Factors Affecting Nitrogen Fixation Rates in Termites (Isoptera: Rhinotermitidae: \u3ci\u3eReticulitermes\u3c/i\u3e) and Termite Nitrogen Contributions of Forest Ecosystems

Termites host symbiotic bacteria that fix atmospheric nitrogen, which makes termites important in the biogeochemical cycling of nitrogen in ecosystems. The objective of this study was to estimate annual nitrogen contributions by termites by examining seasonal rates of nitrogen fixation, seasonal abundance of different termite castes and their nitrogenase activity, the effects of dietary nitrogen on nitrogenase activity, and low oxygen and high carbon dioxide atmospheres on termite nitrogenase activity, and the accuracy of mark-recapture studies to estimate termite population size. Termite nitrogen fixation rates were monitored monthly for 10 Reticulitermes Holmgren colonies from June, 1993, to December, 1995. Rates were highest in fall and spring and lowest in the winter and summer. Workers fixed more nitrogen than other castes and were the most abundant caste. Nitrogenase activity in other nitrogen fixation systems is influenced by dietary nitrogen. The nitrogen content of natural termite food was not correlated with nitrogenase activity in the field, but nitrogenase activity was suppressed in termites on diets enriched with certain types of nitrogen. Nitrogen fixation in other nitrogen fixing systems is oxygen sensitive. Termites foraging within logs and deep in the soil may be exposed to low oxygen and high carbon dioxide conditions. In laboratory experiments, termite nitrogenase activity increased with decreasing oxygen concentration. In contrast, increased carbon dioxide concentrations did not affect termite nitrogenase activity. Although previous studies indicate that termite nitrogenase activity decreases in the laboratory, in this study termites maintained or exceeded their original rates in the laboratory. Initial environmental conditions in the field affected the ability of termites to regain or exceed their original nitrogen fixation rates. Mark-release-recapture experiments were conducted in the laboratory to test the accuracy of the Lincoln index used to estimate termite populations. Population estimates for termite colonies fed 0.1% (w/w) Nile blue were approximately 3 times greater than the actual population size while they were 10 times greater for termites fed 0.05% (w/w). Potential model biases are discussed in reference to mark-release-recapture in termites fed ingestible dye markers. Based on this study and previous termite abundance studies, Reticulitermes is capable of contributing 76.84-384.21 g Nitrogen • ha-1 • yr-1 to temperate forest ecosystems