Nitrogen for Life

The air around you is about 78 percent nitrogen. Yet corn farmers in Virginia usually add nitrogen to their fields, while soybeans next door, with healthy root bacteria, can often get by with little or none.

Dig up a healthy soybean plant and shake off the soil. You will usually find small round bumps on its roots. What are those bumps? And if nitrogen is all around us, why do plants run short of it?

Explore: grow a soybean field and a cornfield

In this simulation you run one growing season on two fields, soybeans and corn. You can turn root bacteria on or off, choose how much nitrogen fertilizer to add, and set how much it rains. Bars show how much the plants grow, how much usable nitrogen is in the soil, and how much nitrogen washes into a nearby stream.

Try this.

1. Turn the root bacteria on and add no fertilizer. Predict which crop will grow better. Then run the season.
2. Predict what will happen to the corn if you add a little fertilizer, then a lot. Test both and watch the soil bar.
3. Add a lot of fertilizer and turn the rain up. Predict where the extra nitrogen goes. Then check the stream bar.

Explain: why living things need nitrogen, and why air is not enough

Every living thing is built partly from nitrogen. A protein is a large molecule that builds cells and does much of their work. Proteins contain nitrogen. DNA, the material that carries a living thing’s instructions, contains nitrogen too. So a plant cannot grow new leaves without it, and you cannot grow muscle without it.

Here is the puzzle. The air is about 78 percent nitrogen, but it is nitrogen gas. Most living things cannot use nitrogen in that form. Corn can stand in nitrogen-rich air all summer and still run short.

Nitrogen gas must first be changed into other forms that plants can use. Three things can change it:

• Bacteria are tiny living things, each made of one cell. Some kinds that live free in the soil change nitrogen gas into a usable form.
• A root nodule is a small bump on the root of a plant such as a bean, pea, soybean or peanut. Bacteria live inside it and change nitrogen gas into a usable form for the plant.
• Lightning, and factories that make fertilizer, also change nitrogen gas. Fertilizer is material that farmers add to soil to supply nutrients such as nitrogen.

The soybean and its bacteria are partners. The plant gives the bacteria sugar it made by photosynthesis. The bacteria give the plant usable nitrogen. Corn has no root nodules, so it depends on usable nitrogen already in the soil.

Explain: the nitrogen cycle

The nitrogen cycle is the movement of nitrogen from the air, through the soil, into living things and back to the air. At each stage, the nitrogen changes form. No new nitrogen is made, and none is used up.

1. Bacteria, lightning and factories change nitrogen gas from the air into usable forms.
2. Plant roots take up usable nitrogen from the soil.
3. Animals get nitrogen by eating plants or other animals.
4. A decomposer is a living thing, such as a fungus or a bacterium, that breaks down dead things and wastes. Decomposers return the nitrogen to the soil in a usable form.
5. Other bacteria in the soil change some usable nitrogen back into nitrogen gas, which goes back into the air.

This cycle is tied to the others. Water moves usable nitrogen through the soil and carries it into streams. Plants use sugar from photosynthesis to build proteins with nitrogen. You met the carbon and water cycles in The Carbon and Water Cycles.

Explain: too little or too much nitrogen

The amount of usable nitrogen changes what can live in a place. In many ecosystems, there is not enough of it, so plants grow less. Less plant growth means less food for animals.

Too much nitrogen causes problems too. Rain can wash extra nitrogen from farms, lawns and other places into streams that flow to the Chesapeake Bay. There it can feed an algal bloom, a fast growth of algae in the water. When the algae die, decomposers break them down and use up oxygen that fish and crabs need. You will see more in Big Changes (a later lesson).

Extra nutrients can also change which plants grow. In an experiment in 40 grasslands on six continents, adding nutrients, including nitrogen, made plants grow more. But fewer kinds of plants grew, mostly because the taller plants shaded the smaller ones. Biodiversity is the variety of living things in an area. So the amount of nitrogen affects biodiversity.

Elaborate: do root bumps add nitrogen?

Scientists in Michigan grew two kinds of soybeans side by side. The plants were nearly the same, except that one kind cannot form root nodules. No fertilizer was added. Late each summer they measured the dried plant parts above the ground.

Source: Snapp, Wilke, Gentry and Zoellner, Agronomy Journal, 2017, field trial at the Kellogg Biological Station, Hickory Corners, Michigan, 2006 to 2008. Each value is a mean over several research plots with different management histories. A hectare is 10,000 square meters, about the size of one and a half soccer fields.

1. In each year, which soybean type held more nitrogen?
2. In 2007, how many more kilograms of nitrogen per hectare did the plants with nodules hold?
3. Find the mean nitrogen for plants with nodules over the three years. Do the same for plants with no nodules.

Design problem

Do root bacteria help pea plants grow in soil with little usable nitrogen? Plan an investigation.

Evaluate: check your thinking

Corn cannot use the nitrogen gas in the air, so it needs usable nitrogen in the soil, and farmers often add it. The bumps on soybean roots are nodules. Bacteria inside them change nitrogen gas into a usable form, so soybeans often need little or no added nitrogen.

Science practices you used: asking a testable question, planning a fair test with a control and repeated trials, reading a data table and finding a mean, and making a claim with evidence and reasoning.