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Why does the ocean lose its nitrogen?

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In the largest ocean area where the oxygen level is so low that it is not measurable, nitrogen escapes to the atmosphere. Located in the South Pacific, this region contributes significantly to the loss of nitrogen from the global ocean. In a study published in  Nature , scientists reveal the main mechanisms involved. 

The human disturbance  of the nitrogen cycle  is particularly important. Anthropogenic flows, related to industry and chemical fertilizers, are as important as natural flows. Industrial activities emit 90 million tonnes of nitrogen oxides (NOx) and ammonia (NH 3 ) per year into the atmosphere. The fertilizer spreading 80 million tons of nitrogen compounds in the soil each year.

Understanding the nitrogen cycle is a key factor in predicting the ocean’s response to climate change , as it is one of the limiting nutrients for life in the oceans. Nitrogenxa exchanges are dominated by microbial transformations, with biomass assimilating it only in the form of minerals (NO3  and NH4+). In the ocean, there are  areas of minimum oxygen  ( OMZ ) where microbes are able to use various sources of dissolved nitrogen. They produce greenhouse gases such as nitrous oxide, or nitrous oxide(N 2 O), 300 times more powerful than CO 2 , which are then released into the atmosphere.

Oceanic zones of minimum oxygen (at the level of the arrows).  The color bar indicates dissolved oxygen concentrations in μM, or 10-6 mol / L (mole per liter), at a depth of 300 m.  About 30-50% of nitrogen losses occur in these areas, which account for only 0.1% of ocean volume.  © World Ocean Atlas 2009

These OMZs contribute 20 to 40% of the total loss of ocean nitrogen and would tend to spread. The largest OMZ in the world is located east of the South Equatorial Pacific. Scientists at the  Max  Planck Institute for Marine Microbiology  have studied this area closely. They sought to identify and interpret nitrogen loss regulation mechanisms. “This research is fundamental to improving current biogeochemical models because so far our models can not faithfully reproduce the nitrogen loss trends that are measured,” says Andreas Oschlies of Geomar of Kiel, Germany.

Bacteria by billions in areas without oxygen

Recently published in the journal  Nature , their results show that the loss of nitrogen in the OMZ of the southern equatorial Pacific is related to the burial of organic matter in coastal waters with low  oxygen . Indeed, this region is home to billions of bacteria. They fix the nitrogen and convert it into gas by two processes, denitrification and ammonization (ie the anaerobic oxidation of the ammonium ion , NH4+ , with a nitrite ion NO2 ) . The NH4+ ion  comes from the mineralization of organic matter.

The factors that regulate nitrogen loss by anammox remained previously unknown. In this study, the researchers estimated the total nitrogen balance of OMZ by measuring nutrient concentrations, nitrogen conversion rates, and from a numerical model that calculates the export rate of nitrogen. ‘nitrogen. They found that nitrogen loss rates peak in   highly productive coastal waters , where large amounts of organic matter are falling from the ocean surface.

The fall of organic matter as a trigger

Anammox was therefore the dominant mode of nitrogen loss at the time of sampling . Overall, nitrogen transformation rates, including anammox, were strongly correlated with the export of  organic matter . The drop of organic matter (and thus the release of ammonium in the water column) as well as the release of benthic ammonium fuels the loss of nitrogen from the zones of oxygen minimum .

These results will make it possible to more realistically estimate the impacts of ocean deoxygenation. They will also help to determine precisely how bacterial productivity changes the nitrogen cycle in OMZs, as well as the rest of the ocean. This is essential to estimate the amount of  CO 2  that can be absorbed by the ocean in the future.

Dr. Monika Mathur

Ph.D Yale University

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