The nitrogen cycle

Last time

Before we start: last time, in The carbon cycle and the nitrogen cycle, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Surrounded, yet short of it

The air is seventy-eight percent nitrogen, yet plants can starve for lack of it. Plants cannot use nitrogen gas. They need nitrate ions from the soil. Today: how nitrogen gets from the air into plants and animals, and back to the air again.

Why nitrogen matters

Organisms need nitrogen to make amino acids, which join to form proteins, and also for DNA. Nitrogen gas makes up about seventy-eight percent of the air, but most organisms cannot use it directly. Plants take in nitrate ions from the soil through their roots.

Nitrogen fixation

Nitrogen fixation turns nitrogen gas into compounds that plants can use, such as nitrates. Lightning does this in the atmosphere, and nitrogen-fixing bacteria do it in soil and in root nodules of legumes. Legumes, such as peas and beans, have a partnership with these bacteria, so they grow even in poor soil.

Through the food chain

Plants use nitrate ions to make amino acids, and then proteins. Animals get their nitrogen by eating plants or other animals, and digesting the proteins. Excess amino acids are broken down by deamination in the liver, forming urea, which is excreted.

Back to the soil

Decomposers break down dead organisms and waste, releasing ammonium ions. This is decomposition. Nitrifying bacteria then change ammonium into nitrite and then nitrate. This is nitrification. These nitrates can be absorbed by plant roots again, and the cycle continues.

Denitrification

In waterlogged soil there is little oxygen. Denitrifying bacteria change nitrate back into nitrogen gas. This removes useful nitrate from the soil, so waterlogged fields grow poorly. The gas returns to the atmosphere, completing the cycle.

Four bacteria jobs

Fixing bacteria: nitrogen gas to nitrogen compounds. Nitrifying bacteria: ammonium to nitrate. Decomposers: dead remains to ammonium. Denitrifying bacteria: nitrate to nitrogen gas. Say the starting substance and the finishing substance, and you will not mix them up.

Fertilisers

When crops are harvested, the nitrogen in them is removed from the field, so the soil loses nitrate each year. Farmers replace it with fertilisers containing nitrate or ammonium, or with manure, or by growing legumes. Too much fertiliser can wash into rivers, which causes pollution. That is the topic of a later lesson.

Lack of nitrogen

A plant short of nitrate cannot make enough amino acids, so it makes less protein. Its growth is stunted, and the older leaves turn yellow.