The marine nitrogen (N) cycle is essential for regulating ocean productivity and global climate, yet human activities and climate change have been disrupting it in unprecedented ways over the past two centuries. How the nitrogen cycle responded to these perturbations remains underexplored. To address this gap, we are using the ocean component of the Community Earth System Model (CESM) coupled with an N isotope-enable Biogeochemical Elemental Cycling (BEC) module to simulate the temporal progression of the marine N cycle and its biological source (i.e., N fixation) and sink processes (i.e., water column and sediment denitrification) from 1850 until present. We are specifically examining the impact of the steadily increasing anthropogenic N input and the role of climate variability vis-à-vis forced change from climate change. Our hindcast simulation reveals substantial variability in key N processes and tracer fields often obscuring potential long-term trends. In this presentation, we aim 1) to distinguish between the impacts of human-source N addition and climate-related forcing; 2) to evaluate changes and variability in N cycle processes and ecosystem functions (e.g., biological pump); 3) to identify the driver of these changes and variability, particularly the influence of climate modes like El Niño‐Southern Oscillation, and 4) to estimate validity of tracers usage in detecting anthropogenic and climate influences. Understanding N cycle variability will improve interpretations of ocean tracer records, such as N* (defined as NO₃ – 16 PO₄ + 2.9 µmol kg^-1) and N isotopes, in the past, present and future ocean. Our results indicate that changes in denitrification are the most pronounced process in the nitrogen cycle, particularly within the ocean’s oxygen minimum zones. We observed trends of up to 60 mmol m⁻² yr⁻¹ yr⁻¹, with variability exceeding 10³ mmol m⁻² yr⁻¹ (up to 60% of the rate) over the study period. Significant variability in nitrogen fixation also persists, especially in the subtropical ocean, where rates can surpass 30 mmol m⁻² yr⁻¹ (up to 10% of the rate). Combined with the addition of anthropogenic nitrogen, these changes in the nitrogen cycle may leave a distinct imprint on tracer fields, complicating the interpretation of current tracer observations.
 

marine nitrogen cycle, anthropogenic nitrogen, decadal climate variability, nitrogen isotope, N*