The longleaf pine forests that once dominated the southeastern coastal plain are now considered one of the most endangered ecosystems in North America. Once covering 90 million acres from Virginia to Texas, longleaf is found today in only about three percent of its historic range. Many of those remaining acres are on U.S. military installations, where land managers are tasked with keeping the forests healthy while meeting their primary objective of supporting military readiness.
To help the Department of Defense accomplish these dual goals, University of Georgia ecosystem ecologist Nina Wurzburger is leading a study exploring how the soil-based process of nitrogen fixation facilitates recovery from physical disturbances, including military training exercises and fire. The five-year project is supported by a $1.39 million grant from the Strategic Environmental Research and Development Program of the U.S. Department of Defense in partnership with the U.S. Department of Energy and the U.S. Environmental Protection Agency.
According to Wurzburger, understanding the natural processes of recovery in longleaf ecosystems is important for many reasons. One of those is conserving biodiversity: longleaf forests harbor a number of endangered and rare species of animals and plants.
Longleaf forests also store a significant amount of carbon. “Longleaf pine forests serve as a strong carbon sink for the region,” Wurzburger said. “And nitrogen fixation provides a continual supply of new nitrogen to the forests, keeping them highly productive.”
Nitrogen fixation—the process by which soil microorganisms take nitrogen from the atmosphere and convert it into usable forms for plants—enhances the growth of the pines, allowing them to keep that much more carbon dioxide out of the atmosphere.
The study is also important because longleaf forests demand management. The longleaf ecosystem depends on periodic fire; without it, fire-intolerant species soon take over.
“These forests require a particular burning frequency,” said Wurzburger. “But there’s a delicate balance between burning enough to help longleaf grow and burning too much and depleting the soils of nitrogen.”
Fire, though necessary, removes nitrogen from soils and places it back in the atmosphere. Fortunately, longleaf forests contain plenty of nitrogen fixers that can replenish these losses. These include leguminous plants like indigo, partridge pea, and prairie clover that associate with nitrogen-fixing bacteria in their roots; soil crusts—thin layers of lichen, algae, mosses, and bacteria that cover the open ground; and certain free-living bacteria in the soil. Training conducted at military installations can impact these nitrogen fixers, however.
“There hasn’t been any systematic examination of nitrogen fixation, what controls it, and how fire and other disturbances influence it,” said Wurzburger. “Longleaf pine forests are ideal systems for understanding these processes.”
Wurzburger and her colleagues will establish a series of 2.5 acre plots at Fort Benning, Georgia, and Eglin Air Force Base, Florida, representing different combinations of fire frequencies and levels of military training impact. They will then measure the rates of nitrogen fixation, with particular attention to how the types of nitrogen fixers respond to the disturbance regimes, and determine whether nitrogen inputs are sufficient to replenish losses from disturbance.
They will also conduct experiments to determine how the combination of different soil nutrients affects nitrogen fixation.
“We know from previous work in the tropics that other nutrients—phosphorus and molybdenum—play a role in regulating nitrogen fixation,” said Wurzburger. Understanding how disturbance influences the interaction of all these nutrients will fill an important knowledge gap for ecologists and land managers alike.
“What we learn will help land managers understand soil ecology, particularly nitrogen fixation, as a natural mechanism for recovery in longleaf ecosystems,” said Wurzburger. “It will help guide better management practices for conservation of these last remaining longleaf stands. So the benefit of this project is really threefold: to protect endangered ecosystems, help maintain the carbon balance in the region, and allow these forests to play a role in military training.”
Wurzburger’s fellow investigators on the project include Robert Mitchell, senior scientist at the Joseph W. Jones Ecological Research Center, and Lars Hedin, professor of terrestrial biogeochemistry at Princeton University.