title: Nitrogen fixation in riverine wetland plant communities
Creators: name: Allan, Caroline Elizabeth
Publisher: name: University of Glasgow
Year: 1996
Subjects: subject: Biogeochemistry, ecology.

description: The occurrence of biological nitrogen fixation was examined in wetland plant communities of three riverine wetland catchments of differing geography, climate and land use in the British Isles. Two of the three were also targeted by an EC project 'Functional Analysis of European Wetland Ecosystems'. Within the three catchment areas, a total of 17 sites were surveyed. Repeated visits were made to the sites during the period 1993-1995. Species abundance (frequency), legume density and cover and a suite of environmental parameters were measured. Indirect measurements of legume and soil (free-living) nitrogen fixation (acetylene reduction assays) were conducted at three sites in Ireland and Scotland. Further detailed studies using both acetylene reduction and 15N methodology were focused on one site at Ring Bog, Loch Lomond, Scotland. By using TWINSPAN to classify sites by their species composition and successfully comparing these groups to NVC and CORINE categories, it was confirmed that several different wetland plant communities had been surveyed. Nine species of legumes were found at the riverine wetlands investigated in this study. Canonical correspondence analysis revealed that the most influential of the measured environmental factors on species distribution were flooding regime, soil wetness class, soil pH and land management. Legume species were ranked along these environmental gradients. Legume cover was negatively correlated with the flooding regime gradient. Decreases in both growth and nodulation in Trifolium pratense and Trifolium dubium in 1994 were attributed to early winter (1993) and late spring (1994) floods. Mowing for hay in 1993 however, was not detrimental to subsequent growth and nodulation in Lathyrus pratensis and Vicia cracca when compared to 1994, when mowing took place after assessments were completed. All the species of legumes examined for nitrogenase activity reduced acetylene to ethylene, illustrating their potential to fix nitrogen. Although no accurate value of nitrogen fixation was assigned to the Irish species (L. pratensis, V. cracca, T. dubium and T. pratense) all had the potential to fix nitrogen and therefore to contribute to the nitrogen economy of the riverine wetlands in which they were found. The cyclic nature of growth and nodulation in Lotus pedunculatus was revealed (1993-1995). Die back and decay occurred in the winter months (thus providing input of biologically fixed nitrogen into Ring Bog), with regrowth the following spring. Root and nodule growth occurred mainly in the above-ground vegetation and litter layer in L. pedunculatus at Ring Bog, a phenomenon not previously recorded. L. pedunculatus at Ring Bog also reduced acetylene to ethylene and more detailed investigations revealed its ability to incorporate 15N2. The rate of nitrogen fixation for L. pedunculatus was estimated conservatively at 11.2 kg N ha-1 yr-1. An isotope dilution experiment conducted in the greenhouse on material collected from the Field gave the percent contribution of atmospheric nitrogen to the tops of L. pedunculatus as 73-76%. Natural abundance of in15N L. pedunculatus and non-legume species of similar growth from in Ring Bog illustrated the unsuitability of using the two source model of Shearer &amp; Kohl (1986) in an ecosystem far removed from the homogenous agricultural situation. In the more detailed study at Ring Bog, the acetylene reduction assay showed the nitrogen fixing potential of surface peat soil and bryophytes. This was reinforced by direct measurements of nitrogen fixation by incorporation of 15N2. Nitrogen fixation by the peat soil was attributed to free-living bacteria and the annual rate of nitrogen fixation in the surface peat soil was estimated conservatively to be 55.8 kg N ha-1. The fixation occurring in the bryophytes may be attributed to an association between the mosses (Sphagnum palustre, Sphagnum recurvum and Polytrichum commune) and cyanobacteria and/or heterotrophic bacteria. (Abstract shortened by ProQuest.).