Stoichiometric approach
| Denitrification rate (stoichiometric tracer analysis) |
|---|
| Approach: deviations from Redfield stoichiometry in DIN, O2, DIC, PO4 |
| Context: in situ |
| Spatial scale: ocean-wide to basin |
| Temporal scale: years and beyond |
| Units: mol N time-1 |
| Community captured: bulk |
| Co-measurements: deviations from Redfield stoichiometry over time |
Method Overview
Denitrification leaves a characteristic stoichiometric fingerprint in the water column: it consumes nitrate without a corresponding change in phosphate, causing a negative deviation of the measured N:P ratio from the Redfield ratio (N* = [NO3-] − 16 × [PO43-] + 2.9, or similar formulations). By mapping the spatial distribution of N* across ocean basins and tracking changes in the tracer over time using hydrographic data, the net rate of denitrification (minus N fixation) can be estimated. When combined with inverse circulation models (INCLUDE LINK TO MODEL PAGE HERE?) and oxygen consumption data, the approach provides basin-scale estimates of denitrification rates[1].
Scale of measurement
Basin to global scale. Temporal resolution is limited by the timescales of ocean circulation (years to centuries). The method cannot resolve point rates or short-term variability.
Data generated
Basin-integrated denitrification rates in mol N time-1 or Tg N yr-1; spatial maps of N* revealing regions of net N loss or gain.
Units & currency
Units are mol N time-1 or Tg N yr-1. The currency is nitrogen.
Sample size
Not applicable. This method uses existing hydrographic datasets.
Repositories & databases
Global ocean hydrographic data: GO-SHIP; World Ocean Atlas.
Limitations
The approach relies on the assumption that the Redfield ratio accurately describes the elemental stoichiometry of organic matter remineralization in the region of interest. In freshwater or coastal systems, variable stoichiometry makes the approach less reliable. Multiple processes can contribute to N* anomalies (N fixation decreases the deficit; denitrification increases it), and separating them requires additional constraints. Requires inverse circulation models to interpret tracer distributions quantitatively.
Example Applications & Protocols
Classic examples
- Li & Peng (2006) Nitrate deficits by nitrification and denitrification processes in the Indian Ocean [1]
Recent applications
- Mashifane (2021) Denitrification and anammox shift nutrient stoichiometry in the Benguela upwelling system [2]
Common calculations/conversions
- N* = [NO3-] − 16 × [PO43-] + 2.9 (µM); negative N* indicates denitrification; positive N* indicates N fixation.
References
- ↑ 1.0 1.1 Li, Y.-H., & Peng, T.-H. (2006). Nitrate deficits by nitrification and denitrification processes in the Indian Ocean. Deep-Sea Research Part I, 53(1), 94–110. https://doi.org/10.1016/j.dsr.2005.09.009
- ↑ Mashifane, T. B., Roychoudhury, A. N., Waldron, H. N., Racault, M.-F., & Moloney, C. L. (2021). Denitrification and anammox shift nutrient stoichiometry and the phytoplankton community structure in the Benguela upwelling system. Journal of Geophysical Research: Oceans, 126(8), e2021JC017816. https://doi.org/10.1029/2021JC017816