Natural Abundance Nitrogen and Oxygen Isotope Steady-State Box Model of the Nitrogen Cycle

Abstract:

Natural abundance nitrogen and oxygen isotopes (δ¹⁵N and δ¹⁸O) of compounds in the nitrogen cycle, including nitrate, nitrite, ammonium, and particulate organic nitrogen, are powerful tools for constraining nitrogen transformations and processes in the environment. Instantaneous rate measurements of processes in the nitrogen cycle are difficult to extrapolate over time and space gradients, and modeling based on natural abundance isotope and concentration measurements offers a more robust solution to understanding nitrogen cycling dynamics across different environmental systems. Here we present a steady-state box model of the nitrogen cycle based on concentration and nitrogen and oxygen isotope mass balances of nitrate, nitrite, ammonium, and particulate organic nitrogen. The model incorporates nine biogeochemical processes of the nitrogen cycle (nitrate and nitrite reduction, nitrite and ammonia oxidation, remineralization, assimilation of nitrate, nitrite and ammonium, and oxygen isotope exchange between water and nitrite) together with their associated isotope effects. First, we constructed a version of the model solving for steady-state isotopic composition of each nitrogen pool prescribed over a range of biogeochemical rates, explicitly exploring connections among rates of coupled aerobic and anaerobic processes. We then inverted this model to use field isotope data from a large river study to place estimates on rates of steady-state biogeochemical nitrogen cycling processes occurring over a ~80 km reach. We will present the conceptual framework of the model and key aspects learned by exploring relationships among variables and relative rates and demonstrate estimates of real-world processes by natural abundance isotopic measurements of nitrogen species.