The effect of 15N to 14N ratio on nitrification, denitrification and dissimilatory nitrate reduction

School of Civil Engineering, The University of Sydney, 2006, Sydney, NSW, Australia.
Rapid Communications in Mass Spectrometry (Impact Factor: 2.25). 02/2012; 26(4):430-42. DOI: 10.1002/rcm.6119
Source: PubMed


Earlier experiments demonstrated that isotopic effects during nitrification, denitrification and dissimilatory nitrate reduction can be affected by high (15) N contents. These findings call into question whether the reaction parameters (rate constants and Michaelis-Menten concentrations) are function of δ(15) N values, and if these can also lead to significant effects on the bulk reaction rate.
Five experiments at initial δ(15) N-NO(3) (-) values ranging from 0‰ to 1700‰ were carried out in a recent study using elemental analyser, gas chromatography, and mass spectrometry techniques coupled at various levels. These data were combined here with kinetic equations of isotopologue speciation and fractionation. Our approach specifically addressed the combinatorial nature of reactions involving labeled atoms and explicitly described substrate competition and time-dependent isotopic effects.
With the method presented here, we determined with relatively high accuracy that the reaction rate constants increased linearly up to 270% and the Michaelis-Menten concentrations decreased linearly by about 30% over the tested δ(15) N-NO(3) (-) values. Because the parameters were found to depend on the (15) N enrichment level, we could determine that increasing δ(15) N-NO(3) (-) values caused a decrease in bulk nitrification, denitrification and dissimilatory nitrate reduction rates by 50% to 60%.
We addressed a method that allowed us to quantify the effect of substrate isotopic enrichment on the reaction kinetics. Our results enable us to reject the assumption of constant reaction parameters. The implications of δ-dependent (variable) reaction parameters extend beyond the study-case analysed here to all instances in which high and variable isotopic enrichments occur.

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Available from: Federico Maggi, Jun 23, 2014
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    • "For example, isotopic effects of biochemically reactive compounds transported in soil samples may stem from physical, chemical, and biological processes, all of which contribute an isotopic effect on the source and product compounds; unlike typical laboratory analyses on pure cultures, environmental samples may show non-stationary isotopic effects because of competitive or synergistic processes that affect the kinetics of targeted compounds or microorganisms (e.g. Menyailo and Hungate 2006), because nutrient concentration or isotopic composition are temporally and spatially variable (Mathieu et al. 2007; Tang and Maggi 2012), or because temperature and other environmental factors may affect the overall bio-mineral soil complex (Riley et al. 2014; Tang and Riley 2014). Even in the simplest case, understanding the impacts of these processes is often complicated by a lack of knowledge of boundary and initial conditions. "
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    • "This process occurs through bacteria, physical processes that produce high temperatures such as lightning and fire, and also human activities including the use of fertilizers and the production of energy (Fogel and Cifuentes 1993) (Figure 3). The δ 15 N of soil can vary significantly on the basis of the processes occurring in the soil such as mineralization, nitrification, volatization, and nitrate reduction or denitrification, each incurring an isotopic fractionation (Mariotti and others 1981; Mariotti and others 1982; Choi and others 2006; Tang and Maggi 2012). The extent of these processes depends on factors such as soil depth, vegetation type, and climate, and therefore, the 15 N natural isotope abundance ranges vary (Persson and Wiren 1995; Malchair and others 2010). "
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