DIRECT MEASUREMENT OF NITROGEN GAS FLUXES FROM CONTINENTAL-SHELF SEDIMENTS

It has been suggested that denitrification in continental shelf and slope sediments is the most important sink in the marine nitrogen cycle 1-4. This conclusion has been reached, not from direct measurements of denitrification in these areas, but rather from indirect estimates derived from pore-water models of diagenetic processes. In highly bioturbated continental shelf and slope sediments with steep pore-water gradients, such indirect estimates may not be applicable 5,6. I have now made direct, in situ measurements of denitrification in sediments of the eastern North Pacific continental margin by determining the flux of molecular nitrogen out of the sediments into the overlying water. Denitrification rates in continental shelf sediments measured in this fashion averaged 3.7 pmol N cm-2 s-1. The flux of nitrate from the overlying water into the sediments was only 1.5 pmol N cm-2 s-1, showing that most of the nitrogen gas production is coupled to nitrification within the sediments. The denitrification rates observed here are four to five times those estimated previously by indirect methods for these same sediments, and indicate the limitations of such indirect estimates. My results suggest that the global denitrification rate in shelf and slope sediments may be greater than previously thought, and confirm the importance of sedimentary denitrification in the marine nitrogen budget.