Tracking the rise of anthropogenic nitrogen

Nitrogen is critical to life on Earth: Every amino acid of every protein molecule in every living thing contains at least one N atom. The N₂ molecules that make up most of the atmosphere, however, are nearly inert, and the element must be converted into some other chemical form before organisms can use it. That conversion can be accomplished in a few ways: lightning; nitrogen fixation by various species of bacteria and algae; or, more recently, industrial synthesis of ammonia for fertilizer. Fossil-fuel combustion, too, adds to the supply of chemically available nitrogen by releasing organic material from long-dead organisms previously trapped underground.

Human activities are known to have a sizeable and deleterious effect on the global N cycle. (See the article by Ann Kinzig and Robert Socolow, Physics Today, November 1994, page 24 .) But the geographic extent of that effect is poorly understood, because data on the N cycle from before the rise of pollution are sparse, particularly in remote areas. Now National Taiwan University’s Haojia Ren and her colleagues have used isotope measurements to derive a 45-year N record from the coral reefs of Dongsha Atoll, an uninhabited group of islands 340 km from land in the South China Sea.

Like tree rings and human hair, the interior of a coral colony is devoid of living tissue, and it remains unchanged from the time it was formed. Though mostly calcium carbonate, the coral skeleton contains a small amount of protein—a valuable record of N conditions over the colony’s lifetime.

The deep-sea nitrogen that naturally feeds marine organisms is slightly enriched in ¹⁵N, because processes that remove N from the ocean have a preference for the lighter ¹⁴N. In contrast, anthropogenic N sources—both fossil fuels and fertilizer—have a smaller fraction of ¹⁵N. By sampling the Dongsha corals (and patching them up again so the colonies could regrow), Ren and company measured the relative ¹⁵N abundance, δ¹⁵N, taken up by the coral from 1968 to 2013. As shown in the graph, they found a noticeable change in δ¹⁵N, beginning around the year 2000, that closely tracks the rise of Chinese coal consumption but appears uncorrelated with fertilizer use.

The results hint that switching from coal to cleaner fuels could mitigate the harm to the open ocean, but it’s hard to draw definitive conclusions from measurements at a single location. Ren and colleagues are working on repeating their measurements in other sites around the world. (H. Ren et al., Science 356, 749, 2017 ; Dongsha photo courtesy of Thomas DeCarlo.)