MEPS 605:87-101 (2018)  -  DOI: https://doi.org/10.3354/meps12755

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.

KEY WORDS: Nitrogen fixation · Stable isotope tracer · Zostera muelleri