Forests, Farms, or Houses?

Impact of Land Use on Greenhouse Gases and Nutrient Cycles

residential
The number of new homes in the southern Appalachians has increased dramatically over the past few decades and will likely continue to grow. Photo by Paul Brennan, courtesy of PublicDomainPictures.

Molecules relentlessly cycle from one form to another. “Simple human activities, such as building homes, can affect these cycles,” says U.S. Forest Service research soil scientist Jennifer Knoepp.

For example, trees growing near streams affect the way nitrogen and other nutrients move from the land to the water. “Riparian zones play a critical role in nutrient cycles,” says Knoepp. “When the land next to a stream is forested, nitrogen is much slower to move into streams.”

The loss of streamside forests is one of the major causes of worsening water quality in the U.S.

Over the past few decades, many new homes have been built in the southern Appalachian Mountains, and this growth will probably continue. Many of these new homes are in areas that were once small farms or scattered houses.

“Current predictions are that 75 percent of new development in the southern Appalachians will be suburban,” says Knoepp. “These new homes will represent higher density residential areas than the previous land use.”

By 2030, almost 70 percent of the development could be on land that was once forested. Knoepp and her colleagues studied the expected impact of land use change on nutrient cycles and on greenhouse gas emissions. The study was led by Peter Baas, who at the time was a researcher at the University of Georgia, and published in the journal Biogeochemistry.

Many studies have explored the relationships between agriculture and the amount of nitrogen in streams, but less is known about how building homes could affect nutrient cycles and greenhouse gas emissions.

farm
Agricultural land emits more greenhouse gases than forests or residential land. Photo by Alex Ford, courtesy of Flickr.

“We expected that development would release extra methane, as well as more nitrogen,” says Knoepp. “However, we found that farms released far more greenhouse gases and nitrogen than housing developments did.” The scientists found that if agricultural land was replaced with housing, greenhouse gas emissions would probably decline.

If homes replace forests, the outlook changes. Forests remove methane from the atmosphere, and if the forests are cut down, the soil emits methane. “Building homes where forests used to be will result in a moderate increase in emissions,” says Knoepp. “Forests use and store methane, but both agricultural and residential land use release methane.”

If the projections about land use change are accurate, greenhouse gas emissions from riparian areas will decrease overall. The decrease will occur as people build new homes on old agricultural land.

However, there are potential impacts on nutrient cycles. Between fertilized lawns and septic systems, residential areas usually produce extra nitrogen. Soils cannot store this extra nitrogen, so rain will wash it into creeks and rivers where it can harm water quality.

“Cutting down riparian forests to build homes could threaten stream ecosystems,” says Knoepp. “Preserving riparian forests can help protect water quality.”

Read the full text of the article.

For more information, email Jennifer Knoepp at jknoepp@fs.fed.us.

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