Assessing Marine Nitrogen Cycle Rates and Process Sensitivities With a Global 3‐D Inverse Model

We present results from a global inverse marine nitrogen (N) cycle model that include nitrate (NO 3 − ) and nitrite (NO 2 − ) concentrations and their N isotopic compositions as constraints on N cycle process rates in marine oxygen deficient zones (ODZs). NO 2 − is an important intermediate in the N cycle, particularly in ODZs where it is a substrate in the N loss processes, denitrification, and anammox. Similar to earlier work, our model yields a total water column N loss rate of 61 ± 10 Tg N/year. However, by including NO 2 − and its N isotopic composition, we are able to assess the relative contributions of denitrification and anammox to N loss and examine some of the potential drivers of that balance. We find that anammox contributes 60% of global water column N loss, dominating N loss along the edges of ODZs, while denitrification is more important in the anoxic ODZ cores. The decoupling of anammox and denitrification is supported by NO 2 − oxidation, which co‐occurs with NO 3 − reduction and anammox in ODZs. High rates of NO 2 − oxidation (up to 400 nM/day), which are tightly coupled to heterotrophic NO 3 − reduction, are required to match NO 3 − and NO 2 − concentration and isotope observations in marine ODZs. Lowering the rate of NO 2 − oxidation in ODZs by adjusting O 2 ‐sensitive parameters results in higher rates of water column N loss, highlighting the role of NO 2 − oxidation in maintaining the marine fixed N inventory.