The Nitrogen Cycle and Biological Nitrogen Fixation – Nutrient Cycles (Senior 3 Biology) | Ganzaa

Nutrient Cycles

Lessons

Senior 3 · Biology

Nutrient Cycles

4 lessons

Lesson 3 of 4

The Nitrogen Cycle and Biological Nitrogen Fixation

25 min

Surrounded by nitrogen, yet short of it

Every breath you take is 78% nitrogen gas, and the air above one hectare of a farm holds tens of thousands of tonnes of it. Yet maize on that farm can turn yellow and stunted for lack of nitrogen, and farmers buy urea to feed it. The reason: plants and animals cannot use nitrogen gas. Nitrogen has to be changed into other compounds before living things can take it in, and almost all of that work is done by bacteria.

Definition

Nitrogen cycle: the continuous movement of nitrogen between the air, the soil, water and living things, in forms such as nitrogen gas $\mathrm{(N_2)}$ , ammonium $\mathrm{(NH_4^+)}$ , nitrite $\mathrm{(NO_2^-)}$ , nitrate $\mathrm{(NO_3^-)}$ and proteins.

Nitrogen fixation: changing nitrogen gas into nitrogen compounds (ammonium or nitrate) that living things can use. Biological nitrogen fixation is done by bacteria.

Ammonification (putrefaction): the breakdown by decomposers of proteins and urea in dead remains and wastes into ammonium.

Nitrification: the change of ammonium into nitrite and then into nitrate by nitrifying bacteria in well-aerated soil.

Denitrification: the change of nitrate back into nitrogen gas by denitrifying bacteria, mainly in waterlogged soil with little oxygen.

Why living things need nitrogen

Nitrogen is part of every amino acid, so every protein (enzymes, muscle, haemoglobin, antibodies) contains it. It is also in DNA, in chlorophyll and in ATP. Plants take in nitrogen as nitrate (and some ammonium) through their root hairs and use it to make amino acids. Animals get their nitrogen ready-made, by eating plants or other animals.

The nitrogen cycle drawn

Figure 1: The nitrogen cycle. Follow the numbered arrows with the key below.

Arrow	Process	Done by
1	Biological nitrogen fixation: $\mathrm{N_2}$ → ammonium, used by the plant	Rhizobium bacteria in root nodules of legumes (beans, peas, groundnuts, soya); free-living soil bacteria such as Azotobacter and cyanobacteria also fix nitrogen
2	Fixation by lightning: $\mathrm{N_2}$ and $\mathrm{O_2}$ combine; rain carries nitrates into the soil	the energy of lightning
3	Absorption of nitrate; plants make amino acids and proteins	root hairs of plants
4	Feeding: proteins pass along food chains	animals
5, 6	Death, fallen leaves, faeces and urine (urea)	plants and animals
7	Ammonification: proteins and urea → ammonium	decomposers (bacteria and fungi)
8	Nitrification: ammonium → nitrite	nitrifying bacteria (Nitrosomonas)
9	Nitrification: nitrite → nitrate	nitrifying bacteria (Nitrobacter)
10	Denitrification: nitrate → nitrogen gas	denitrifying bacteria, in waterlogged soil
11	Industrial fixation: fertilisers such as urea and ammonium nitrate, made from $\mathrm{N_2}$ in factories (the Haber process)	people

Nitrate dissolves easily, so heavy rain can also wash it down through the soil and into streams: this leaching removes nitrogen from farms and can pollute water.

Worked example 1: how much nitrogen is in urea?

Urea, $\mathrm{CO(NH_2)_2}$ , is a common nitrogen fertiliser. (a) Calculate the percentage of nitrogen in pure urea. (b) A farmer wants to give a 0.5-hectare maize field 60 kg of nitrogen per hectare. Fertiliser urea is 46% nitrogen. What mass of urea is needed? (Relative atomic masses: H = 1, C = 12, N = 14, O = 16.)

Relative formula mass: $12 + 16 + 2 \times (14 + 2 \times 1) = 28 + 32 = 60$ .
(a) Nitrogen: $2 \times 14 = 28$ , so $\%\,\mathrm{N} = \frac{28}{60} \times 100\% = 46.7\%$ (fertiliser urea is sold as 46% N because it is not quite pure).
Nitrogen needed: $60 \times 0.5 = 30$ kg.
(b) Urea needed: $\frac{30}{0.46} = 65$ kg.

Worked example 2: maize after beans

A farmer grew maize on two plots with no fertiliser. On plot A the previous crop was beans; on plot B it was maize. Plot A gave 3.2 tonnes of maize per hectare and plot B 2.4 tonnes per hectare. (Illustrative figures.) (a) Calculate the percentage increase. (b) Explain the difference using the nitrogen cycle.

(a) $\frac{3.2 - 2.4}{2.4} \times 100\% = \frac{0.8}{2.4} \times 100\% = 33\%$ .
(b) Beans are legumes. Rhizobium in their root nodules fixed nitrogen gas from the soil air. When the bean roots and leaves decayed, decomposers and nitrifying bacteria turned their proteins into nitrate, which the next maize crop absorbed and used to make proteins and grow more. After a maize crop, which fixes no nitrogen, less nitrate was left.

This is why rotating or intercropping cereals with beans, a common practice in Rwanda, improves soil fertility.

Waterlogged soil and the nitrogen cycle

Nitrifying bacteria need oxygen. In waterlogged soil, such as marshland rice fields, air spaces fill with water, nitrification slows and denitrifying bacteria turn nitrate into nitrogen gas, which escapes to the air. Good drainage and turning the soil (which lets air in) keep nitrate in the soil. Legumes and fertilisers replace the nitrogen that crops remove when they are harvested.

Warning

Common mistakes. (1) "Plants take in nitrogen gas through their leaves": plants cannot use $\mathrm{N_2}$ ; they absorb nitrate through their roots. (2) Mixing up nitrifying and nitrogen-fixing bacteria: fixing bacteria turn $\mathrm{N_2}$ into ammonium; nitrifying bacteria turn ammonium into nitrate. (3) Forgetting that denitrification removes nitrate from the soil.

Carbon and Nitrogen Cycles (Amoeba Sisters). Watch the nitrogen cycle section and note which step each kind of bacteria carries out.

Tip

Memory hook: "Fix, Feed, Rot, Nitrify, Denitrify". Fix ( $\mathrm{N_2}$ → ammonium), Feed (proteins along food chains), Rot (ammonification), Nitrify (ammonium → nitrite → nitrate), Denitrify (nitrate → $\mathrm{N_2}$ ). Nitrification adds oxygen at each step: $\mathrm{NH_4^+}$ → $\mathrm{NO_2^-}$ → $\mathrm{NO_3^-}$ .

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Practice: the nitrogen cycle

Why can plants not use the nitrogen gas that makes up 78% of the air?
Name the bacteria found in the root nodules of beans, and state what they do.
Put in order: nitrate, ammonium, protein in a dead leaf, nitrite.
Name the process, and the kind of organism, for each change: (a) urea → ammonium; (b) nitrite → nitrate; (c) nitrate → nitrogen gas.
Calculate the percentage of nitrogen in ammonium nitrate, $\mathrm{NH_4NO_3}$ . (H = 1, N = 14, O = 16.)
A 50 kg bag of fertiliser contains 46% nitrogen. What mass of nitrogen does it supply?
Explain why a waterlogged field loses nitrate.
A farmer says: "I don't need to grow beans, because I can buy urea." Evaluate this, using the nitrogen cycle.

Show answersHide answers

Nitrogen gas is very unreactive; plants have no enzymes to break it apart. It must first be fixed into ammonium or nitrate.
Rhizobium: they fix nitrogen gas into ammonium compounds, which the plant uses to make amino acids; in return they get sugars from the plant.
Protein in a dead leaf → ammonium → nitrite → nitrate.
(a) ammonification, by decomposers; (b) nitrification, by nitrifying bacteria (Nitrobacter); (c) denitrification, by denitrifying bacteria.
$M_{r} = 14 + 4 + 14 + 48 = 80$ ; nitrogen $= 28$ ; $\frac{28}{80} \times 100\% = 35\%$ .
$0.46 \times 50 = 23$ kg.
Water fills the air spaces, so there is little oxygen: nitrifying bacteria (which need oxygen) work slowly, and denitrifying bacteria turn nitrate into nitrogen gas, which escapes. Nitrate may also be leached.
Urea supplies nitrogen quickly but costs money every season, and excess can be leached into water. Beans fix nitrogen for free, give food and protein, and leave nitrogen in the soil for the next crop. Most farmers benefit from using both: beans in the rotation, and fertiliser where soils are poor.

Key points

Nitrogen is needed for proteins, DNA and chlorophyll; plants absorb it as nitrate.
Fixation (Rhizobium, free-living bacteria, lightning, factories) → absorption → feeding → ammonification → nitrification (Nitrosomonas, Nitrobacter) → denitrification.
Legumes enrich the soil with nitrogen; waterlogging and leaching remove it.

Sources and credits

20.2: Biogeochemical Cycles, from Concepts of Biology by Samantha Fowler, Rebecca Roush and James Wise (OpenStax), via LibreTexts Biology, licensed under CC BY 4.0. Adapted by Ganzaa. (section "The Nitrogen Cycle")
Photos: Compost pile, Ksd5, CC0; 2013-08-07 04-23-26-foudre-belfort, Thomas Bresson, CC BY 3.0; both via Wikimedia Commons.
Video: Carbon and Nitrogen Cycles, Amoeba Sisters, embedded from YouTube.

Written by Ganzaa: Figure 1, the key, the worked examples (the maize yields are illustrative) and the practice questions.

1Surrounded by nitrogen, yet short of it

2Why living things need nitrogen

3The nitrogen cycle drawn

4Waterlogged soil and the nitrogen cycle

Surrounded by nitrogen, yet short of it

Why living things need nitrogen

The nitrogen cycle drawn

Waterlogged soil and the nitrogen cycle