Group post: The effect of OM stoichiometry on nitrogen loss

There has been a long debate about which process is the most important for the removal of nitrogen from the oceans anammox (anaerobic ammonium oxidation), the sole source of oxidation of POM-dervied ammonium in ODZs, or denitrifiation. This is purely more than just for scientific interest as denitrification both consumes and produces NO, a virulent greenhouse gas, whereas anammox doesn’t. The balance between the two therefore has the potential to influence global climate change. The processes are thought to be controlled by organic matter (OM) stoichiometry but the predicted value of 30% anammox is rarely seen.

In order to investigate the relationship between anammox, denitrification and OM stoichiometry, Babbin et. al (2014) derived a theoretical balance between the two processes from generic OM stoichiometry using predictions based on previous theoretical and experimental studies. They then performed incubation experiments at the top of the ODZ and the secondary nitrite maximum in a coastal and open water location in the Eastern Tropical Pacific. They used isotypically labeled nitrite as a tracer and various OM treatments which varied  in their C:N ratio.

The results showed that anamox and denitrification rates increase by different proportions in response to specific OM additions. Higher proportions of anammox corresponded to a lower C:N ratio in the OM. This also confirmed that OM stoichiometry was the major controlling factor in the ratio between anammox and denitrification with the predicted values matching closely with the observed for each OM treatment.

These models predicting water column denitrification rates using POM fluxes and C/N ratios can help provide information on present and past fixed N losses in marine suboxic regions giving us further insight into the organisms and processes involved in these areas. It is additionally helpful in evaluating the effects of ODZ expansion on future climate, potential shifts in average C/N ratios with changing atmospheric CO₂ concentrations and coastal eutrophication. Nitrogen loss can also act as negative feedback to global primary production through limiting the fixed nitrogen available. This in turn can effect the amount of food available for higher trophic levels and the amount of CO₂ absorbed from the atmosphere. Therefore understanding the processes that remove fixed nitrogen is of high importance.

Although the predicted values on these experiments matched the observed values quite well, it may be worth performing repeat experiments in other ODZ areas in order to assess the reliability of this model.

Their model did not take into consideration other elements which can make up OM and perhaps some compounds which may have a similar make up of C and N to those found in this study but a different chemical composition such as more recalcitrant OM may favour one nitrogen loss process over the other. This is an interesting and important area of research that allows us to understand the balance between the two main nitrogen loss processes. As mentioned before this balance can significantly impact the production and consumption of greenhouse gases and the availability of fixed nitrogen in the water.

Babbin, A. R., Keil, R. G., Devol, A. H., & Ward, B. B. (2014). Organic Matter Stoichiometry, Flux, and Oxygen Control Nitrogen Loss in the Ocean. Science, 344(6182), 406-408.
http://www.princeton.edu/nitrogen/publications/pdfs/babbin_science2014.pdf