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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 (1–3). 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).