Chemical Clues to a Forest’s Past in Nitrogen Isotope Ratios

In Appalachian hardwood forests, the pattern of nitrogen isotopes in the soil can indicate past disturbances. Photo by MrSparkle17, courtesy of Wikimedia Commons.
In Appalachian hardwood forests, the pattern of nitrogen isotopes in the soil can indicate past disturbances. Photo by MrSparkle17, courtesy of Wikimedia Commons.

From the depths of the soil to the top of the atmosphere, nitrogen is everywhere. It is also indispensable to plants and animals. The vast majority of nitrogen atoms contain the same number of uncharged particles. However, a few atoms are ‘stable’ isotopes that have one extra uncharged particle. Although the extra particle adds a miniscule amount of weight – much less than one trillionth of an ounce – living things prefer compounds that contain the lighter nitrogen atom to the heavier isotope.

“Measuring the ratio of heavy and light nitrogen in the trees and soils tells us about the nitrogen cycle in different ecosystems,” says U.S. Forest Service soil scientist Jennifer Knoepp, soil scientist at the Forest Service Southern Research Station (SRS) and lead author of a study on how forest disturbances affect nitrogen isotopes recently published in the Soil Science Society of America Journal.

Knoepp and her colleagues studied the amount of heavy and light nitrogen in longleaf pine-wiregrass forests on the Joseph W. Jones Ecological Research Center in Georgia, and in the Appalachian hardwood forests at the SRS Coweeta Hydrologic Laboratory in North Carolina where Knoepp is stationed. The six study plots in the Appalachians had been clearcut 25 years before sampling, while control plots had been unmanaged since a timber harvest in 1927. In the longleaf pine stand, prescribed fires had been conducted every 1 to 3 years for decades. Adjacent unburned areas were used as control plots.

“The ratio of heavy to light nitrogen reflects atmospheric inputs as well as the complex series of processes that transform organic and inorganic forms of nitrogen to compounds that are available for plants,” says Knoepp. Disturbances, including timber harvesting and prescribed fire, affect the amount of nitrogen available to plants.

In Appalachian forests, small trees, branches, and stumps are often too small to sell during a timber harvest and are left in the forests to decompose. As they decompose, they create large pulses of organic matter. Longleaf pine forests require prescribed fire, which tends to consume organic matter. However, both management regimes increase soil nitrogen availability, and also promote the growth of nitrogen-fixing plants that can convert the nitrogen gas in the air to a form the plant can use.

In both sites, scientists sampled roots and soils from several depths, as well as tree increment cores and foliage. In the longleaf pine stand, soils and vegetation showed no changes after treatments. However, in the Appalachian hardwood stand, the isotope values in clearcut soils from 8 to 24 inches below the surface were lower than those in the reference forest. “Our study suggests that in Appalachian hardwood forests, the pattern of nitrogen isotopes in the soil indicates past disturbance,” says Knoepp.

The scientists found that in Appalachian forests, the amount of nitrogen isotopes stored in wood and leaves depends on the plant species. Additionally, plant species that can convert atmospheric nitrogen into forms other plants can use become more common after disturbances. However, the data suggest that these plants play little role in soil or plant isotope content.

Read the full text of the article.

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

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