Warming trend boosts nitrogen in Boulder watershed

Trend occurs despite lower rainfall levels, which should lower nitrogen levels, CU-Boulder researchers conclude

By Lara Herrington Watson

Funded by a grant from the National Science Foundation, University of Colorado Boulder alumna Rebecca Barnes conducted research in 2010 that showed inorganic nitrogen levels are increasing in water catchments in Colorado’s Green Lakes Valley, and thawing glacial features are the culprit.

When Barnes began her post-doctoral dissertation at CU-Boulder in conjunction with the United States Geological Survey, she was perplexed to find that nitrogen exports from Green Lakes Valley had not responded to the recent drought, which caused decreases in “atmospheric nitrogen deposition.”

The Green Lakes Valley is a 700-hectare protected area that supplies Boulder’s drinking water. Its lakes are fed in part by glaciers.

The question is, how can you put less nitrogen in and get more out?”Both a nutrient and a pollutant, nitrogen makes up 78 percent of the atmosphere and is oxidized when humans burn fuel. Seventy-five percent of the nitrogen in the rain and snow that falls in the Front Range comes from human activities, like using fertilizers or combusting fuel.

When rainfall deposits nitrogen, plants and microbes consume it, but it doesn’t go away. Because of the regional drought affecting Colorado, less nitrogen has been deposited in recent years.

“Nitrogen per raindrop hasn't changed, and there is less water,” Barnes says, so you’d expect there would be less nitrogen in alpine lakes, snow melt and water catchments.

Instead, Barnes and her collaborators found that in some locations there has been even more nitrogen export than there was before rainfall decreased.

“The question is, how can you put less nitrogen in and get more out?” Barnes says.

Drawing on nearly 40 years of environmental data from the Niwot Ridge Long Term Ecological Research network, Barnes hypothesized that increased nitrogen levels are linked to thawing cryosphere, which includes permafrost and glaciers.



“The key here is long-term ecological research, which shows directional change in things like export in nitrate,” says CU Professor Mark Williams, who collaborated with Barnes and is the principal investigator of LTER. “Nitrogen is hard to study. It’s dynamic.”

To test her hypothesis, Barnes compared the water chemistry from different catchments. The most interesting sites, Green Lake 4 and Martinelli, are close enough that they should have similar nitrogen output. The main difference between them is that Green Lake 4’s drainage contains cryosphere, whereas Martinelli does not.

“We can't take an ecosystem into the lab and control it, so we have to do comparisons like this,” Barnes says.

She found the Martinelli site hasn’t experienced an increase in nitrogen. “So, Martinelli responds the way you'd expect: you put less nitrogen in and get less out,” Barnes says.

The Green Lake 4 site exported 40-percent more nitrate over the last 10 years, compared to previous decades. This, combined with an increase in stream flow in Green Lake 4 in the late summer during the driest years, suggested thawing permafrost and glaciers could be the source of the increased nitrate export.

While the reduced precipitation has caused fewer nitrogen deposits, this has been counteracted by warming trends that thaw soils, providing a new source of nitrogen to microbes.

“We have less pollution in the system, but any benefit that we potentially would have seen has been counteracted, because, due to warming, there’s more nitrogen. For sites experiencing warming, even if we fixed the air pollution problem leading to the nitrogen increase, it might not change the nitrogen balance of the system," Barnes says.

Although her research and the accompanying paper invoke climate change, “until we delved into this, I didn't start by saying I want to see how warming was influencing ecosystems. It points to a feedback from climate change I wasn't expecting,” Barnes says.

While increasing nitrate concentrations cause acidification in surface waters, Williams says the nitrification phenomenon will be short-lived.

“Climate change and permafrost melt have made the nitrogen pollution worse, but there’s a tipping point, because eventually the world will run out of permafrost and glaciers,” Williams says.

“There could be a water-quality and quantity effect further down the road. Mountain ecosystems are water resources for all people. The Rocky Mountains contain the headwaters for the Colorado River, which provides water to places like California,” says Barnes.

For now, the subalpine and montane ecosystem is doing its job.

“As water flows into Boulder Creek, the trees are using nitrogen. There’s a steady decline in nitrogen concentration as you move down the slope,” Barnes says.

The critical management issue her research points to is the future water supply, which is currently being subsidized by melting glaciers, Barnes says.

“When we run out [of melting features] in a few decades, it will be worse than people think,” Williams says. “We have a Congress full of climate deniers, but people in Colorado get it now. They’re very sophisticated when it comes to water.”

He is encouraged by levels of communication between groups in Colorado who control water.

When it comes to managing nitrogen and mitigating pollution, Barnes suggests using less or no fertilizer, which is rich in nitrogen.

Lara Herrington Watson is a CU alumna ('07) and freelance writer who splits her time between Denver and Phoenix. 

June 25, 2014