Adverse Effect of Excess Nitrogenous Fertilizer on Environment

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Agriculture is faced with the challenge of providing healthy food for a growing population at minimal environmental cost. Meanwhile, crop production will need to be double (2.4% annual increase) to feed the burgeoning population and meet global demand. Agricultural intensification, whereby higher yields per unit of land area are realized, is considered necessary to achieve this goal.  However, intensification can have environmental costs such as non-point source pollution and increased greenhouse gas (GHG) emissions. Increased production also implies increased demand for water and nutrient. So, farmers are applying huge amount of inorganic fertilizer to achieve higher yield. Among different inorganic fertilizers, nitrogenous fertilizer is one of the most important nutrient sources, which is an essential element for all organisms and basic building blocks of our food, and constituents of amino acids, proteins (enzymes), nucleic acids, and chlorophyll in plants. It is usually the most yield limiting nutrient in crop production. So, farmers are applying tonnes of nitrogen in the form of inorganic fertilizer, compost and organic manures to the field in order to get higher yield.

Farmers are applying double dose of recommended nitrogen fertilizer in order to get higher yield. But, in case of nitrogen fertilizer the nitrogen use efficiency (NUE) is very less when compared to other fertilizers. It is in the range of 15-25 %. So, remaining applied nitrogen fertilizer will be lost through volatilization, leaching, denitrification and runoff. This practice is costly for farmers and harmful to the environment. The nitrogenous fertilizer is lost through evaporation, seepage and percolates into groundwater, lakes and oceans, where it can feed massive algal blooms that consume oxygen and suffocate aquatic organisms, which further affects aquatic vegetation. This leads to ‘Eutrophication’, which creates aesthetic problems, deplete oxygen content of the water as they eventually die and decompose and also emit more methane gas.

The problems posed to the environment occur when excess nitrogen in soils is carried away with surface runoff and water moving through soils and then finds its way to water and other ecosystems, which can also receive nitrogen from precipitation. Much of the nitrogen from the decomposed algae and aquatic vegetation remains in the water to nourish new organisms, thus continuing the cycle of growth, decomposition, and oxygen depletion. High levels of ammonium are toxic to fish and use up dissolved oxygen in the water as the ammonium converts to nitrate. Nitrate poses a particular environmental concern because of its mobility. Nitrate readily moves with water moving through soils. It can contaminate groundwater to the point at which it becomes a health risk (10 ppm). The primary health concern is methemoglobinemia (called the “blue baby" syndrome). Human infants and some animals are especially susceptible. In this condition, the digestive system reduces nitrate to nitrite, which interferes with the blood's ability to carry oxygen.

Most of this nitrogen is in soluble forms, such as nitrate and ammonium. Nitrogen enters the soil from a variety of sources. Unlike other nutrients, only a small amount of N is contributed from the mineral part of the soil (i.e., rocks). Humans also increase the N in soils by fertilizing with either chemicals or organic fertilizers. Once in the soil, the N will transform through the processes of mineralization, immobilization, volatilization, nitrification, denitrification, and plant uptake. Two of these transformations—volatilization and denitrification—result in losses of N from the soil, through leaching and runoff. These losses led to increase in green house gases in the atmosphere and contaminate the ground water, lakes and ponds etc.

Author: T K Vanitha, JRF, Bioengineering and Environmental Sciences (BEES) Lab, CEEFF, CSIR-IICT, India

Image source:

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