Nitrification fractionation effect

This method of establishing retention time is valid only if isotopic fractionation effects during nitrogen transformations are unimportant. For example, nitrification results in a 15N enrichment of the residual NH4 + pool... 

The main processes involved in the biologieal utilization of nitrogen are all associated with kinetic fractionation effects, nevertheless they exhibit different fractionation factors. The isotope effect of nitrogen fixation (a = 1.000 to 1.004) is small relative to the effects of bacterial nitrification, denitrification, or anammox (a = 1.02 to 1.04) (see Montoya et al. 1994 Table 1). The results by Miyake and Wada (1971) indicate that little overall isotope fractionation occurs in the... 

Note Because TKN hydrolyzes to produce NH4, [5nh ] may also represent [TKN]) is Xj for nitrification [SqJ is the concentration of dissolved oxygen is the half-velocity constant for oxygen CpH and is a fractional correction factor due to the effect of pH. 

Better rate measurements Although the currently available methods for direct assessment of nitrification rates and the geochemical constraints provide internally consistent estimates of the overall rates and distributions of nitrification, improvements in direct rate methods are sorely needed to investigate the effects of environmental variables on rates under realistic conditions. These improvements will likely come with the application of even greater sensitivity isotope tracer methods, which require less substrate perturbation for detection of signal in the product pool. In addition, stable isotope methods that do not require tracer addition may possess the sensitivity to detect small fractionation... 

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