OCv 



c c 

 On 



u 'E 



D O 

 ■° ~ 



0) o 

 DC E 



ea 



6 8 



Nitrate, vM 



10 



12 



-3 h -l 



Figure 4. --Rate of NOo reduction (nmol NO3 cm"- 3 h _i ) vs. concentration of 



NO3 (yM) in sediment experiment. 



rates measured in the presence 

 source of NO3 for denitrificat 

 as opposed to diffusion of NO3 

 in the pore waters were much g 

 concentrations we measured in 

 high rates of nitrification, 

 trations were shown to decreas 

 pore waters when nitrification 

 cation rates were indicated to 

 NO3 concentrations in the pore 

 nitrification must also be rap 



of acetylene and the NO3 reduction rates. The 

 ion must come from nitrification in the_sediments 



from the overlying water, since the NO3 concentrations 

 reater than in the overlying water. The NO3 

 the pore waters must be maintained by relatively 

 Two lines of evidence support this: (1) NO3 concen- 

 e rapidly below concentrations measured in the 

 was inhibited by acetylene, and (2) denitrifi- 

 be high in the sediments and thus, to maintain 

 waters with active denitri fication occurring, 

 id. 



The rates of decrease of NO3 we observed in lagoon sediments were much 

 greater than the rates of N2O production. One would expect them to be equal if 

 all of the NO3 decrease was due to denitri fication (see Taylor, 1983, for a 

 review of the problems associated with these measurements). We can presently 

 only speculate as to why the discrepancies occurred (figs. 3A and 3B). For 

 example, acetylene blockage may not be complete or may not yield stoichiometric 

 quantities of N2O. Alternatively, NO3 could be reduced assimi lati vely, e.g., 

 by foraminiferans (Webb and Wiebe, 1978). 



We emphasize that the results presented here are preliminary for several 

 reasons. First, we did only a limited number of experimental measurements and 

 feel that a more detailed study is needed. Second, the techniques we used to 

 measure denitrif ication are adequate for a preliminary survey in the field like 

 this one but should be supplemented by the preferred, direct measurement of 

 denitrification (Seitzinger, et al . , 1980). We consider the use of sediment 



205 



