EFFECT OF PLANT PHOTOSYNTHESIS, CARBON-SOURCES AND AMMONIUM AVAILABILITY ON NITROGEN-FIXATION RATES IN THE RHIZOSPHERE OF ZOSTERA-NOLTII

Rates of nitrogen fixation (measured as acetylene reduction) in the rh izosphere of the seagrass Zostera noltii were highly dependent upon pl ant photosynthetic activity being significantly stimulated at elevated CO2 concentrations and by light, both in the short-term and over diur nal cycles. Stimulation by light became insignificant when 5 mM sucros e was added to the sediment porewater, indicating that in the absence of added carbon sources, light stimulation was due to direct inputs of plant photosynthate to the rhizosphere. Addition of a range of carbon sources to the rhizosphere sediment stimulated rates of acetylene red uction, with this stimulation being significant for sucrose and lactat e. Surprisingly, whilst low additions of ammonium to the sediment pore water (10 to 50 mu M) inhibited 50% of acetylene reduction activity, a pproximately 30% of this activity persisted in the presence of 1 mM am monium chloride; this indicating that in at least a proportion of the N-fixing community, nitrogenase activity was not regulated in the shor t term by the availability of alternative nitrogen sources.