PerspectiveOceans

Nitrogen pollution knows no bounds

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Science  19 May 2017:
Vol. 356, Issue 6339, pp. 700-701
DOI: 10.1126/science.aan3242

Nitrogen isotope data from Dongsha Atoll in the South China Sea provide evidence for rising levels of anthropogenic pollution.

PHOTO: REUTERS/NICKY LOH/ALAMY STOCK PHOTO

In recent decades, the flow of fixed—that is, biologically usable—nitrogen from human activities into the environment has grown substantially. The sources include excess production and use of nitrogen fertilizers; ammonia emitted from animal husbandry and sewage; and nitrogen oxides emitted by automobiles, airplanes, and fossil fuel power plants. The resulting nitrogen flux into the ocean may approach the magnitude of natural sources (13). However, it is difficult to specify the integrated increase over natural sources precisely because there is very little data for when nitrogen sources were mostly natural. On page 749 of this issue, Ren et al. use the nitrogen isotope composition of a 50-year coral core from the South China Sea to show that the natural upwelling flux of fixed nitrogen has risen by 20% during the past two decades (4).

Some natural archives—such as ice cores, peat bogs, and lake deposits—support the premise of higher global fixed-nitrogen deposition as a result of human activities. Examples include increases of nitrate and ammonia in ice cores (5, 6), organic nitrogen in remote lake sediments (7), and a decline in the 15N/14N ratio in Chinese tree rings (8). However, these archives are too sparse and remote to quantify the global deposition of anthropogenic nitrogen emissions into the ocean. Ren et al. now report a half-century record of increasing anthropogenic fixed-nitrogen emissions, derived from measurements of the nitrogen isotope composition of proteins trapped in the aragonite skeletons of an annual banded coral from Dongsha Atoll, just off of China in the South China Sea (see the photo). The extended temporal record provides a perspective from an era before recent increases in Chinese nitrogen emissions.

Nitrogen isotope measurements are helpful for resolving processes that transport fixed nitrogen through the ocean. The main source of fixed nitrogen to open-ocean phytoplankton is the upwelling and upward mixing of nitrate ions dissolved in deep-ocean waters. Below the uppermost layers, deep-sea nitrate has a relatively narrow range in the 15N/14N ratio [or δ15N, defined as (15/14sample/15/14standard − 1)*1000], which is +5.0 to +5.5 per mil (‰) in the South China Sea. The ocean is depleted in the lighter nitrogen isotope relative to atmospheric gaseous nitrogen (the δ15N = 0‰ reference point) because water-column denitrification preferentially uses the lighter isotope. In the warm stratified ocean, nitrate that mixes up to the surface is almost completely consumed by phytoplankton and eventually converted into sinking biological organic matter that balances the upward flux, having the same nitrogen isotope ratio as the upward-mixing deep-ocean water.

In the modern era, three sources deliver fixed nitrogen with isotope compositions lighter than that of deep-ocean nitrate to the surface ocean (see the figure). Lightning provides relatively minor inputs of fixed nitrogen (δ15N = −0.5 to +1.4‰). A larger natural source is biological nitrogen fixation—that is, the conversion of N2 gas to biologically usable forms of nitrogen. This occurs both on the continents by commensal bacteria on plant roots in soils, some of which can be transported to the ocean by rivers and atmospheric dust, and in the marine phytoplankton community by organisms such as Trichodesmium, which use solar energy to fix N2. Biologically fixed nitrogen has a δ15N of ∼−1‰. In the modern era, nitrogen is fixed by many human activities, either deliberately during the Haber-Bosch production of nitrogen fertilizers or inadvertently during the high-temperature combustion of fossil fuels. These anthropogenic mechanisms produce fixed nitrogen that is about −2.7‰, in contrast to the ∼+5‰ in the oceanic nitrate reservoir.

The use of the organic matter in corals as an indicator of the nitrogen isotope composition of nitrogen sources to the surface ocean is based on the premise that the trace proteins that are incorporated into the coral aragonite calcium carbonate skeleton have an isotopic composition that is offset from that of the source fixed nitrogen by a fixed value (about +2‰). This assumption is supported by data on the nitrogen isotopic composition of corals (9).

The observation that the Dongsha Atoll coral has lighter organic skeletal nitrogen now than during the previous half-century could be caused by three processes: relatively large reductions in the upward mixing of thermocline nitrate in this area, an increase in biological nitrogen fixation in the ocean, or an increase in the anthropogenic fixed-nitrogen sources. Reduced upward mixing is inconsistent with long-term observations of a relatively stable chlorophyll concentration in this region. Historical marine nitrogen fixation measurements do not support an increase in regional nitrogen fixation. Hence, Ren et al. conclude that the lighter isotope ratio of coral-bound organic nitrogen is due to an increase in the deposition of light anthropogenic nitrogen in this region.

The Dongsha Atoll data imply that anthropogenic inputs have increased the flux of nitrogen to the surface of the South China Sea. Although substantial, this estimate is near the lower end of the range of previous estimates from atmospheric measurements. The timing of the anthropogenic fixed-nitrogen increase is later than anticipated from the history of many Chinese anthropogenic fixed-nitrogen sources, such as agriculture and fertilizer production. The late rise does, however, coincide with a major increase in coal consumption and vehicle use (10), suggesting that these fossil fuel sources may account for the increased anthropogenic nitrogen flux into the South China Sea.


Embedded Image
Sources of fixed nitrogen in surface waters

The isotope signatures of fixed-nitrogen sources, given in per mil (‰), can be used to estimate the flux strengths. Upwelling of deep-ocean nitrate is the main natural source of fixed nitrogen. Coral skeleton data now show the rising contribution of anthropogenic emissions.

GRAPHIC: K. SUTLIFF/SCIENCE

One consequence of the recognition of substantial anthropogenic additions of isotopically light nitrogen is that some light upper-ocean nitrate, which has been attributed solely to natural biological nitrogen fixation in the ocean (11), is actually anthropogenic. Better understanding and modeling of the integrated rates of biological nitrogen fixation in the ocean will require a clearer quantification of the anthropogenic sources throughout the ocean (12).

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