The stages of the nitrogen cycle
In a milpa in Oaxaca, corn, beans, and squash have grown in the same furrow for centuries. Farmers notice that the corn looks greener and stronger when the beans are nearby. What do the beans give the corn? To answer, we have to follow the journey of a very special element: nitrogen.
Plenty of nitrogen, but out of reach
Almost 4 out of every 5 parts of the air (about 78%) is nitrogen gas, $\text{N}_2$. However, plants and animals can't use it in that form. The two nitrogen atoms are joined by a very strong bond, and roots can't absorb that gas.
Living things need nitrogen to make proteins (which form muscles, enzymes, and leaves) and nucleic acids (DNA). That's why it is an essential nutrient.
The solution comes from certain bacteria. Nitrogen fixation is the process in which certain bacteria convert the $\text{N}_2$ in the air into nitrogen compounds (such as ammonium) that can be used.
There are two types of nitrogen-fixing bacteria:
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Those that live freely in the soil or in water.
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Those that live inside small lumps (nodules) on the roots of legumes: beans, fava beans, lentils, peas, alfalfa. The plant gives them sugars and shelter; they give the plant usable nitrogen. It is a relationship in which both benefit.
In addition, lightning storms fix a little nitrogen, and industry makes fertilizers with it.
Explain why plants cannot use nitrogen gas (N₂) from the atmosphere even though it is very abundant, and name the organisms that "fix" it by turning it into compounds…
Explain why plants cannot use nitrogen gas (N₂) from the atmosphere even though it is very abundant, and name the organisms that "fix" it by turning it into compounds that roots can absorb.
From ammonium to nitrate: nitrification
The ammonium ($\text{NH}_4^+$) left by nitrogen-fixing bacteria also appears when remains decompose. Other soil bacteria transform it into nitrites and then into nitrates ($\text{NO}_3^-$). This process is called nitrification.
Nitrates dissolve in the water in the soil, and roots absorb them. With them, the plant makes its proteins: nitrogen moves from the soil into a living thing.
Through food chains and back to the soil
An herbivore eats the plant and uses its nitrogen to make its own proteins. A carnivore eats the herbivore, and the nitrogen keeps moving along.
When organisms die, or when they excrete waste (urine, manure), decomposers (fungi and bacteria) break down their remains and return the nitrogen to the soil in the form of ammonium. This is decomposition (or ammonification). That ammonium can be nitrified again and absorbed once more.
Back to the atmosphere: denitrification
If nitrogen only stayed in the soil, the air would run out of it. Certain bacteria, in very wet or oxygen-poor soils (such as waterlogged soils or swamps), transform nitrates back into $\text{N}_2$, which rises into the air. This is denitrification, and it closes the cycle.
In the diagram, follow the numbers and the arrows: 1 fixation (air → ammonium), 2 nitrification (ammonium → nitrates), 3 absorption (nitrates → plants), 4 decomposition (animals → ammonium, dashed line), and 5 denitrification (nitrates → air).
Order the following stages of the nitrogen cycle into a logical sequence and explain what happens in each one: denitrification, fixation, absorption by plants,…
Order the following stages of the nitrogen cycle into a logical sequence and explain what happens in each one: denitrification, fixation, absorption by plants, decomposition of remains, nitrification.
The milpa: knowledge that uses the cycle
The Indigenous peoples of Mexico planted corn, beans, and squash together for centuries. Without having the vocabulary of microbiology, they took advantage of the nitrogen cycle:
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The bean is a legume: its nodules with bacteria fix nitrogen and leave ammonium and nitrates in the soil.
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The corn needs a lot of nitrogen and absorbs it from the soil that the beans enrich.
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The squash covers the ground with its leaves, retains moisture, and limits weeds.
This reduces the need for industrial fertilizers, which are expensive and can pollute water.
In the Mexican milpa, corn, beans, and squash have been planted together for centuries. Beans, like other legumes, host nitrogen-fixing bacteria in their roots. Explain,…
In the Mexican milpa, corn, beans, and squash have been planted together for centuries. Beans, like other legumes, host nitrogen-fixing bacteria in their roots. Explain, using what you know about the nitrogen cycle, why corn grows better when planted next to beans.
Summary
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The air has a lot of $\text{N}_2$, but plants can't use it directly.
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Fixation: bacteria (free-living or in the nodules of legumes) convert $\text{N}_2$ into ammonium.
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Nitrification: other bacteria transform ammonium into nitrates, which roots absorb.
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Nitrogen passes to animals when they eat; decomposition returns ammonium to the soil.
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Denitrification: bacteria convert nitrates into $\text{N}_2$ and close the cycle.
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In the milpa, the beans fix nitrogen and benefit the corn.
Go back to the beginning: the beans give the corn nitrogen that has already been fixed by the bacteria in their roots.