SEASONAL RATES OF NITROGEN FIXATION BY TERMITES IN A TIDAL WETLAND FOREST AT THE VCR

D. Waller & A. Curtis, Department of Biological Sciences, Old Dominion University, Norfolk, VA 23529 USA

Termites are abundant soil arthropods in forests where they degrade woody debris. Their creation of extensive subterranean galleries serves to aerate soils and translocate minerals. In addition, termites may contribute significant amounts of nitrogen (N) to terrestrial ecosystems through the fixation of atmospheric N by their hindgut bacteria. The objective of the present study is to quantify amounts of N fixed by termites in a pine forest at Brownsville, Virginia, by examining seasonal rates of nitrogenase activity and determining termite biomass at the site. Experiments have been ongoing since June 1993. Each month, we measure rates of N fixation in ten field colonies of the subterranean termite Reticulitermes (Rhinotermitidae) by using the acetylene-reduction assay. During the first year, we collected data on the dietary N content and pH values of the termites' wood diet. Every month we measure air temperature and the temperature of the host log. The effects of dietary N, food pH and temperature on termite survivorship, food consumption and N fixation rates are also being investigated in laboratory experiments.

Preliminary studies suggest that termite N fixation rates vary with the concentration and form of combined N in the diet (Curtis & Waller, unpublished). In contrast, food pH appears to have little effect on termites in nature. A survey of wood eaten by termites at the site revealed a range of pH values from 3.0 -5.0. However, in laboratory studies, termite food consumption and N fixation rates were affected by pH values of 2.0, but not by pH values of 3.0 or higher. Temperature appeared to have the greatest impact on termite N fixation rates in both the field and the laboratory. Natural N fixation rates peaked in fall and spring when temperatures were moderate, and dropped during temperature extremes during the winter and summer. In the laboratory, N fixation rates in termites maintained for three weeks in incubators were highest for termites held at 26^oC and lower for those at 22^oC and 32^oC. Results were similar for termites collected in April, after a cold winter, and those collected in August, following a hot summer.

Our initial studies suggest that termite workers fix significantly more N than soldiers, nymphs, larvae and alates. We are therefore collecting data on seasonal caste composition in order to more accurately estimate colony rates of N fixation when we begin to sample termite biomass next year.

Acknowledgments

This work was supported by NSF grant # DEB 9220857 to DAW. We thank the Nature Conservancy for permission to do field work at the VCR Headquarters in Brownsville.