Nonsymbiotic and Symbiotic Nitrogen Fixation in a Weakly Minerotrophic Peatland

The acetylene reduction assay was used to measure nonsymbiotic and symbiotic nitrogen fixation in a weakly minerotrophic peatland throughout the ice-free season. Nonsymbiotic nitrogen fixation was found in surface materials and subsurface peat. In surface materials, nitrogenase activity measured in the field contributed about 0.6 kg N ha-' yr-', was closely associated with Sphagnum, but was not correlated with temperature between 12 and 27 C. No cyanobacteria were found in association with Sphagnum. In subsurface peat, nitrogenase activity measured in situ contributed no more than 0.4 kg N ha-' yr-' and was closely correlated with temperature between 7 and 21 C. There were uncertainites in these measurements due to presence of ethylene oxidizing activity and a long time lag. Symbiotic nitrogen fixation was found only in actinomycete-induced root nodules of Myrica gale L. Legumes were absent and the few lichens present lacked nitrogenase activity. Based on acetylene reduction assays, Myrica gale fixed about 35 kg N ha-' yr-'. Nitrogenase activity in Myrica gale showed a strong seasonal pattern which varied little during three consecutive years even though water levels varied substantially. Nitrogen input to the peatland from nonsymbiotic nitrogen fixation was only 15% the amount contributed by bulk precipitation. Symbiotic fixation, in contrast, contributed approximately six times the amount in bulk precipitation. RELATIVELY LITTLE IS KNOWN about annual inputs of nitrogen into peatlands (mires) via biological dinitrogen fixation. Related studies concerning flooded soils, especially rice paddies, have been more numerous and have recently been reviewed (Buresh, Casselman and Patrick, 1980). In peatlands nitrogenase activity occurs in heterotrophic bacteria, free-living cyanobacteria (blue green algae), quasi-symbiotic cyanobacteria associated with Sphagnum, lichens, actinorhizal (actinomycete-nodulated) plants, and legumes. Nitrogenase activity due to heterotrophic bacteria is present in a wide variety of peatlands in Europe and North America (Granhall and Selander, 1973; Blasco and Jordan, 1976; Waughman and Bellamy, 1980). Nitrogenase activity due to freeliving and quasi-symbiotic cyanobacteria, as well as lichens containing cyanobacteria, has also been found in Europe and North America (Granhall and Selander, 1973; Alexander and Schell, 1973; Blasco and Jordan, 1976; and others). Actinorhizal plants, namely Myrica gale and Alnus spp., also grow in both Euro1 Received for publication 13 September 1982; accepted 7 January 1983. I thank Lynn D. Disney and Susan A. Lancelle for technical assistance and John D. Tjepkema for helpful suggestions. This work was supported by NSF Grant No. DEB81-06952 and a Charles Bullard Fellowship. 2 Present address: Department of Botany and Plant Pathology, University of Maine, Orono, ME 04469. pean and North American peatlands, where they can have substantial nitrogenase activity (Akkermans, 1971; Akkermans and van Dijk, 1976; Sprent, Scott and Perry, 1978; Schwintzer, 1979). In addition a legume, Lathyrus palustris, occurs in some strongly minerotrophic peatlands in North America (Curtis, 1959). In the present study, I examined all components of a weakly minerotrophic peatland containing vigorous stands of Myrica gale for nitrogenase activity to determine the relative amounts of nigrogen fixed by Myrica gale and other agents of biological nitrogen fixation. Seasonal variations in nitrogenase activity were also examined. MATERIALS AND METHODSStudy areaThe study site is located in Tom Swamp near Harvard Pond (42?30'N, 72?12'W, elevation approximately 252 m) in the Harvard University Forest at Petersham, Massachusetts. Most observations were made in a 0.7-ha area dominated by Myrica gale and designated "Open Mat" in an earlier study (Schwintzer, 1979). This site is located in an extensive open peatland (about 400 m X 400 m) on a partially floating mat at the northern end of the lake. Measurements of subsurface nitrogenase activity in the peat were made in adjacent parts of the open peatland lacking Myrica gale. The vegetation of the Open Mat site is dominated by Myrica gale (mean cover 53%) and