Consequences of Denitrification

Two stable isotopes of nitrogen exist, (99.634%) and (0.366%). Because of this mass difference the heavier isotope reacts at a sHghtly slower rate, producing a kinetic isotope fractionation in many biological reactions. The increase in the in the reaction products is typically expressed as the parts per 

The isotopic fractionation is easily seen in 8 N03 and 8 N2 distributions in the major open-ocean denitrification zones (Altabet et al., 1999 Brandes et al., 1998 Cline and Kaplan, 1975). Typical open ocean values ofsub-euphotic zone nitrate are about 5%o (Lehmann et al., 2005 Sigman et al., 2000 Wu et al., 1997) but within the ODZ they climb to upwards of 15%o. Concomitant with this increase is a decrease in the 8 N2 from about 0.6%o to 0.2%o (Fig. 6.15). The large enrichment of N-N03 and the mirror image decrease in N-N2 is undoubtedly due to fractionation during denitrification. It is also possible to derive a fraction factor, , from the isotope distributions in the ODZ if one makes some assumption about the amount of nitrate that has been removed by denitrification, i.e., the nitrate deficit. For the eastern tropical North Pacific Brandes et al. (1998) assumed a Raleigh fraction mechanism and both open (advection-reaction) and closed (diffusion-reaction) systems to derive fractionation factors from the data, in Fig. 6.15. (Raleigh fractionation 8 N03 = where 8 N03 is the isotopic composition 

Sigman et al. (2003), and Voss et al. (2001) have similarly used observed isotopic values and nitrate deficits to estimate from ODZs (Fig. 6.16) and all of these estimates cluster around an of 25%o. 

The small isotopic effect expressed during sedimentary denitrification could be due to diffusion limitation as proposed by Brandes and Devol (1997), but it could also result from a balance between the isotopic effects of denitrification being offset by an input of low N03 from sedimentary nitrification. However in an 

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In the case of nitrogen, some is transferred to the atmosphere as a consequence of denitrification, which takes place in soils, sewage treatment plants, and natural waters. Some nitrogen is also volatilized as ammonia from animal feedlots. 

This review wiU cover various pathways of denitrification, important oceanic sites of denitrification, isotopic consequences of denitrification, and the role of... 

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