Isotopic Insight Into Nitrate Sources and Cycling in Polar Sea Ice

Abstract Sea ice alters nitrate (NO 3 − ) availability in polar oceans, yet the influence of sea‐ice type on NO 3 − sources and cycling remains poorly constrained. We analyzed NO 3 − concentrations and isotopic compositions ( δ 15 N, δ 18 O, Δ 17 O) in Antarctic one‐year and Arctic multi‐year sea ice to investigate NO 3 − sources and transformation processes. Mean NO 3 − concentrations were substantially higher in Antarctic one‐year sea ice (3.89 ± 17.75 μmol L −1 , with the large standard deviation driven by a few bottom‐layer samples with elevated concentrations) than in Arctic multi‐year sea ice (0.50 ± 0.22 μmol L −1 ). Δ 17 O‐based source apportionment showed that atmospheric NO 3 − contributed 37% ± 11.64% in Arctic multi‐year sea ice, compared with 4% ± 2.69% in Antarctic one‐year sea ice. Despite much lower total NO 3 − concentrations, Arctic multi‐year sea ice exhibited a higher atmospheric NO 3 − contribution, indicating better preservation of atmospheric NO 3 − . Non‐atmospheric NO 3 − , derived from in situ nitrification and/or underlying seawater, accounted for 96% ± 2.69% and 63% ± 11.64% of the NO 3 − pool in Antarctic and Arctic sea ice, respectively. Isotopic compositions and nutrient concentrations indicate the occurrence of microbial NO 3 − cycling in both Antarctic one‐year and Arctic multi‐year sea ice, with more pronounced cycling in Antarctic one‐year sea ice. Collectively, these results suggest that sea‐ice type influences NO 3 − concentrations, sources, and cycling, highlighting the role of sea ice in NO 3 − availability in polar marine environments. We acknowledge that nitrite (NO 2 − ) in sea‐ice samples could introduce a degree of uncertainty into isotope interpretations; however, given the relatively low NO 2 − /(NO 2 − + NO 3 − ) fractions, this uncertainty is unlikely to have affected our main conclusions.