With every gas-powered car and every traditional wastewater treatment plant, a little nitrogen pollution gets released into the atmosphere. But that doesn’t mean it stays there.
Nitrogen pollution in the atmosphere affects how forests grow. Photo: stpaulgirl (flickr)
“We were curious as to where all this nitrogen was going,” said Alan Talhelm, a researcher at the University of Idaho.
And scientists say the nitrogen that settles from the atmosphere into the soil is changing the ecosystems of sugar maple forests.
“The extra nitrogen we’ve been adding has slowed the decay of sugar maple leaf litter, and that’s caused the forest floor to build up and get thicker,” said Donald Zak, a professor at the University of Michigan.
A thicker forest floor can be a good and bad thing. It can store more carbon from the atmosphere, but can also lead to runoff, biodiversity loss and fewer tree seedlings.
Zak is working with other scientists from Michigan Technological University and the University of Idaho on a century-long study to see how increased nitrogen in soil will affect sugar maple forests. They add extra nitrogen to forests throughout Michigan to see how they will react to the pollution levels expected in 2100.
They found that sugar maple seedlings have a hard time stretching through the bulkier forest floor to reach soil and sunlight.
“In the experimental plots that receive extra nitrogen, there are fewer sugar maples regenerating,” Zak said.
But while young sugar maples fight to take root, older trees grow taller. Nitrogen is usually the first nutrient used up by plants in sugar maple forests, so when there’s more available, the trees grow faster.
That creates a carbon sink. When those trees grow taller and the leaf litter decays slowly, more carbon is stored in the forest and not in the atmosphere, a good thing for slowing global climate change.
And taller, faster growing trees could be good for the forest products industry, Talhelm said.
But despite the carbon storage benefit, which may be temporary if tree seedlings have trouble taking root, extra nitrogen raises other environmental concerns, like decreased biodiversity and runoff into lakes and streams.
“There are plants that are specialized to live in low-nitrogen environments, so when you have a lot of extra nitrogen floating around they can be out-competed,” Talhelm said.
When those low-nitrogen plants lose out, that leads to decreased biodiversity, or variation of life, in the forest.
And when plants don’t use up nitrogen in the soil, it eventually will find its way to surface water, said Dan Button, physical scientist at the U.S. Geological Survey Science Center in Ohio.
When nitrogen runoff mixes with phosphorus runoff, it can lead to algae growth that can create toxic algae blooms, Talhelm said.
Button said when all that algae dies off and decays, oxygen is removed from the water. The oxygen-deficient environment stresses aquatic life and can even lead to fish kills.
“It depends on concentrations,” Button said. “If you have higher concentrations of nitrogen in surface water and all other conditions are right, like temperature and light, you’re going to get algal growth.”