Modeling oceanic nitrate and nitrite concentrations and isotopes using a 3-D inverse N cycle model

Abstract

Nitrite (NO -2 ) is a key intermediate in the marine nitrogen (N) cycle and a substrate in nitrification, which produces nitrate (NO -3 ), as well as water column N loss processes denitrification and anammox. In models of the marine N cycle, NO -2 is often not considered as a separate state variable, since NO -3 occurs in much higher concentrations in the ocean. In oxygen deficient zones (ODZs), however, NO -2 represents a substantial fraction of the bioavailable N, and modeling its production and consumption is important to understand the N cycle processes occurring there, especially those where bioavailable N is lost from or retained within the water column. Improving N cycle models by including NO -2 is important in order to better quantify N cycling rates in ODZs, particularly N loss rates. Here we present the expansion of a global 3-D inverse N cycle model to include NO -2 as a reactive intermediate as well as the processes that produce and consume NO -2 in marine ODZs. NO -2 accumulation in ODZs is accurately represented by the model involving NO -3 reduction, NO -2 reduction, NO -2 oxidation, and anammox. We model both 14 N and 15 N and use a compilation of oceanographic measurements of NO -3 and NO -2 concentrations and isotopes to place a better constraint on the N cycle processes occurring. The model is optimized using a range of isotope effects for denitrification and NO -2 oxidation, and we find that the larger (more negative) inverse isotope effects for NO -2 oxidation, along with relatively high rates of NO -2 , oxidation give a better simulation of NO -3 and NO -2 concentrations and isotopes in marine ODZs.