The Nitrogen Cycle and Environmental Security
17 Feb, 2002 · 702
Col PK Gautam underlines the importance of the nitrogen cycle to soil and water security and offers few suggestions to preserve it
Mankind has altered the natural carbon and nitrogen cycle. Global warming is caused by excessive emission of Green House Gases (GHG) like carbon-dioxide which is a serious emerging threat to global security. Anthropogenic energy pattern has upset the natural carbon cycle. Carbon dioxide concentration has increased by about 31 per cent. But there is another natural cycle that is also being affected by humans, which is the nitrogen cycle.
Goa
in 2000, it was found that fertilizer run-off has led to a vast reservoir of nitrous oxide near the west coast of
India
. This would be a large contribution by
India
to GHGs. The contribution may well be about 21% of the global total. More research is being done on this aspect. The problem is that N2O must be retained in the soil and not be lost as run-off to pollute water bodies or escape into the atmosphere to become GHG.
India
show wide variations. The reasons are complex and need to factor the type of crops, fertilizer input, soil type and irrigation/tillage practices.
Nitrogen comprises 78-79 % of the earth’s atmosphere and is an important constituent of plant and animal protein. Nitrogen in the atmosphere cannot be directly used by plants and other animals. It must be converted into nitrates and nitrites. This process of conversion is called nitrogen fixation comprising of –
(a) Atmospheric fixation: High temperature due to lightning allows nitrogen to combine with oxygen to form oxides of nitrogen, which get fixed in the soil.
(b) Biological Fixation: Nitrogen is fixed by bacteria and blue green algae. For example, root nodules of leguminous plants like peas and beans have nitrogen fixing bacteria which directly absorb nitrogen to form nitrogen compounds.
(c) Industrial Fixation: This is artificial fixation brought about by science and technology. Ammonia is formed which is oxidised to form nitrates, that are used to synthesize nitrogenous fertilizers. Due to its increased and widespread use, it is estimated that nearly one third of all nitrogen in human body now comes from artificial sources.
Prior to the Green Revolution, nitrogen was being supplied to soil by way of atmospheric and biological fixation. Multi-cropping was common and farmers sowed legumes like peas with wheat for biological fixation. But with rising population there was need to more than double food production. Starting around the 1960s we saw a change in agricultural practices with the use of hybrid seeds, chemical fertilizers like urea, and increased irrigation. All three provided us food security. But, the excess use of chemicals led to other problems that affected land and water security. The use of nitrogen (N) was responsible for the problem. Excess of N causes nitrate-leaching which is bad for water as it causes pollution. N fertilizers are also based on inputs like petroleum, and are therefore ecologically and economically expensive.
Fertilizer use in agriculture is the single most important source of emitted Nitrous oxide (N2O) contributing 70% of man-made inputs to excess nitrogen cycle. N2O is also a GHG which continues for 166 years and is 30 times more potent than methane and 200 times more potent than carbon dioxide. It constitutes 6% of anthropogenic GHG and contributes to depletion of stratospheric ozone. But N2O does not have only negative features: if retained, it enriches soil with nutrients and is of great benefit to farmers and the ecology.
In a discovery by the National Institute of Oceanography at
Agricultural lands in the tropics are large emitters of N2O due to favourable moisture, temperature and high N fertiliser use. Emissions from soils used for different crops in different seasons in
Agricultural strategies include matching N supply with crop demand, optimizing irrigation, drainage and tillage, use of advanced fertilizer techniques and controlling the N flow cycle. Another option is to reduce reliance on industrial and rely more on biological N fixation. Research on rice with better N use efficiency is in progress. Investment and information technology in the agricultural sector would require these aspects of nitrogen fixation being taken care of to ensure soil and water security, and reduce the emissions of GHG to control global warming and ozone depletion.
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