Dual isotopes of nitrite in the Amundsen Sea in summer

Nitrite (NO₂^-) is a key intermediate in the nitrogen (N) cycle, and its transformation is accomplished by microbial communities. However, due to few studies on the nitrite cycle, a clear assessment of the contribution to the marine biogeochemical cycle is missing. Here, we present data on nitrogen and oxygen isotopic composition of NO₂^- in the Amundsen Sea in summer, and explore the biogeochemical processes that influence the NO₂^- cycle. Extremely low δ¹⁵N_NO2 and abnormally high δ¹⁸O_NO2 were found in the upper waters of the Amundsen Sea, with δ¹⁵N_NO2 as low as -58.4 ‰ and δ¹⁸O_NO2 as high as 44.4 ‰. Enzymatic isotopic exchange reactions between nitrate and nitrite have been proposed to be responsible for these isotopic anomalies. The mirror-symmetrical variation between δ¹⁵N_NO2 and δ¹⁸O_NO2 suggests that the isotopic fractionation effects of nitrogen and oxygen are opposite in isotope exchange reactions. Dual isotopes of nitrite indicate that ammonia oxidation is the main source of nitrite, thus nitrification plays an important role in the formation of primary nitrite maximum in the upper Amundsen Sea. The nitrogen and oxygen isotopic compositions of nitrite provide support for clarifying multiple processes of marine nitrogen cycle.