Nature s check and balance
Nitrogen is the most abundant element on Earth: the total amount present in the earth's atmosphere, hydrosphere and biosphere is estimated to be 4 X 1021 gm. It forms just 1.5 per cent of a living being's constitution, but is a fundamental component of the nucleic acids which determine the genetic character of all beings and the enzymes which drive the tiny metabolic machinery of cells. However, more than 99 per cent of this nitrogen is not available to living organisms. The reason? The form in which nitrogen usually exists on Earth.
Usually, a nitrogen molecule consists of two atoms very strongly bonded together and therefore non-reactive. It is, literally, useless. The kind that plants can absorb, the chemically active kind, is called reactive nitrogen. Some of the major reactive nitrogen compounds are ammonia, ammonium, nitric acid and various oxides of nitrogen such as nitrous oxide, nitrogen dioxide and nitrate.
The reactive nitrogen form is also present several organic compounds such as urea, amines, proteins and amino acids. There was a time reactive nitrogen got to be produced only through lightning and biological nitrogen fixation. So produced naturally, it did not accumulate in environmental reservoirs. This was due to a process called dinitrification, where certain other set of microbes convert reactive nitrogen back into non-reactive form. However, human activities in the last century have considerably upset this equilibrium. Scientists say the global rate of increase in reactive nitrogen was slow from 1860 to 1960 but since then:
• Cultivation-induced reactive nitrogen production increased from approximately 15 teragrammes (Tg) per year to nearly 33 Tg per year in 2000 (One terragramme is equal to one million tonnes)
• Reactive nitrogen production because of fossil fuel combustion increased from less than 1 Tg per year in 1860 to approximately 25 Tg in 2000
• Reactive nitrogen creation through the Haber-Bosch process to manufacture fertilisers increased from 0 in 1910 to more than 100 Tg per year in 2000.