Carbon and Nitrogen Cycling in Intertidal Sediments near Doel, Scheldt Estuary

Carbon and nitrogen cycling in intertidal mud flat sediments in the Scheldt Estuary was studied using measurements of carbon dioxide, methane and nitrous oxide emission rates and pore- water profiles of Sigma CO2, ammonium and nitrate. A comparison between chamber measured carbon dioxide fluxes and those based on Sigma CO2 pore-water gradients using Fick's First law indicates that apparent diffusion coefficients are 2 to 28 times higher than bulk sediment diffusion coefficients based on molecular diffusion. Seasonal changes in gaseous carbon fluxes or Sigma CO2 pore water concentrations cannot be used directly, or in a simple way, to determine seasonal rates of mineralization, because of marked seasonal changes in pore- water storage and exchange parameters. The annual amount of carbon delivered to the sediment is 42 mol m(-2), of which about 42% becomes buried, the remaining being emitted as methane (7%) or carbon dioxide (50%). Each year about 2.6 mol N m(-2) of particulate nitrogen reaches the sediment; 1.1 mol m(- 2) is buried and 1.6 mol m(-2) is mineralized to ammonium. Only 0.42 mol m(-2) yr(-1) of the ammonium produced escapes from the sediments, the remaining being first nitrified (1.2 mol m(-2) yr(-1)) and then denitrified (1.7 mol m(-2) yr(-1)). Simple calculations indicate that intertidal sediments may account for about 14% and 30% of the total estuarine retention of nitrogen and carbon, respectively. [KEYWORDS: Carbon cycling; nitrogen cycling; estuarine sediments; methane; nitrous oxide; mineralization; carbon dioxide Coastal marine basin; organic-carbon; disturbed sediments; water interface; kinetic-models; diffusion; methane; sulfur; bay; diagenesis