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 815NNO2 and abnormally high 818ONO2 were found in the upper waters of the Amundsen Sea, with 815NNO2 as low as -58.4 parts per thousand and 818ONO2 as high as 44.4 parts per thousand. Enzymatic isotopic exchange reactions between nitrate and nitrite have been proposed to be responsible for these isotopic anomalies. The mirror-symmetrical variation between 815NNO2 and 818ONO2 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.