Maarg Manthan · Topic 5.5

Nitrogen Cycle and Human Impact on It

Environment & Ecology › Nutrient Cycling (Biogeochemical Cycles) · Topic 5.5

The nitrogen cycle is the movement of nitrogen between the atmosphere, soil, water and living things. Nitrogen makes up about 78 percent of air, but plants and animals cannot use the inert gas directly. It must be converted into usable forms in a series of steps. Nitrogen fixation turns atmospheric nitrogen into ammonia, by symbiotic bacteria such as Rhizobium in legume root nodules, free-living bacteria such as Azotobacter and cyanobacteria such as Anabaena, and by lightning and industry. Ammonification releases ammonia from dead matter. Nitrification, by Nitrosomonas and Nitrobacter, converts ammonia into nitrite and then nitrate. Plants absorb nitrate (assimilation), and denitrifying bacteria such as Pseudomonas return nitrogen gas to the air. Human fertiliser use, fossil fuel burning and farming have doubled the reactive nitrogen entering the biosphere.

6 min read · UPSC Prelims · Environment & Ecology · Nutrient Cycling (Biogeochemical Cycles)

At a Glance

Air share About 78 percent
Fixation N2 to ammonia
Nitrification Ammonia to nitrate
Denitrification Nitrate to N2
Type Gaseous cycle
Exam link UPSC Prelims, Environment & Ecology: Nutrient Cycling (Biogeochemical Cycles)

Where Does It Fit?

This topic follows the water cycle. Nitrogen is the nutrient most often limiting plant growth, so its cycle is central to agriculture and is closely linked to pollution topics later. Previous topic: Hydrological (Water) Cycle: Steps and Storage. Next topic: Oxygen Cycle.

What Are the Steps of the Nitrogen Cycle?

Step	Process and organisms
Nitrogen fixation	N2 converted to ammonia. Symbiotic: Rhizobium in legumes. Free-living: Azotobacter, Clostridium. Cyanobacteria: Nostoc, Anabaena (rice fields), also Azolla with Anabaena. Abiotic: lightning, and industrial Haber-Bosch process
Ammonification	Decomposers (bacteria and fungi) convert organic nitrogen of dead matter and waste into ammonia
Nitrification	Nitrosomonas convert ammonia to nitrite, and Nitrobacter convert nitrite to nitrate
Assimilation	Plants absorb nitrate and ammonium and make proteins, which animals obtain by eating plants
Denitrification	Anaerobic bacteria such as Pseudomonas and Thiobacillus convert nitrate back to nitrogen gas, completing the cycle
Leaching and run-off	Nitrate, being soluble, washes into groundwater and water bodies
Anammox	Anaerobic ammonium oxidation, a pathway in the sea and wetlands that converts ammonium and nitrite to nitrogen gas

How Do Humans Disturb the Nitrogen Cycle?

Chemical fertilisers: urea and ammonium compounds; the Haber-Bosch process now fixes more nitrogen than all natural land processes
Fossil fuel burning: makes nitrogen oxides that cause smog and acid rain
Eutrophication: excess nitrate and phosphate cause algal blooms and oxygen loss in water
Nitrous oxide: a long-lived greenhouse gas from fertilised soils and livestock manure, also an ozone depleting substance
Blue baby syndrome: nitrate in drinking water can cause methaemoglobinaemia in infants
Soil acidification: from excess ammonium fertiliser and acid deposition
Loss of biodiversity: nitrogen-loving species displace others
Dead zones: oxygen-poor coastal areas, as in the Gulf of Mexico

What Else Should You Remember?

Legumes: peas, beans, gram and groundnut enrich soil through Rhizobium
Root nodules: swellings with bacteria and leghaemoglobin, a pigment that protects the oxygen-sensitive nitrogenase enzyme
Nitrogenase: the enzyme that fixes nitrogen
Biofertiliser: Rhizobium, Azotobacter, Azospirillum, Azolla and blue-green algae are used to enrich soils
Azolla: a water fern used in rice fields in Asia
Green manure: crops like Sesbania ploughed in to add nitrogen
Crop rotation: growing legumes with cereals restores nitrogen
Neem-coated urea: slows nitrification, raising nitrogen use efficiency
Nano urea: liquid fertiliser promoted by IFFCO to cut conventional urea use
Nitrogen use efficiency: Indian cereals use under half of applied nitrogen
Indian nitrogen assessment: a report on India’s reactive nitrogen found fertiliser use to be the main driver of pollution
Reactive nitrogen: all forms other than N2, such as ammonia, nitrate and nitrous oxide
Nitrogen oxides: NOx, from vehicles and power plants
Ammonia emissions: from fertilisers and livestock
Acid rain: nitric acid from NOx adds to acidity
Ground-level ozone: formed from NOx and volatile compounds
Kyoto Protocol: lists nitrous oxide among greenhouse gases
Colombo Declaration: a 2019 call to halve nitrogen waste by 2030
Link to previous: see Hydrological (Water) Cycle: Steps and Storage
Link to pollution: see Air, Marine, Noise and Other Pollution macro
Link to agriculture: see Land, Forest and Soil Resources macro
Caution: lightning fixes only a small share of nitrogen
Mycorrhizae: fungi that help plants take up nutrients, though not fixers of nitrogen
Lichen and mosses: some lichens with cyanobacteria fix nitrogen in cold areas
Exam hint: match organism to step: Rhizobium fixation, Nitrosomonas and Nitrobacter nitrification, Pseudomonas denitrification
Haber-Bosch process: developed in the early twentieth century, it combines nitrogen and hydrogen under high pressure and temperature to make ammonia, and supports roughly half of the world’s food supply today
Nitrogen in oceans: cyanobacteria such as Trichodesmium fix nitrogen in tropical seas, and nitrogen often limits marine phytoplankton
Industrial nitrogen compounds: ammonium nitrate is both a fertiliser and an explosive, and its storage needs strict safety rules
Nitrate pollution limit: drinking water standards set a limit on nitrate, with the World Health Organization guideline at 50 milligrams per litre for nitrate; check the latest national standard
Soil organic matter: holds most soil nitrogen, which microbes slowly mineralise, so humus-rich soils supply nitrogen steadily
Waterlogged soils: low oxygen favours denitrification and loss of nitrogen as gas, which is why rice farmers lose much fertiliser
Sewage and manure: are major point sources of ammonia and nitrate to rivers and groundwater
Peas and beans in rice systems: Azolla and green manure are low-cost alternatives to urea for small farmers
Legume share in India: pulses cover a large area and bring free nitrogen to the cropping system, which is one more reason for promoting them

Why Does It Matter for the Exam?

Steps: fixation to denitrification
Organisms: Rhizobium, Nitrosomonas, Pseudomonas
Impact: eutrophication, nitrous oxide
Current affairs link: nano urea and neem-coated urea
Common errors: swapping nitrification and denitrification or saying animals fix nitrogen

As an analytical point, nitrogen is both the nutrient that feeds the world and a major pollutant. Improving fertiliser efficiency through biofertilisers, neem-coated urea and balanced use can cut losses while protecting yields.

Why Is This Topic Important?

Steps Fix, nitrify, denitrify
Bacteria Rhizobium and others
Impact Fertiliser pollution

What Came Next?

The next topic covers the oxygen cycle. Next topic: Oxygen Cycle.

Key People and Terms

Nitrogen fixation N2 to ammonia
Nitrification Ammonia to nitrate
Denitrification Nitrate to N2
Ammonification Organic N to ammonia
Reactive nitrogen Biologically usable nitrogen
Eutrophication Nutrient overload of water

Exam Corner

Points to Remember

Air is 78 percent nitrogen.
Rhizobium fixes in legume nodules.
Nitrosomonas then Nitrobacter.
Denitrifiers return N2.
N2O is a greenhouse gas.

Do Not Confuse With

Nitrification and denitrification Ammonia to nitrate versus nitrate to gas
Nitrogen fixation and assimilation Gas to ammonia versus uptake by plants
Nitrous oxide and nitric oxide Greenhouse gas versus smog precursor

Memory Hook

Fix it, rot it, nitrify it, eat it, denitrify it: gas to plant and back to gas.

Mains Angle

Questions ask for steps and organisms. These points help.

Steps Cycle
Impact Fertiliser
Remedy Efficiency

A Question You May Face

An original practice question, not a past paper question.

“Describe the nitrogen cycle and discuss how excess use of nitrogen fertilisers affects the environment.”

How to Answer

Steps Five.
Organisms Bacteria.
Impact Pollution.
Remedy Efficiency.

GS Relevance

Prelims: steps, bacteria, fixation, human impact. GS Paper 3: agriculture, pollution and environment.

Frequently Asked Questions

What is the nitrogen cycle?

The nitrogen cycle is the series of processes by which nitrogen moves between the atmosphere, soil, water and organisms. Bacteria fix nitrogen gas into ammonia, other bacteria turn it into nitrate, plants take it up and animals eat plants, and denitrifying bacteria finally return nitrogen gas to the air.

What is nitrogen fixation and who does it?

Nitrogen fixation is the conversion of inert atmospheric nitrogen into ammonia. It is done by symbiotic bacteria like Rhizobium in legume root nodules, free-living bacteria like Azotobacter, cyanobacteria like Anabaena and Nostoc, by lightning, and industrially by the Haber-Bosch process used to make fertilisers.

What is the difference between nitrification and denitrification?

Nitrification is the oxidation of ammonia to nitrite and then nitrate by bacteria such as Nitrosomonas and Nitrobacter, making nitrogen available to plants. Denitrification is the reduction of nitrate to nitrogen gas by anaerobic bacteria such as Pseudomonas, returning nitrogen to the atmosphere.

How does nitrogen fertiliser harm the environment?

Excess nitrogen fertiliser leaches into groundwater and runs into rivers and lakes, causing eutrophication and algal blooms, and nitrate pollution of drinking water. Fertilised soils also release nitrous oxide, a potent greenhouse gas, and ammonia, which add to air pollution and soil acidification.

Why are legumes grown in crop rotation?

Legumes such as gram, pea and groundnut have root nodules containing Rhizobium bacteria that fix atmospheric nitrogen into ammonia. When the plants die or are ploughed in, this nitrogen enriches the soil, so rotation with legumes restores fertility and reduces the need for chemical fertilisers.

PYQ Practice — Statement Analysis

1 Rhizobium fixes nitrogen in legume root nodules.

True

A symbiotic relationship.

2 Nitrobacter converts nitrate to nitrogen gas.

False

It converts nitrite to nitrate.

3 Denitrification returns nitrogen to the atmosphere.