Mass Balance

• Page authors: PRIMO
• Responsible curator: Hagen Buck-Wiese

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Denitrification rate (marine N mass balance)
Approach: synthesis of ocean nitrogen inventory data over time
Context: in situ, data synthesis
Spatial scale: ocean-wide to basin
Temporal scale: years to decades
Units: Tg N yr-1
Community captured: bulk
Co-measurements: N inventory over time; N fixation rates

Method Overview

The marine nitrogen mass balance approach constrains total ocean denitrification by requiring that inputs to the fixed nitrogen (N) pool balance outputs at steady state. Inputs consist primarily of nitrogen fixation and riverine/atmospheric deposition; outputs consist of denitrification in water column oxygen minimum zones (OMZ denitrification) and sediments (benthic denitrification). If the budget is in steady state and other inputs/outputs are constrained, then total denitrification = total inputs − other sinks. Alternatively, observed changes in the ocean fixed nitrogen inventory over time can directly constrain whether the ocean N budget is in balance.

Scale of measurement

Ocean-wide synthesis: The approach does not provide spatial or temporal resolution of denitrification within the ocean but constrains its global-scale magnitude.

Data generated

Estimated total marine denitrification in Tg N yr^-1. Uncertainties in individual input and output terms propagate into the estimate.

Units & currency

Units are Tg N yr^-1 or mol N yr^-1. The currency is nitrogen.

Sample size

Not applicable: synthesis approach.

Limitations

The fundamental assumption is that the ocean is at steady state with respect to fixed nitrogen, which is increasingly questioned given evidence for imbalances between N fixation and denitrification on decadal timescales. Changes in N₂ fixation rates are difficult to constrain globally, adding uncertainty to the balance. The approach requires accurate estimates of all terms in the N budget, each of which carries its own large uncertainty.

Example Applications & Protocols

Classic examples

• Battaglia & Joos (2018) Marine N₂O emissions from nitrification and denitrification constrained by modern observations

Recent applications

• DeVries et al. (2013) Marine denitrification rates determined from a global 3-D inverse model

Common calculations/conversions

• At steady state: denitrification = N fixation + riverine N input + atmospheric deposition − other sinks.