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Nitrogen Fixation in a Gulf Coast Salt Marsh¹

ABSTRACT

The main sites of nitrogen (dinitrogen) fixation were determined in an emergent Spartina alterniflora salt marsh of Barataria Basin, Louisiana. Nitrogenase activity was observed on leaves and stems of the plants, in vegetated marsh soils, in unvegetated stream sediment, and on the soil surface of a mudflat. The vegetated marsh soil, particularly the upper 10 cm, comprised the main site of nitrogen fixation. Seasonal variations of nitrogenase activity in both streamside and inland marsh soils followed the general pattern of plant metabolism. This finding was further supported by the similarity between vertical nitrogen fixation profiles and root distributions. An inverse relationship between nitrogenase activity and extractable ammonium concentrations in marsh soils was found, indicating the role that plant roots may play in assimilating native soil ammonium and creating local nitrogen depletion. Differences in nitrogen fixation rates of streamside and inland soils were attributed to differences in extractable ammonium concentrations, root density, and plant production. A total nitrogen fixation rate of 15 and 4.5 g N m^–2 year^–1 was measured in the streamside and inland marsh, respectively.

¹ Contribution from the Laboratory for Wetland Soils & Sediments, Center for Wetland Resources, Louisiana State Univ., Baton Rouge, LA 70803.

² Research Associate, Boyd Professor, and Research Associate IV, respectively, Laboratory for Wetland Soils & Sediments, Center for Wetland Resources, Louisiana State Univ., Baton Rouge, LA 70803.

Received for publication June 11, 1980. Accepted for publication September 24, 1980.