Development and N₂‐Fixing Activity of the Benthic Microbial Community in Transplanted Spartina alterniflora Marshes in North Carolina

Growth and maturation of transplanted salt marshes is often limited by the availability of nitrogen (N). We examined the role of N 2 ‐fixing benthic microbial assemblages (microalgae and associated bacteria) in two restored marshes (1‐year‐old and 6‐year‐old marsh) and a natural salt marsh in the Newport River Estuary, North Carolina. Benthic N 2 fixation (nitrogenase activity, NA), chlorophyll a (Chl a ) concentration, Spartina alterniflora (smooth cordgrass) stem counts, and sediment organic matter content were determined in the three marshes. Significant differences were observed between sites for both Chl a and NA. The 1‐year‐old marsh always exhibited the highest levels of NA and Chl a . Sediment organic matter content was lowest in the 1‐year‐old marsh (∼2%), intermediate in the 6‐year‐old marsh (∼5%), and highest in the natural marsh (∼10%). Carbon and nitrogen analyses were also performed on the 1‐year‐old marsh sediments, which were depleted in N. A positive correlation was observed between surface sediment N and Chl a . Remineralized, microbially derived N may provide growth‐limiting inorganic N to Spartina transplants. N 2 ‐fixing microbial assemblages in the 1‐year‐old marsh may also be an important food source for marsh infauna. Benthic N 2 ‐fixing microbial assemblages play a key role in the N economy of restored salt marshes.