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 (NO3-) and nitrite (NO2-) concentrations and their N isotopic compositions as constraints on N cycle process rates in marine oxygen deficient zones (ODZs). NO2- 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 NO2- 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 NO2- oxidation, which co-occurs with NO3- reduction and anammox in ODZs. High rates of NO2- oxidation (up to 400 nM/day), which are tightly coupled to heterotrophic NO3- reduction, are required to match NO3- and NO2- concentration and isotope observations in marine ODZs. Lowering the rate of NO2- oxidation in ODZs by adjusting O-2-sensitive parameters results in higher rates of water column N loss, highlighting the role of NO2- oxidation in maintaining the marine fixed N inventory. Plain Language Summary We have created a global nitrogen (N) cycle model to study the transformations between different forms of N in marine oxygen deficient zones (ODZs). In these regions, oxygen concentrations are very low and microbial processes remove bioavailable N from the ocean. Understanding N loss is important because low N availability can limit carbon uptake in the surface ocean. Previous research has implied that an oxygen-requiring process, nitrite oxidation, may help prevent N loss in and around ODZs. Using our model, which accurately models the N concentrations and isotopes in the ocean and in ODZs, we have shown that nitrite oxidation is indeed necessary in ODZs in order for the model results to match existing measurements. Removing nitrite oxidation from ODZs results in rates of N loss that are too high and concentration and isotope profiles that do not match observations.