SEASONAL-VARIATIONS IN NITROGEN-FIXATION (ACETYLENE-REDUCTION) AND SULFATE-REDUCTION RATES IN THE RHIZOSPHERE OF ZOSTERA-NOLTII - NITROGEN-FIXATION BY SULFATE-REDUCING BACTERIA

Nitrogen-fixation (acetylene reduction) rates were measured over an an nual cycle in meadows of the seagrass Zostera noltii Hornem in the Bas sin d'Arcachon, south-west France, between March 1994 and February 199 5, using both slurry and whole-core techniques. Measured rates using t he slurry technique consistently overestimated those determined on who le cores, probably due to the release of labile organic carbon sources as a result of root damage during preparation of the slurries, Thus, the whole-core technique may provide a more accurate estimate of in si tu activity, since disturbance of physicochemical gradients of oxygen, sulphide, nutrients and the relationship between the plant roots and the rhizosphere microflora is minimised. Rates measured by the whole-c ore method were 1.8- to 4-fold greater (dependent upon season) in the light than those measured during dark incubations, indicating that org anic carbon diffusing from the plant roots during photosynthesis was a n important factor in regulating nitrogen fixation in the rhizosphere. Additions of sodium molybdate, a specific inhibitor of sulphate-reduc ing bacteria (SRB) inhibited acetylene-reduction activity by > 80% as measured by both the slurry and whole-core techniques throughout the y ear, inferring that SRB were the dominant component of the nitrogen-fi xing microflora. A mutualistic relationship between Z. noltii and nitr ogen-fixing SRB in the rhizosphere, based on the exchange of organic c arbon and fixed nitrogen is proposed. Acetylene- and sulphate-reductio n rates showed distinct summer peaks which correlated with a reduced a vailability of ammonium in the sediment and the annual growth cycle of Z. noltii in the basin. Overall, these data indicate that acetylene r eduction (nitrogen fixation) activity in the rhizosphere of Z. noltii was regulated both by the availability of organic carbon from the plan t roots and maintenance of a low NH4+ concentration in the vicinity of the plant roots due to efficient assimilation of NH4+ by Z. noltii du ring the growth season. Nitrogen-fixation rates determined from acetyl ene-reduction rates measured using the whole-core technique ranged fro m 0.1 to 7.3 mg N m(-2) d(-1), depending on season, and were calculate d to contribute between 0.4 and 1.1 g N m(-2) yr(-1), or 6.3 to 12% of the annual fixed nitrogen requirement of Z. noltii.