Tracking the Nitrogen Cascade

This isn't quite pure ecology (we'll get there, I promise), but I thought I'd post it today.

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Global warming is in the news. Small increases in the amount of carbon dioxide in the atmosphere are changing Earth’s climate, with unpredictable results. By now, almost everybody has heard of this human impact on the carbon cycle. Yet impacts to the nitrogen cycle are even more dramatic. We have doubled the amount of biologically available nitrogen annually cycling through the environment, with complex and sometimes subtle consequences.

It has been said that the first law of human ecology is, “we can never do merely one thing”. This is the major challenge in thinking and writing about impacts to the nitrogen cycle. Effects form a complex cascade that has to be considered as a whole. In addition, there may be effects we are not even aware of yet.


Consider a molecule of nitrogen, N₂. The two atoms making it up are held together by a strong and stable triple bond, making the nitrogen inaccessible to the vast majority of living things. Yet this element is the fourth most common one in living things, necessary to make the amino acids that make the proteins that make most structures inside you, your goldfish and the fly on the wall.

The reason all three of you are alive is that a few kinds of bacteria called nitrogen fixers have mastered the trick of fixing inert atmospheric nitrogen, turning it into forms they and everyone else in the biosphere can use. Nitrogen fixers live both in the ocean and on land. They commonly associate with certain plants, forming root nodules on clover, beans, acacia trees and other members of the legume family. Using special enzymes to speed up a process that would otherwise be unbelievably slow, they combine it with oxygen or hydrogen to form nitrate or ammonia. These compounds find their way into plants and from plants go to animals, fungi and bacteria.

Eventually, they reach the denitrifiers. These bacteria live in oxygen-poor waters and sediments, where they use dissolved nitrate ions to extract energy from food. They give off nitrogen gas as a waste product, lowering the amount of biologically available nitrogen in the biosphere.

Following this cycle one nitrogen atom at a time, we may travel from the nitrogen-fixing nodules on an alder’s roots to a moose, a wolf, and denitrification by bacteria in a marsh. Another journey might take us from a pea plant to a human, bacteria in a sewage treatment plant, algae, an insect larva, more bacteria and, as always, eventual denitrification. Until less than a century ago, this would have summed up the basics of the nitrogen cycle.

At that time, farmers, who often need extra nitrogen for their crops, applied animal or human manure to their fields or planted nitrogen-fixing crops. People also got some nitrogen, for both industry and agriculture, from Chilean deposits of guano (aka bird poop). At the turn of the 20th century, Chile led the world in commercial nitrogen production. This, of course, gave Chile – and whoever could control access to Chile – much political and economic power.

Nitrogen compounds are used not only for agriculture but also to make explosives and gunpowder. So when German chemist Fritz Haber discovered how to make ammonia from nitrogen in the air, he made possible both modern agriculture and modern warfare. Working with engineer Carl Bosch, Haber developed mass production of ammonia. Without the Haber-Bosch process, Germany would have run out of food and munitions by 1916, with who knows what consequences for the modern world.

Even more important, though, is the Haber-Bosch process’s impact on our food supply. It has been estimated that 40% of the people alive today owe their lives to artificial fertilizer. Humans are now responsible for more nitrogen fixation than all natural processes combined. The environmental consequences have been correspondingly enormous.


Let us now take a step back from nitrogen and think about how elements move through the biosphere. All living things take in some substances and excrete others. From these small flows, cycles emerge. For an example, think about food. Eat a slice of bread and as your body combines it with oxygen to release energy, you exhale CO₂. A wheat plant can take that CO₂ and use it, with water and sunlight, to make seeds that then become flour for your bread. The wheat also releases oxygen that you inhale and then release as water and CO₂.

Scale up these processes over a large area or even the whole Earth. You will now see biogeochemical cycles in which water, air, life and even the slow movements of geology transport the chemicals of life throughout the world. Without knowing all the details of every process involved, we can get an idea of how much of what substance flows from where to where each year. How much carbon from the soil to the atmosphere? How much phosphorus from land to ocean? How much nitrogen from the atmosphere to nitrogen-fixing bacteria?

Globally, these numbers remain roughly constant year to year, at least on time scales meaningful to us. But the amount of biologically available nitrogen cycling through Earth’s systems has doubled in about a hundred years. Think of putting twice as much electricity as usual through the wiring in your house. You don’t know what will be damaged by the power surge, but you know something will.


What if we could follow a single nitrogen atom fixed in the Haber-Bosch process? Part of a nitrate ion in a bag of fertilizer, it is bought by a corn farmer in Iowa and applied to his field. An atom of nitrogen stands only a 14% chance of becoming part of a corn plant and helping feed someone.

Our atom is not among the 14%. Instead, it sinks into the groundwater, where it has plenty of company. A well supplies water to the farmhouse, but its water now contains high levels of nitrates. If the farmer’s baby drank this water, bacteria in her stomach would convert the nitrates to nitrites, which can bind to hemoglobin and cause a potentially serious condition called methemoglobinemia or blue baby syndrome. Luckily, the community knows about the threat and residents give infants bottled water.

Some water from the well goes on the family’s vegetable garden and our atom comes along for the ride. The garden receives its own fertilizer, much of which gets washed into a pond after a summer rain. Algal growth goes haywire from this bounty. In a process called eutrophication, the algae grow, die and decay, robbing the water of oxygen and killing fish.

A heavier rain comes and the farm pond overflows. The nitrate ion with our atom of nitrogen flows from stream to stream, eventually reaching the Mississippi River. Here, more and more nitrogen is added as you go downstream. Wetlands along the river’s edge that could absorb some of this excess were destroyed long ago. Eventually, the water with its load of fertilizer reaches the Gulf of Mexico.

Here, eutrophication takes place on a huge scale, creating a dead zone thousands of square miles in area. Fish and other marine organisms die or are driven out, with devastating impacts on both biological and, potentially, human communities. While fishermen have, so far, largely managed to work around the dead zone, in the end, people cannot fish where there are no fish! In some places, however, the problem is even worse because algae there produce toxins that accumulate in shellfish and can sicken or kill people who eat the shellfish.

Finally, the nitrate ion is denitrified and becomes a nitrogen atom that spends several years floating around in the atmosphere as part of a nonreactive N₂ molecule. Eventually, though, it reaches Beijing and gets sucked into a businessman’s car engine, where high temperatures combine it with oxygen to form a gas called nitric oxide, NO. Even more NO forms from the nitrogen present in gasoline, as does nitrous oxide, NO₂. If nitrous oxide reaches the upper atmosphere, it contributes to ozone depletion and acts as a greenhouse gas 200 times more potent than CO₂.

NO comes out through the car’s exhaust pipe and becomes part of the city’s infamous smog. It also contributes to the formation of ground-level ozone, a respiratory irritant. Sitting in traffic, the businessman coughs.

A little later, wind blows the NO a few miles out of the city, when rain begins to fall. The molecules containing our atom and a few million of his closest pals dissolve in rainwater and change into nitric acid. When this acid rain falls on the ground, it injures plants. When lakes become acidic, fish and amphibians die.

The root of a plant absorbs our nitrogen atom. This is another, more subtle, effect of nitrogen deposition. In many ecosystems, nitrogen abundance limits plant growth. Plants must compete for this critical nutrient and the competition helps maintain diversity. When more nitrogen is added, it becomes easier for a few species to take over. If that happens, the ecosystem can become less able to resist shocks like droughts.


What can we do to get the nitrogen cascade described earlier under control? There are three major types of strategies: reducing the amount of biologically available nitrogen we create, keeping it where we want it and increasing denitrification.

Reducing fossil fuel use is an important component of the first strategy. In addition, technologies that greatly reduce nitrogen oxide emissions are available right now. Their universal use would make human contribution to the atmospheric concentration of these chemicals quite minor. Preventing emissions is generally better (and, in the long run, cheaper) than cleaning them up at the end of a pipe.

Keeping nitrogen where we want it means working with farmers to reduce fertilizer use and improve its efficiency. Farmers certainly don’t benefit from wasting money on fertilizers that aren’t even taken up by plants! However, they sometimes use more than the recommended amount as a sort of gamble. Most years, water availability limits how much benefit a fertilized crop can derive from nitrogen. Some years, however, are wetter than usual and the crop could grow more with extra nitrogen. Farmers don’t want to miss out on that kind of chance, so they apply more fertilizer than recommended by the USDA.

One proposed solution is a kind of insurance policy. Under a program providing such insurance, farmers would apply the USDA recommended amount of fertilizer to most of their land but fertilize some small patches as much as they want. If there was a substantial difference in yields, the insurance company would compensate the farmer. This is being tried on a small scale in the US.

Improving the efficiency of irrigation can help prevent fertilizer runoff. So can more ambitious plans, such as a switch to year-round cropping to keep soil nutrients in place. Individuals can reduce or eliminate fertilizer use on their own lawns and gardens.

The final strategy, encouraging denitrification, typically boils down to encouraging wetlands. Protection of existing wetlands is important, as is restoration, particularly along streams and rivers. Constructing wetlands on farmland is another promising approach because it puts denitrification close to the nitrogen runoff source.


The nitrogen cascade is a beautifully complex problem that makes global warming seem straightforward. Solving it requires us to think on multiple scales and in terms of whole systems. In the end, we can’t double the amount of an important nutrient flowing through the biosphere each year without expecting consequences.

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Sources

Carlisle, Elizabeth. 2000. “The Gulf of Mexico Dead Zone and Red Tides”.

Charles, Dan. 2002. “The Tragedy of Fritz Haber”.

Fields, Scott. 2004. “Global Nitrogen: Cycling Out of Control”. Environmental Health Perspectives 112(10). (Slightly technical, but excellent.)

Fisher, David E. and Marshall Jon Fisher. 2001. “The Nitrogen Bomb”. Discover. 22(4)

Galloway, James N., et al. 2003. “The Nitrogen Cascade”. BioScience. 53:341-356. (Comprehensive but technical.)

Pafko, Wayne. "Nitrogen: Food or Flames". "The History of Chemical Engineering".

Raloff, Janet. 2004. “Dead Waters”. Science News. 165:360

Raloff, Janet. 2004. “Limiting Dead Zones”. Science News. 165:378. (This two-part series on dead zones is aimed at interested non-scientists.)

Science Museum of Minnesota. “The Gulf of Mexico Dead Zone”. (Mostly for kids, but works for anyone.)

Vitousek, Peter M., et al. 1997. “Human Alteration of the Global Nitrogen Cycle: Causes and Consequences”. Issues in Ecology 1. (Excellent introduction. Somewhat technical, but aimed at general public.)