Experimental investigation of nitrogen and oxygen isotope fractionation in nitrate and nitrite during denitrification

Abstract

In batch experiments, we studied the isotope fractionation of nitrogen and oxygen during denitrification of two bacterial strains (Azoarcus sp. strain DSM 9056 and Pseudomonas pseudoalcaligenes strain F10). Denitrification experiments were conducted with succinate and toluene as electron donor in three waters with a distinct oxygen isotope composition. Nitrate consumption was observed in all batch experiments. Reaction rates for succinate experiments were more than six times higher than those for toluene experiments. Nitrogen and oxygen isotopes became progressively enriched in the remaining nitrate pool in the course of the experiments; the nitrogen and oxygen isotope fractionation varied between 8.6-16.2 and 4.0-7.3%, respectively. Within this range, neither electron donors nor the oxygen isotope composition of the medium affected the isotope fractionation process. The experimental results provide evidence that the oxygen isotope fractionation during nitrate reduction is controlled by a kinetic isotope effect which can be quantified using the Rayleigh model. The isotopic examination of nitrite released upon denitrification revealed that nitrogen isotope fractionation largely follows the fractionation of the nitrate pool. However, the oxygen isotope values of nitrite are clearly influenced by a rapid isotope equilibration with the oxygen of the ambient water. Even though this equilibration may in part be due to storage, it shows that under certain natural conditions (re-oxidation of nitrite) the nitrate pool may also be indirectly affected by an isotope equilibration.

Denitrification and inference of nitrogen sources in the karstic Floridan Aquifer

“…8a, b) support both the inference of denitrification in the UFA and the hypothesis that variation in isotopic signatures is primarily driven by denitrification rather than differential contribution from organic and inorganic sources. Fractionation coefficients ( 15 ε) derived from both the cross-spring analysis and (5.27-7.49) the Ichetucknee River springs time series (7.37) are within the lower range of values reported for other aquifers (Mariotti, 1986;Bohlke et al, 2002;Green et al, 2008), and other marine and freshwater environments (Sigman et al, 2005;Granger et al, 2008), but are lower than some recent experimental values (Knoller et al, 2011). These relatively low values may indicate diffusion-constrained NO in the UFA (Sebilo et al, 2003), but could also be an artefact of mixing between distinct water sources (Green et al, 2010).…”

Identification of nitrogen sources and transformations within karst springs using isotope tracers of nitrogen

“…Work by Sigman et al (2005) and Granger et al (2004) with laboratory cultures of denitrifying bacteria resulted in slopes near 1 and it has been hypothesized that older methods for determining d 15 N-NO 3 and d 18 O-NO , (sealed tube combustions or pyrolysis rather than denitrifying bacteria) may have produced biased d 18 O-NO 3 data ). However, in experiments using two strains of denitrifying bacteria, Knöller et al (2011) produced a mean ratio of 0.5 for multiple laboratory experiments (with a range of 0.33-0.79) and Kendall et al (2007) state that they have obtained slopes of \1 in groundwater where denitrification occurs. Figure 4 provides additional support for denitrification being a significant nitrate sink in portions of the Florida aquifer during dry years.…”

Nitrogen isotope dynamics and fractionation during sedimentary denitrification in Boknis Eck, Baltic Sea

“…Nitrite exchanges oxygen atoms with ambient water, so any reoxidation should be visible in oxygen atoms in nitrite. Different recent studies (Knöller et al, 2011;Wunderlich et al, 2012) closely investigated the potential for nitrite reoxidation, and concur that there is no re-oxidation of nitrite during dissimilatory nitrate reduction within bacterial cells under anoxic conditions.…”