Nitrogen Cycle Dynamics Revealed Through δ18O-NO3− Analysis in California Groundwater

Abstract

Nitrate is a significant water-quality issue in California, the United States as a whole, and the world. Critical to addressing nitrate contamination is understanding the presence and extent of denitrification, and further refining the techniques used to identify nitrate sources. The use and understanding of nitrate isotopic signatures to identify nitrate sources have advanced tremendously; however, knowledge gaps remain concerning specific fractionation pathways and the role of denitrification in altering source values. Using a large unique database of California groundwater nitrate isotopic compositions, we explored the utility of nitrate–oxygen isotope ratios in determining specific nitrate origins. Lawrence Livermore National Lab (LLNL) samples were supplemented by United States Geological Society (USGS) data to create a dataset of over 1200 dual-isotope results. Methods used at LLNL allowed for the determination of δ15N-NO3−, δ18O-NO3−, δ18O-H2O, δ2H-H2O, excess air, major dissolved gases, and excess N2. Results were examined for the degree to which δ18O-NO3− conforms to the model of nitrification in which two atoms of oxygen are sourced from ambient water and one from the atmosphere. Almost 80% of the results fall within one standard deviation of predicted values. However, 19% of samples had significantly higher values, suggesting the preservation of a synthetic nitrate source signature, mixing of sources, or widespread denitrification. Results were examined with respect to general land-use classifications and, while nitrate concentrations followed the expected pattern of being higher in agricultural settings, δ18O-NO3−patterns are complicated by application of N-fertilizer in various forms, and subsequent N cycling in the soil zone. We found that the current understanding of oxygen isotope-fractionation mechanisms cannot yet explain the prevalence of oxygen-isotope compositions with higher than predicted δ18O values, but when paired with related data such as land use and indicators of denitrification, oxygen-isotope compositions of nitrate can help to assess nitrogen cycle dynamics.

Seasonality of nitrogen sources, cycling, and loading in a New England river discerned from nitrate isotope ratios

“…This empirical metric, however, demonstrably overlooks substantive isotope effects associated with O-atom incorporation into the NO − 3 molecule during nitrification and isotopic exchange of the nitrite intermediate with water, which otherwise give way to nitrified NO − 3 whose δ 18 O NO 3 value is close to that of ambient water (Sigman et al, 2009;Casciotti et al, 2008;Buchwald and Casciotti, 2010;Snider et al, 2010;Boshers et al, 2019). This consideration explains frequent observations of relatively low δ 18 O NO 3 in some soils and saturated systems, which are not explained by simple fractional contribution of reactants (Hinkle et al, 2008;Xue et al, 2009;Fang et al, 2012;Veale et al, 2019). Thus, we posit that the O isotope composition of the NO − 3 imported into the river with increased discharge, which is typical of that in soils and shallow groundwater, does not strictly indicate that shallow-flow NO − 3 originated from proximate nitrification therein, as generally presumed.…”

Vadose zone flushing of fertilizer tracked by isotopes of water and nitrate

“…Further supporting the idea that decreases in δ 15 N-NO 3 − were caused by a source shift rather than fractionation changes in nitrification, the measured δ 18 O-NO 3 − values remained steady during fertilizer signal periods (typically all below 0‰; Figure S1). They were also outside the range expected of nitrification processing in the system (+1.5‰ to +2.6‰) based on the assumption that oxygens from source water (δ 18 O-H 2 O; Figure S3) and atmospheric oxygen (δ 18 O-O 2 ; assumed to be constant at +23.5‰) would be incorporated in a 2:1 ratio during nitrification (Amberger & Schmidt, 1987;Boshers et al, 2019;Snider et al, 2010;Veale et al, 2019). A change in Vadose Zone Journal nitrification fractionation would have shifted the isotopic values of δ 18 O-NO 3 − closer to that of the soil water, but this was not the case.…”

A Review on the Application of Isotopic Techniques to Trace Groundwater Pollution Sources within Developing Countries

“…Furthermore, nitrogen fractionations are negligible in N-limited systems [60]. With regard to δ 18 O − NO 3 , several isotope studies demonstrated that NO − 3 produced by nitrification obtains two oxygen atoms (O) from water molecules and one O atom from the atmosphere [61,62]. Therefore, the δ 18 O of microbial nitrate can be calculated using Equation (6) if the δ 18 O of water and that of atmospheric O 2 are known.…”

“…Conversely, Synthetic NO − 3 fertilizer is not produced by this same nitrification reaction. It derives all of its oxygen from atmospheric O 2 ; this results in significantly enriched values of δ 18 O − NO 3 ranging from +17 to +25‰ [62,65]. Thus, δ 18 O − NO 3 values may be effectively used to distinguish between ammonia in fertilizer and precipitation from that of nitrate fertilizer.…”