Nitrate-Nitrite Isotopic Exchange: Unveiling Its Influence on Isotope Effect Estimates in Nitrate Assimilation in the Cosmonaut Sea, Antarctica

Recent insights into isotopic exchange between nitrate and nitrite have introduced complexities to our understanding of the nitrogen cycle in the Southern Ocean. This study highlights unusual isotopic compositions in the mixed layer of the Cosmonaut Sea characterized by notably low δ¹⁵N and high δ¹⁸O values in nitrite. These anomalies challenge our expectations regarding isotopic behavior within cycling pathways, highlighting the significant role of isotopic exchange between nitrate and nitrite. Interestingly, incorporating this exchange reaction significantly altered the estimated isotope effect of nitrate assimilation for both nitrogen and oxygen based on the Rayleigh model in the nitrate-only system. This raises questions about the reliability of previous estimates in assessing nitrate consumption in the Southern Ocean, suggesting that the nitrate + nitrite system may provide a more accurate representative of nitrate uptake. Additionally, spatial variations in the nitrate assimilation isotope effect (${\epsilon }_{{\text{NO}}_3+{\text{NO}}_2}$) both for nitrogen and oxygen were also observed, with higher ${}^{15}{\epsilon }_{{\text{NO}}_3+{\text{NO}}_2}$ and ${}^{18}{\epsilon }_{{\text{NO}}_3+{\text{NO}}_2}$ values north of the southern boundary (SB) and lower values to the south. The reduced ${}^{15}{\epsilon }_{{\text{NO}}_3+{\text{NO}}_2}$ and ${}^{18}{\epsilon }_{{\text{NO}}_3+{\text{NO}}_2}$ values south of the SB may be primarily driven by elevated iron concentrations as indicated by the positive relationship between ${}^{18}{\epsilon }_{{\text{NO}}_3+{\text{NO}}_2}$ and △Si/△N, a proxy for iron limitation, rather than by nitrification, phytoplankton composition, vertical mixing, or light availability. This study uses dual isotopes of nitrate and nitrite to evaluate isotopic exchange effects on nitrate assimilation, specifically for oxygen, enhancing our understanding of the Southern Ocean's role in the global nitrogen cycle.