Diel Rates of N 2-fixation and Denitrification in a Transplanted Spartina alternifloraMarsh: Implications for N-flux Dynamics

Abstract

Magnitudes and diel patterns of N 2-fixation (acetylene reduction, AR) and potential denitrification (acetylene block technique, DNF) rates were determined in a transplanted Spartina alternifloramarsh. Rates of AR in transplanted marsh surface sediments exceeded rates in an adjacent natural marsh by 5- to 10-fold. In late May, surface sediments from two sites in the transplanted marsh were inhabited by two distinct cyanobacterial communities, each exhibiting a unique diel pattern of AR. Sediments dominated by filamentous, non-heterocystous cyanobacterial species exhibited a nighttime peak in AR, supplying an estimated N input of 50 mg N m −2 day −1. Sediments dominated by heterocystous and coccoid cyanobacterial species exhibited a daytime peak in AR and an estimated N input of 145 mg N m −2 day −1. In August, surface sediments from five sites in the transplanted marsh were dominated by non-heterocystous cyanobacteria and diatoms. At this time, all sites exhibited a similar diel pattern in AR rates, with a midday minimum and dawn or dusk maxima. The estimated average daily N input was 37 mg N m −2 day −1. There was an inverse relationship between hourly AR and CO 2fixation (estimated by 14CO 2uptake) rates, suggesting an inhibitory effect of oxygenic photosynthesis on AR. Oxygen also appeared to inhibit DNF; in May, low DNF rates were noted during the day with higher rates occurring at night. Overall, DNF rates were three orders of magnitude less than N 2-fixation rates; thus, the balance between these processes results in a significant input of N to the transplanted salt marsh. We hypothesize that the reduced vascular plant canopy and coarser sediment characteristic of transplanted marshes may enhance the development and production of benthic microalgal communities dominated by cyanobacteria. Microbially-mediated N-cycling in young, transplanted salt marshes may differ significantly from values recorded from mature, natural salt marshes.