Nitrogen Fixation by Legumes (Graduate Project)

Research Team: Mark Hainds (Graduate Student), Kevin Heirs (Graduate Student), Dr. Lindsay R. Boring (Director), Joe Hendricks (State University of West Georgia), and Dr. Robert J. Mitchell (Scientist)

Collectively, native legumes make up one of the most important groups, or functional guilds, of understory plants that naturally occur in frequently burned longleaf pine forests. From a landowner's perspective, native legumes are extremely important to forest and wildlife management. Most of the legumes that occur naturally are perennials so that they persist for a long time after establishment. They are adapted to the climate and low-fertility soils of the southeastern U.S., and most are tolerant of shade that open-canopy longleaf pine forests provide. Native legumes have an extremely high nutrient content and seed production, which makes them suitable for a wide range of wildlife. Introduced legume species like Lespedeza bicolor (bush clover) and Lespedeza cuneata (Chinese lespedeza) do provide some wildlife value. Sometimes, though, exotic legumes cause problems for areas where they are unwanted. Native legumes provide structure in the forest by growing abundantly and providing wildlife cover in the understory. They provide function by being a very important food source for wildlife, by fixing nitrogen from the atmosphere and replenishing that lost from prescribed fire, and making these nutrients available to plants through rapid decomposition. This last characteristic is extremely important to ecosystem function because of the role that fire plays in maintaining this ecosystem. The Jones Center has undertaken a number of studies that have addressed the importance of legumes from a management and biological perspective. Future work will determine how, when, and where legumes replace nitrogen after a fire, and which legumes might be utilized more actively in forest and wildlife management.

Before research at the Jones Center began little was known about whether native legumes fixed nitrogen, and if so how much they fixed. Several studies have recently been completed. These studies use stable (non-radioactive) isotopes of nitrogen to measure fixation. By enriching the soil with a heavy isotope of nitrogen (¹⁵N) and growing non-fixing plants (like milkweed) with legumes one can estimate the amount of nitrogen that comes from the atmosphere (almost no ¹⁵N occurs in the atmosphere naturally). Non-fixing plants pick up the heavy nitrogen from the soil as will legumes if they are using soil nitrogen. If they are fixing nitrogen from the atmosphere, then the amount of heavy nitrogen is diluted. By calculating the dilution of heavy nitrogen compared to non-fixers the amount of nitrogen derived from the atmosphere can be determined. In initial studies of nitrogen fixation by six legume species at Ichauway, we found that five species (Cassia nictitans, Crotalaria rotundifolia, Centrosema virginianum, Tephrosia virginiana, and Rhynchosia reniformis) fixed 30-90% of the nitrogen found in their leaves and stems. One species, Lespedeza virginica, fixed less than 10%. On sites with higher soil moisture they fixed about 10% more. Other studies with Tephrosia spicata, Lespedeza procumbens, and Desmodium viridiflorum indicate that they also fix significant quantities of nitrogen. Nitrogen fixation in some species was not affected by tree shade or by variation in burn season. Thus, native legumes derive much of their nitrogen from the atmosphere, but species differ in their nitrogen fixing abilities. Future work will determine which species do so most effectively and relate fixation rates to fire and impacts on maintaining forest productivity with burning.

Additional Information:

Hainds, MJ, Mitchell RJ, Palik BJ, Boring LR, Gjerstad DH. Distribution of native legumes (Leguminoseae) in frequently burned longleaf pine (Pinaceae)-wiregrass (Poaceae) ecosystems. Am J Bot 1999;86(11):1606-14.

Hendricks JJ, Boring LR. N2-fixation by native herbaceous legumes in burned pine ecosystems of the southeastern United States. For Ecol Mgmt 1999;113:167-77.

Hiers, JK, Mitchell, RJ, Broing, LR, Hendricks, JJ, Wyatt, R. In Press Legumes native to longleaf pine savannas exhibit capacity for high N₂-fixation rates and negligible impacts due to timing of fire. New Phyt.

Funded by: The Joseph W. Jones Ecological Research Center