Like all organisms, marine phytoplankton require nitrogen to live and grow. Although the majority of the air we breathe is N2, the nitrogen in the atmosphere is not available for use by most phytoplankton; certain bacteria and cyanobacteria that are capable of breaking the strong N–N triple bond represent exceptions. Before organisms can use nitrogen, N2 gas must first be converted to a more chemically available form, such as ammonium, nitrate, or organic nitrogen. The inert nature of N2 means that biologically available nitrogen is often in short supply in the ocean, limiting phytoplankton growth.
Within the last century, humans have become an important source of fixed nitrogen. In the early 20th century, Fritz Haber and Carl Bosch developed an industrial process to produce reactive nitrogen from atmospheric N2 stores. The Haber-Bosch reaction soon became an important means of producing nitrogen-rich fertilizers. Meanwhile, the adoption of internal combustion engines and other industrial burning processes led to widespread releases of oxidized nitrogen (NOx) to the atmosphere. Through these activities, humans have more than doubled the amount of fixed nitrogen that is pumped into the biosphere every year. Most of this fixed nitrogen is deposited on land, but some escapes and travels long distances. Modeling studies suggest that the oceans far away from the continents are not immune to the impacts of humankind's nitrogen fertilization experiment. However, little evidence exists to support this hypothesis. The signal of anthropogenic nitrogen deposition is diluted by ocean mixing, and its impact may also be counteracted by organisms capable of fixing atmospheric N2.
Anthropogenic sources of nitrogen are often isotopically lighter than the nitrogen that circulates in ecosystems due to natural processes. Using 15N/14N stable isotope analysis, Ren et al. track the appearance of this isotopically light nitrogen in seasonally resolved coral from Dongsha atoll, a semi-closed circular coral reef atoll located 300 km from the nearest continental mass in the northern South China Sea. The authors find that the light-nitrogen signal increased just before 2000, coincident with massive increases in fossil fuel combustion in Asia but decades later than predicted by modeling work. The amplitude of change suggests that, by 2010, the atmospheric deposition of anthropogenic N represented approximately one-fifth of the annual input of N to the surface ocean in this region. This proportion appears to be at the lower end of other estimates.
This study provides two insights into human amplification of the nitrogen cycle in the South China Sea. 1) Fossil fuel use (including coal consumption and vehicle exhaust) may have been the major driver of increasing anthropogenic nitrogen deposition to the open ocean, and their effects may have been immediate. 2) The differences between the observations and previous modeling studies highlight gaps in our knowledge of the sources and pathways by which nitrogen deposition occurs in the open ocean.
This assessment of the extent to which humankind's unintentional nitrogen release experiment has spread adds urgency to recent concerns about the multidimensional planetary boundaries that humanity is pressuring. It also highlights the urgency of monitoring the ocean environment over large areas and through time to better understand the extent of human influences on the open ocean. This study points out the tremendous potential value of a network of coral-based N isotope records from ocean islands and offshore reefs to deliver this information.
The paper, "21st-Century Rise in Anthropogenic Nitrogen Deposition on a Remote Coral Reef," was published in the May 19, 2017, issue of the journal Science.
Link to the full article: http://science.sciencemag.org/content/356/6339/749
The article has been highlighted in Science: http://science.sciencemag.org/content/356/6339/700
Figure 1. (A) Most of southeastern China was covered by a thick, grayish shroud of aerosol pollution in January 2002. The smog is so thick that it is difficult to see the surface in some regions of this scene, which was acquired on January 7, 2002. The city of Hong Kong is the large brown cluster of pixels toward the lower left-hand corner of the image (indicated by the faint black box). The island of Taiwan, due east of mainland China, is also blanketed by the smog. This true-color image was captured by the Moderate-resolution Imaging Spectroradiometer (MODIS) sensor onboard NASA’s Terra satellite. This image is from NASA (https://visibleearth.nasa.gov/view.php?id=57534). Dongsha is the green circular ring located to the southwest of Taiwan in the South China Sea; it is marked with a red circle.
(B) The increase in the 14N signal observed in coral from Dongsha atoll occurred concurrently with the increase in fossil fuel combustion (including coal consumption and vehicle use) in China.
Reference
Haojia Ren, Yi-Chi Chen, Xingchen T. Wang, George T. F. Wong, Anne L. Cohen, Thomas M. DeCarlo, Mira A. Weigand, Horng-Sheng Mii and Daniel M. Sigman (2017). 21st-century rise in anthropogenic nitrogen deposition on a remote coral reef. Science, 356(6339), 749-752. DOI:10.1126/science.aal3869.
Assistant Professor Haojia Abby Ren
Department of Geosciences
abbyren@ntu.edu.tw