The nitrogen cycle of an East Coast, U.K. saltmarsh: II. Nitrogen fixation, nitrification, denitrification, tidal exchange

Abstract

Measurements of nitrogen fixation (acetylene reduction) showed greatest rates in the saltmarsh pans with a benthic layer of cyanobacteria present. The smallest amount of nitrogen fixation occurred on the marsh surface where a Puccinellia maritima/Halimione portulacoides plant association shaded the underlying sediment. Phototrophic nitrogen fixation was always greater than dark, chemotrophic, bacterial fixation. Only a small proportion of the total amount of ammonium, which was formed during detrital breakdown, was nitrified to nitrate. Although there is a high capacity for bacterial nitrate reduction in these sediments, the process is limited by low nitrate availability and most nitrate upon reduction is converted to ammonium rather than being denitrified to gaseous products. Denitrification does not, therefore, result in any great loss of nitrogen from the saltmarsh. There was little net import or export of nitrogen on an annual basis, although nitrate and organic-N in small particulate material was removed from tidal water by the marsh, and there was net annual export of ammonium, dissolved organic-N and organic-N in large particulate material. Losses of nitrogen by the small net tidal export and by denitrification were approximately balanced by nitrogen fixation. It was concluded that the nitrogen cycle of the Colne Point saltmarsh was balanced on an annual basis, with most nitrogen being recycled within the marsh. The saltmarsh did not apparently act as a net source of nitrogen for the adjacent estuary, although it may act as an important processor of nitrogen, removing some forms of nitrogen such as nitrate from tidal water while exporting other forms of nitrogen such as dissolved organic-N.