high amount of chemical fertilizers, the amount of it is increasing gradually above that threshold, making water unfit for human consumption. The high amount of nitrate in drinking water is dangerous not only for human but for animal consumption also.

Table: Fertilizer consumption (kg/ha) in some states of India (1997-98)

() Indicate the increase (in times) in nitrogen consumption in 1997-98 (kg/ha) as compared to 1989-90 [Source: Fertilizer Statistics (1997-98)].

Impact on human health: The high concentration of NO₃^- can cause methemoglobinemia which is also known as blue-baby syndrome, in this syndrome High concentrations of nitrate in drinking water can interfere with the transport of oxygen in the blood and can cause harm to human health, especially in infants less than 1 year of age. Suspected cases of in drinking water were first reported in the 1940s and 1950s. Babies may become sick from “blue baby syndrome” when nitrate in the blood reduces the oxygen carrying capacity. Because of this, WHO have set a standard on the limiting amount of nitrate in drinking water i.e. at 10–11.3 mg nitrate -N L^-1 (World Health Organization 1984).Exposure to nitrates over time could also potentially impact the health of adults as well, especially the elderly. Studies have also been conducted on the possible links between nitrate intake in drinking water and stomach cancers in adults and childhood diabetes. As mentioned above, the certain regulatory limit for nitrate in drinking water is set to protect infants against methemoglobinemia, but other health effects are not considered. Nitrate is present in many food specifically in green leafy and root vegetables. The average daily intake of nitrate from food are in range of 30-130 mg NO₃^- per day. Drinking water containing nitrate is absorbed in the upper gastrointestinal tract, from where it get distributed in the whole body. When it reaches to the salivary glands, it get transported from blood to saliva. The levels may be up to 20 times higher in the later than plasma. In the oral cavity 6-7% of the total nitrate get converted into nitrite and further into its compounds. It is mainly done by nitrate reducing bacteria. From oral cavity, this converted forms of nitrate again enters into the gastrointestinal tract. The acidic environment of the stomach protonate nitrite to nitrous acid (HNO₂) and further to dinitrogen trioxide (N₂O₃), nitric oxide (NO) and nitrogen dioxide (NO₂). The NO formed is involved in many physiological effects. It is involve in regulation of blood pressure and blood flow by mediating vasodilation, the maintenance of blood vessel tonus, the inhibition of platelet adhesion and aggregation, modulation of mitochondrial function. Other than these, health risks in pregnancy cases has been recorded. Spontaneous abortion, fetal deaths, prematurity, intrauterine growth retardation, low birth weight, congenital malformation and neonatal death cases has been seen as ill effects of drinking water nitrate intake during pregnancy. Some animal studies showed that inhibition of iodine uptake and induce hypertrophy of the thyroid gland can also be caused by high concentration of nitrate in drinking water. Drinking water containing high amount of nitrate can also cause type 1 diabetes (T1D) in children, blood pressure and acute respiratory tract infection in children.

            Potential benefits of nitrate ingestion include lowering of blood pressure due to production of nitric oxide in acidic stomach conditions and subsequent vasodilation, antithrombotic and immuno-regulatory effects.

Strategies to mitigate NO2 leaching: Many sources may lead to nitrate leaching and is affected by soil, management and environmental factors. One trick or method is not there which can mitigate this problem at once, a number of strategies and integrated approach has been given to solve this problem. Integrated approach involving the development and adoption of ‘best management practices’ (BMP) to optimize the N use efficiency of plants for optimum production, while minimizing NO leaching. Strategies to mitigate this problem are as follows:

• Many incidences of high NO2 leaching losses have been associated with high N inputs in excess of plant demand. Reducing total N input is thus one of the effective options for reducing NO2 leaching.
• Synchronising N supply with plant N demand together with a proper application rate is the best way to avoid the accumulation of mineral N in the soil. This approach requires a good understanding of the plant’s N requirement at different times of the year, and the capacity of the soil to meet that demand.
• Most of the times, autumn- winter season possess low evaporation and high drainage. So, Limited N applications prior to or during high leaching season can also be useful to ensure less leaching of nitrate.
• The cover crops should be established early in the autumn so that significant amounts of N can be taken up before the winter leaching season.
• Nitrification inhibitors (e.g. dicyandiamide, DCD) have been used to reduce the conversion rate of NH₄⁺ to NO₃^- , thus reducing NO₃^- leaching.
• Urine returns from grazing animals late in the autumn can result in large amounts of NO₃^- leaching losses. This impact may be reduced by taking the stock off the farm earlier in the autumn– winter period and feed the animals in-house. A delay in irrigation after grazing by a few days may also reduce the leaching potential of urine N.
• Buffer zones between production land and water bodies have been used to reduce the migration of NO₃^- from the production land to the water.
• Whole farm budgeting is a method where the amount of NO₃^- leaching losses is calculated on the basis of N inputs and outputs for an individual farm.
• Computer models also can be used to mitigate nitrate leaching. Mechanistic models are usually constructed by coupling an N cycling model with a solute transport model and, sometimes, with a crop growth model.