Comparison of dinitrogen fixation rates in two subtropical seagrass communities

Abstract

Seagrass morphology varies greatly between species and can impact their nitrogen (N) cycling capacity, including dinitrogen (N 2 ) fixation. We used a 15 N—N 2 stable isotope technique to measure N 2 fixation rates in different zones of two morphologically diverse seagrass communities, Zostera muelleri and Halophila ovalis in Moreton Bay, Australia. Isotope label additions were made to both the surface and rhizosphere of sediment cores containing whole plants over an artificial diurnal cycle. In both species the highest rates of N 2 fixation were found in the leaves (including epiphytes) in the light with areal leaf related N 2 fixation rates higher in the Z. muelleri (46 ± 26 μmol N 2 m −2 h −1 ) community compared to H. ovalis (11 ± 7.2 μmol N 2 m −2 h −1 ). There was a switch in the location of N 2 fixation from the leaves to the sediments in the dark in the Z. muelleri community which was not observed in H. ovalis. The change in the active site of N 2 fixation in the Z. muelleri community is likely related to the reduction in oxygen supply and associated increase in sulphate reducing bacterial activity at night. Overall the Z. muelleri community fixed N 2 at ~3 times (~75 μmol N m −2 h −1 ) the rate of H. ovalis (~25 μmol N m −2 h −1 ). The results of this study illustrate how plant morphology, and its associated influence on sediment biogeochemistry, can regulate N 2 fixation rates in seagrass communities.