Nitrogen pollution on Niwot Ridge, Colorado: The environment is slowly changing around us.

Niwot Ridge (University of Colorado photo)

Niwot Ridge (University of Colorado photo)

for Research Experiences for Undergraduates, University of Colorado at Boulder
July 1997

The haze is thick in Denver on this broiling day in June. But west of Boulder, atop 11,500-foot Niwot Ridge, it is cool and bright beneath an achingly blue sky. Gray peaks surround the ridge, which is carpeted with tiny alpine plants. Wildflower season has scattered all shades of purple, yellow, pink and blue like confetti among new leaves. The dirty remnants of the snowpack, still quite deep in places, keep a chill on the wind. The silence of the tundra is startling. No car horns, no barking dogs. Just a distilled quiet where all is still.

Or so it seems. Beneath the tundra, life is surprisingly busy. Plants are growing faster than ever recorded, and microbes are thriving from an overabundance of nutrients falling from the sky.

This may seem like good news, but it's not. Carried on the wind and in the snow, nitrogen in air pollution has deceived tundra life into a false sense of prosperity. The nitrogen, which comes from cars, power plants, and agriculture, has been accumulating in recent years at a rate four times that first measured in the late 1970s. If scientists studying the phenomenon are right, it can't keep doing so much longer. On Niwot Ridge, in nearby Rocky Mountain National Park, and in other high-altitude ecosystems in Colorado, nature is nearing the threshold of its ability to take up all the extra nitrogen it is receiving. If that threshold is passed, nitrogen will leach nutrients from the soil, threaten fragile tundra plants, and poison rivers and streams.

Tundra plants are growing fast, but the Front Range is growing faster. More cars than ever are driving Colorado's roads, and that number is only expected to increase. This means that nature is moving ever quicker to its nitrogen limit and there's no cure in sight. The buildup of pollution will accelerate as Colorado continues to grow.

But nitrogen studies are so new to environmental science that researchers are just beginning to understand the pollution's impact on the ecosystem of the Niwot Ridge. Tim Seastedt at the University of Colorado's Institute of Arctic and Alpine Research (INSTAAR) is working with colleagues to answer some of the broad and complex questions nitrogen pollution raises. While pollution's long-term consequences are unknown, researchers are certain that too much nitrogen upsets the natural balance, both at Niwot Ridge and around the world. "It's causing wholesale changes," Seastedt says.

Before man came along, nitrogen acted like a speed limit in nature. An ingredient of enzymes and amino acids, it was only available to plants when lightning liberated it from the atmosphere or when microbes altered it in the soil. There was never too much of the stuff, and this low supply kept growth in check. Plants adapted to nitrogen's scarcity by recycling it, or by forming partnerships with microbes that got it for them.

But since the industrial age, humans have profoundly altered the nitrogen cycle by burning fossil fuels and applying nitrogen fertilizers to agricultural lands to stimulate crop growth.

Nearly a third of the land on Earth, and half of Colorado, is devoted to farms and agriculture. More than ever, farmers rely on commercial fertilizers to boost productivity. But if crops can't use all the fertilizer they're given, the surplus runs into the groundwater or evaporates into the atmosphere. Livestock also contribute to the escape of excess nitrogen. There's a whole lot of manure lying around Colorado, and as it dries, nitrogen in the form of ammonium (NH4) rises into the atmosphere. The large amounts of agricultural ammonium produced on farms out on the plains tend to stay near its source.

But in the area around Niwot Ridge, where Seastedt conducts his research, the overwhelming majority of nitrogen pollution comes from cars to the east, and from power plants to the west. The nitrates (NOx) from motor vehicles and smoke stacks travel long distances, entering areas uphill and upstream from their sources.

Nitrogen returns to earth in snow, wind, or rain, and its impact has been classified by researchers into a process with four stages. A stage 0 ecosystem is in balance: all nitrogen received is recycled, and none leaks into surface water. Stage 1 areas leak nitrogen into surface water during the non-growing season. Nitrogen is present in surface water all year round in a stage 2 ecosystem, and in a stage 3 area, more nitrogen flows out than in.

Seastedt points out that all high-elevation ecosystems have always been at stage 1 because their harsh environment demands that they shut down in winter, thereby allowing any nitrogen that falls to pass on to the water table. Human influence, however, has boosted Niwot Ridge's status closer to stage 2.

The difference may seem a subtle one, but geologist Mark Williams of INSTAAR notes that in the early 1980s, nitrate levels at Niwot Ridge were 30 times greater than in the pre-industrial era. Now, in the 1990s, that already elevated amount has doubled. Where will we be in another ten years? Or another hundred?

Back on the tundra, Seastedt shakes bags of nitrogen pellets onto numbered plots. During his five-year project, he will overfertilize local plants to simulate total nitrogen saturation. The project is one year old, and after just one application of fertilizer, applied last summer, the change is remarkable. Grasses burst from the meter square plots. Everything within their borders is distinctly greener than the unfertilized areas surrounding them. The effect is that of a badly sewn quilt thrown across the tundra, with Seastedt's green squares haphazardly strewn among the last of the snowpack.

Seastedt expects that at the end of the experiment, four years from now, natural selection will favor species that can tolerate high nitrogen levels. Grass will dominate the plots, and the most fragile species will have disappeared. If his image is a preview of the distant future, a fully saturated tundra would be rife with grasses and non-native weeds. Downslope, nitrogen running off into streams would cause algae to flourish, reducing the amount of oxygen available to fish. And the high nitrogen levels would encourage aluminum to move out of the soil (where it is harmless) and into the water (where it is deadly), killing fish and their food source, plankton.

Too much nitrogen can also drag calcium and magnesium out of the soil. Essentially nutrients to plant life, calcium and magnesium accumulate slowly and cannot be easily replaced. According to Gregory Lawrence (USGS Forest Hydrologist in Albany), less than 50 years' worth of auto emissions had leached more than 500 years' accumulation of calcium from some forest soils. So plants are pulled in two directions at once, causing stress. As the excess nitrogen fools them into growing faster, they have fewer other nutrients to sustain that growth. It's like drinking coffee on an empty stomach: you may feel more energetic, but you'll fall on your face when you stop.

Niwot Ridge is just one place among countless others where these effects are threatening ecosystems or are already causing harm. The world is awash in nitrogen. Industry, cars and modern agriculture are spreading over the planet and as they do, nitrogen pollution is worsening. According to biologist Peter Vitousek of Stanford University, "more commercial fertilizer was used between 1980 and 1990 than all that previous was applied in history." Fertilizer has already saturated much of the Earth's farmland.

In the United States, the Finger Lakes region of upstate New York has suffered from nitrogen pollution wafting in from the Midwest for decades. Byproducts from steel mills and other industries have drenched the area in NOx-laden acid rain, creating dangerous levels of released aluminum in the water. the Finger Lakes are so saturated with nitrates and sulfuric acid that the fish have died out completely according to Seastedt. The lakes, once some of the most popular tourist destinations of the Northeast, are sterile.

So if predictions of a nitrogen-saturated Colorado were to come true decades or centuries from now, how would the average resident be affected? Colorado doesn't suffer to the degree of the Finger Lakes, damage can still be done. In the most severe cases, human health is jeopardized when nitrogen poisons the drinking water. In some parts of the world, including the Finger Lakes, nitrate levels are so high in groundwater that the medical condition methemoglobinemia, or "blue babies," threatens infants. Concentrated in milk, nitrates oxidize the iron in red blood cells, causing infants to slowly suffocate.

Nitrates also contribute to Denver's infamous "brown cloud," which degrades visibility and, some believe, aggravates medical conditions like asthma. But although the brown cloud is worse than in the past, and some of the Front Range's groundwater is already nitrogen-polluted, the levels aren't nearly high enough to cause serious health problems, at least not yet Seastedt says.

Lastly, if Colorado's forests, tundra, and waterways become nitrogen-saturated, the nitrogen-sensitive plankton that fish depend on will die, causing serious harm to Colorado's recreational fishing industry. If plankton populations decrease, so would those of the cutthroat and rainbow trout.

Lastly, if Colorado's forests, tundra, and waterways become nitrogen-saturated, the nitrogen-sensitive plankton that fish depend on will die, causing serious harm to Colorado's recreational fishing industry. If plankton populations decrease, so would those of the cutthroat and rainbow trout.

Solving the nitrogen problems will be a formidable task. Back when power plants were discovered as the source of acid rain, their smokestacks were fitted with filters that reduced the problem. But reducing nitrogen pollution is much more complex. There are many more cars than power plants in the world, and correcting the emissions from every one would be difficult and expensive.

Some scientists say the problem can be solved by altering the catalytic converter. Installed in automobiles to cut carbon monoxide emissions, the device inadvertently created a nitrate problem. Ideally, a car's computer tells the converter to take in equal amounts of carbon monoxide and nitrates. The converter then spits it out as carbon dioxide and harmless nitrogen gas in its natural state (N2). But few converters work this smoothly, and nitrates still escape.

Vic Cooper of Colorado's Emissions Technical Center says that the catalytic converter cannot do its job well if the engine is not running the way it should. "The catalytic converter only does its magic of stripping the oxygen from the nitrogen if (the engine is) perfectly tuned," he says. So solving the problem may be as simple as making mechanics aware of how important proper tuning is to controlling pollution.

As of last year, the federal government tightened its standards on engine emissions and required that all automotive computers detect poorly functioning catalytic converters. It's too soon to tell if these measures will be effective. If they're not, an alternative solution may lie in futuristic engines that burn hydrogen, says the Technical Center. The byproduct of the hydrogen engine is water, almost too good to be true, but the technology is only in the earliest stages of development.

The prognosis is brighter on the agricultural front. Seastedt considers agriculture easier to fix than automobiles, because wasted nitrogen is wasted money to farmers. Slow-release fertilizers and improved sewage system which convert ammonium (agricultural nitrogen) to atmospheric nitrogen gas have been developed. Knowing the exact nitrogen needs of a crop helps by avoiding overfertilization, so the use of soil testing systems is on the rise. And livestock owners can now use reduced-protein feed, which results in less manure-borne methane and ammonium.

Though agriculture contributes a much smaller percentage of nitrogen pollution to Niwot Ridge than do automobile emissions, both are major contributors on Colorado's plains, where agriculture causes nitrogen to build up in soil and in rivers. For decades, excess fertilizer and livestock waste ran unchecked into groundwater. But with tighter regulations and increased awareness on the part of the agricultural community, that may be changing.

Some farmers, such as Dick Mercer, who runs Double M Farms in Kearney, Nebraska, have had success in reducing wasted nitrogen. Mercer manages 3000 acres of irrigated corn, and at any given time he oversees 3000 head of cattle. In 1996, his advanced composting techniques won him a National Cattleman's Beef Association's Environmental Stewardship Award.

He works with the city of Kearney to turn city sewage and his own farm manure into field fertilizer, and says it's just as effective as commercial forms. "Nitrogen is nitrogen," he says. And natural fertilizer goes one step better: it returns organic matter to the soil, up to 800 pounds per ton of fertilizer, whereas commercial fertilizer is composed only of nitrogen products, robbing soil of natural nutrients and upsetting the pH balance. In addition, Mercer monitors the nitrogen needs of his soil, so that less fertilizer is wasted. Nitrate levels in his watershed have steadily decreased over the years. "The pendulum has swung the other way," he says.

But agriculture as a whole has a long way to go, according to Mercer. Nebraska's Central Platte watershed has had nitrate levels above federal limits for at least 40 years, and Mercer doesn't expect the problem to be solved anytime soon. The expense and effort required to manage farmland wisely means that not everyone will be able to run their farms as he has. But Mercer feels the time will come when farming no longer harms its environment. "We've all become much more knowledgeable that the environment is fragile," he says. "I think most of us understand that we have an obligation."

Seastedt of INSTAAR feels that the government should address nitrogen pollution for economic reasons, if nothing else. Should nitrogen cause Colorado's weeds to flourish and fish to die sometime in the next decades, the government will find it much costlier to save what's left. Endangered-species preservation programs are much more expensive than maintaining stable biodiversity in the first place, he says. "It's the old ounce of prevention versus the pound of cure."

INSTAAR researcher Williams considers Niwot Ridge an early-warning system for nitrogen saturation in this country. Unlike more urban areas downslope, Niwot Ridge is not saturated yet, so monitoring the area is a good way to determine how much closer the ecosystem is to saturation and its consequences: acidified soil, poisoned waterways, and species driven to extinction. Pollution has "thrown a biological switch," he says, the consequences of which we can only begin to guess.

The good news about nitrogen pollution is that it spends less time in the soil than toxic wastes like radioactive sludge. And if pollution can be stopped, nature has the ability to reverse the effects of nitrogen saturation. "Most soils can restore themselves," Seastedt says, though it could take a millennia in the worst cases. Fortunately, stage 2 areas like Niwot Ridge can still turn back before damage is done. But until nitrogen pollution is reduced, the high tundra will remain threatened by the demands of a growing population.