Nitrogen Fixation Activity (Acetylene Reduction) in the Rhizosphere of Salt Marsh Angiosperms, Georgia, U.S.A.

Abstract

The potential importance of biological N fixation to the productivity of salt marsh macrophytes was investigated in a Georgia estuary. Total N content, soil density, and N fixation activity as measured by C2H2 reduction were measured in the rhizosphere of macrophytes [Juncus roemerianus, Spartina alterniflora (short, tail forms), Salicornia virginica, Borrichia frutescens, Sporobolus virginicus, Spartina patens, Distichlis spicata]. Monthly ethylene production measurements showed maximum activity in May-June followed by a secondary activity peak in early autumn. At most sites the activity declined significantly in July. The highest mean monthly activity of 40-80 .mu.moles C2H4 m-2 h-1 occurred in the tall Spartina alterniflora along the creek bank at a soil depth of 20-30 cm. The rates of ethylene production were not related to the N content of the soil. Total N content in various soils was quite different but changed very little over the year. Significant correlations were found between ethylene production and macro-organic matter in the soil, soil density and net aerial primary production in each habitat, suggesting the importance of primary production and root exudation as a source or organic Ca for N fixation or, conversely, N fixation as a source of combined N for primary production. N fixation in Duplin River Estuary supplies an estimated 14.8 g Nm-2 annually, equivalent to the amount of N required by major salt marsh macrophytes.