Opening Opportunities for High-Resolution Isotope Analysis - Quantification of δ15NNO3 and δ18ONO3 in Diffusive Equilibrium in Thin-Film Passive Samplers.

Sophie A Comer-Warner,Daren C Gooddy,Sarah K Wexler,Stefan Krause,Jan Kaiser,Sarah A Bennett

doi:10.1021/acs.analchem.7b00028

Sophie A Comer-Warner, Daren C Gooddy + Show 4 more

Open Access

https://doi.org/10.1021/acs.analchem.7b00028

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Abstract

The fate of nitrate transported across groundwater-surface water interfaces has been intensively studied in recent decades. The interfaces between aquifers and rivers or lakes have been identified as biogeochemical hotspots with steep redox gradients. However, a detailed understanding of the spatial heterogeneity and potential temporal variability of these hotspots, and the consequences for nitrogen processing, is still hindered by a paucity of adequate measurement techniques. A novel methodology is presented here, using Diffusive Equilibrium in Thin-film (DET) gels as high-spatial-resolution passive-samplers of δ15NNO3 and δ18ONO3 to investigate nitrogen cycling. Fractionation of δ15NNO3 and δ18ONO3 during diffusion of nitrate through the DET gel was determined using varying equilibrium times and nitrate concentrations. This demonstrated that nitrate isotopes of δ15NNO3 and δ18ONO3 do not fractionate when sampled with a DET gel. δ15NNO3 values from the DET gels ranged between 2.3 ± 0.2 and 2.7 ± 0.3‰ for a NO3- stock solution value of 2.7 ± 0.4‰, and δ18ONO3 values ranged between 18.3 ± 1.0 and 21.5 ± 0.8‰ for a NO3- stock solution of 19.7 ± 0.9‰. Nitrate recovery and isotope values were independent of equilibrium time and nitrate concentration. Additionally, an in situ study showed that nitrate concentration and isotopes provide unique, high-resolution data that enable improved understanding of nitrogen cycling in freshwater sediments.

Highlights

Diffusive Equilibrium in Thin-film (DET) gels as high-spatialresolution passive-samplers of δ15NNO3 and δ18ONO3 to investigate nitrogen cycling
Recognizing the limitations of inferring biogeochemical cycling and nutrient dynamics from concentration data alone, we propose the use of DET gels as a high resolution, in situ sampler, of nitrate isotopes in addition to concentration data
Nitrate concentrations were recovered from the DET gels with ranges between 3.6 ± 0.1 and 3.7 ± 0.1 for a 3.3 ± 0.0 60ga ..90N4.8±Oan±30d−.00L6.−0f.8o1 rgs±taNo7cO0k.0.13s−±oflLou0r−t.i11oasngt7,oN.bc0ekOt±ws3o−el0euL.nt0−io15gn.s2,tNo±bceOkt0w3.s−3oeelaLunnt−di16o.5n7s.,p3±aik±ned0d0.16.1r.3iavfnoe±drr sample (Table S-1)

Summary

■ RESULTS AND DISCUSSION

Horizontal patterns along the flume indicate a general trend of denitrification with an increased observation of low nitrate concentration samples from gel 3 through to gel 1, combined with an increased frequency of high δ15NNO3 and δ18ONO3 values. This longitudinal profile is overlain by local effects, where hotspots of biogeochemical reactivity can be seen, thought to be influenced by the vegetation effect described previously. The nitrate concentrations required limit this technique to sediments in nonpristine environments.[3,32]

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES