Nitrogen is an essential element which every organism needs for adequate growth and function. Nitrogen is found in all amino acids, incorporated into proteins and present as bases that make up nucleic acids such as deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). In plants, much of the nitrogen is used in chlorophyll molecules which are essential for photosynthesis and further growth. Although 78 per cent of the atmosphere comprises of nitrogen gas, it is relatively inert. It cannot, therefore, be used directly by most living organisms until it is converted into nitrates or other nitrogen compounds. However, certain bacteria in the soil and cyanobacteria in the oceans are among the few organisms that are independently able to carry out this conversion.
The conversion of atmospheric nitrogen into a form readily available to plants and hence to animals and humans is an imperative survival step in the terrestrial nitrogen cycle (Fig. 1). There are four ways to such conversion.
Biological fixation – Symbiotic bacteria, often associated with leguminous plants, along with few free living bacteria such as Azotobacter are able to fix nitrogen and assimilate it as organic nitrogen. An example of mutualistic nitrogen fixing bacteria is the Rhizobium, which lives in plant root nodes. Another case in point is the photosynthetic cyanobacteria that often live as free living organisms in pioneer habitats such as desert soils or as a symbiont with lichens in other pioneer habitats. They also form symbiotic associations with other organisms such as the water fern Azolla, and Cycads.
Industrial N fixation – Fritz Haber and Carl Bosch discovered a process wherein a nitrogen fixation reaction of nitrogen and hydrogen gas, over an enriched iron or ruthenium catalyst is used to industrially produce ammonia in 1909. The Haber-Bosch process converts nitrogen together with hydrogen gas into ammonia fertilisers.
Combustion of fossil fuels – Automobile engines and thermal power plants release NOx.
Other processes – Additionally, nitrogen can be added to the soil as a result of electrical discharge during thunderstorms. The energy from lightning causes oxygen and nitrogen gases to combine with water vapour, forming weak nitric acid. This is washed down in rain and contributes to the nitrogen content of the soil.
Nitrates taken in by plant roots are incorporated into large organic molecules, which are transferred to animals when they eat the plants. In plants which have a mutualistic relationship with Rhizobium, some nitrogen is assimilated in the form of ammonium ions from the nodules. All plants however, can absorb nitrate from the soil. These are then reduced to nitrite ions and then ammonium ions for incorporation into amino acids and hence protein, which forms part of the plants or animals who eat them. The wastes and remains of both plants and animals contain organic nitrogen compounds which are broken down by decomposers and converted into inorganic compounds such as ammonium ions. Nitrifying bacteria convert these compounds back into nitrates in the soil, which can be taken in again by plants and cycled through the ecosystem once more.
Denitrification is the process which converts the nitrates (NO3) back to nitrogen gas. Denitrifying bacteria are found in waterlogged soils where they release nitrogen gas causing the soil to lose its nitrogen. Over-irrigation thus should be avoided as it leads to loss of cultivable land.
Crops can use more nitrate than is present in most soils. Artificially produced nitrogen which is the basic ingredient of fertilisers is now more abundant than the nitrogen from natural sources. Agricultural yields thus have improved dramatically. In fact the annual transfer of nitrogen into a biologically available form has nearly tripled with the cultivation of legumes, creation of chemical fertilisers and pollution emitted by biomass burning, vehicles, industrial plants and humans.
Modern crops such as wheat and rice require high levels of nitrogen to sustain their rapid growth. Once harvested the nitrogen within the crop is not returned to the soil and farmers have to add fertilisers such as ammonium nitrate. Too much fertiliser particularly during the monsoons leach out of the soil and passes into water courses and eventually ends up in a river or pond where it stimulates the growth of freshwater algae. The algae grow rapidly to form a green blanket over the surface of the water, called algal bloom. It blocks the light to plants in the water, inhibiting their growth and causing the loss of oxygen content in the water body. The resulting suffocation causes many species of aquatic fauna including fish to die and as a whole is detrimental for aquatic and wetland habitats. The process is referred to as eutrophication. When plant communities turn saturated with nitrogen, the soil can also become acidified, making it inhospitable.
Burning fossil fuels and wood contributes to a large amount of nitric oxide in the atmosphere. It is a reactant in the atmosphere, where it acts as an aerosol, decreasing air quality and clinging on to water droplets. These large quantities of nitric oxide combine with oxygen to form nitrogen dioxide, which reacts with water vapour to form nitric acid and precipitates out of the atmosphere in the form of acid rain. The acid can damage trees and kill fish. Nitrous oxide (N2O) also has deleterious effects in the stratosphere (second atmospheric layer from the surface), where it breaks down and acts as a catalyst in the destruction of atmospheric ozone. Fossil fuel combustion has contributed to a 6 or 7 fold increase in NOx flux to the atmosphere. Nitric oxide actively alters atmospheric chemistry and is a precursor of tropospheric (first atmospheric layer from the surface; weather zone) ozone production, which contributes to smog, acid rain, and increases nitrogen inputs to ecosystems.
Inputs from Ecology and Environment by Sally Morgan and Mike Allaby.