Nitrification isotopic effect

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... 

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... 

Figure 3 Schematic diagram of the processes and pools central to the internal cycling of N in the ocean. The isotope effects shown here are based on laboratory studies. Dashed arrows represent assimilation of dissolved species into particulate matter, and solid arrows represent remineralization. Complete consumption of the ammonium pool by assimilation in the surface ocean or by nitrification in the ocean interior causes the relatively high isotope effects associated with these processes to have little effect on N isotope dynamics. However, in regions where ammonium assimilation and nitrification co-occur, their isotope effects will impact the <5 N of their respective products, PN and nitrate. In nitrification, ammonia (NHs), rather than the protonated form ammonium (NH4 ), is oxidized. However, ammonium is the dominant species in seawater, and there is isotope discrimination in the ammonium-ammonia interconversion. Thus, the isotope effects for ammonia oxidation given here and elsewhere in the text refer specifically to consumption of ammonium. The processes surrounding DON production and utilization are not well understood from an isotopic perspective but are thought to play an important role in N cycling.

Figure 5 The effect of different marine N cycle processes on nitrate <5 N and concentration, assuming an initial nitrate <5 N of 5%o. The trajectories are for reasonable estimates of the isotope effects, and they depend on the initial nitrate <5 N as well as the relative amplitude of the changes in nitrate concentration (30% for each process in this figure). A solid arrow denotes a process that adds or removes fixed N from the ocean, while a dashed line denotes a component of the internal cycling of oceanic fixed N. The effects of these two types of processes can be distinguished in many cases by their effect on the concentration ratio of nitrate to phosphate in seawater. The actual impact of the different processes on the N isotopes varies with environment. For instance, if phytoplankton completely consume the available nitrate in a given environment, the isotope effect of nitrate uptake plays no major role in the <5 N of the various N pools and fluxes the effect of nitrate generation by organic matter degradation and nitrification, not shown here, will depend on this dynamic. Similarly, the lack of a large isotope effect for sedimentary denitrification is due to the fact that nitrate consumption by this process can approach completion within sedimentary pore waters.

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... 

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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