Dinitrogen (N2) fixation rates in a subtropical seagrass meadow measured with a direct 15N-N2 tracer method

Abstract

This study used a direct 15N stable isotope labelling technique to measure rates of dinitrogen (N2) fixation within above- and belowground loci of a subtropical seagrass meadow (Zostera muelleri). The total rate of N2 fixation (i.e. sum of the above- and belowground rates) was ∼38 μmol N2 m-2 h-1, similar to other rates measured in subtropical systems. Rates of N2 fixation were higher when the 15N-N2 label was added to the surface water compared to when it was added to the sediments. Furthermore, the lowest rates of N2 fixation were observed in the root/ rhizome material regardless of whether the 15N-N2 label was added directly to the rhizosphere (0.12 μmol N2 m-2 h-1) or the overlying water column (7.3 μmol N2 m-2 h-1). These results suggest that there was active transport of fixed N from the leaves to the roots of the seagrass plants, in contrast to other studies in which N2 fixation was more active in the rhizosphere. Our study demonstrates the utility of the direct 15N-N2 tracer approach for quantifying the spatial heterogeneity of N2 fixation in complex seagrass environments.