As a key limit of marine primary productivity, oceanic nitrogen inventory exerts a fundamental control on biological carbon uptake. The nitrogen inventory is regulated by the balance between nitrogen input through biological N-fixation and atmospheric deposition, and the dominant N loss though denitrification. Over the past 150 yrs, atmospheric nitrogen deposition driven by human activity, mainly including fossil fuel combustion, has increased dramatically, rising by more than 24 TgN/yr between 1850 and 2000. However, increase atmospheric nitrogen deposition may drive a compensatory decrease in N-fixation, as the additional external nitrogen supply reduces the ecological advantage of nitrogen-fixing organisms. Also, enhanced deposition may lead to higher productivity and thereby stimulate denitrification. It therefore remains a key question whether changes in anthropogenic emission exert a significant net effect on the open ocean nitrogen inventory.
In this study, we conducted bulk δ¹⁵N and δ¹³C analyses on the organic skeletons of several deep-sea corals from the South China Sea and the western Pacific. Both regions exhibit a coherent decrease of over 1‰ in δ¹⁵N and δ¹³C over the past 150 years. The decline in δ¹³C reflects the transport of the Suess effect into the deep ocean. Meanwhile, compound-specific amino acid isotopic analyses on coral skeletons indicate that the observed δ¹⁵N changes are dominated by variations in the isotopic signals of the upper-ocean source as well. Using a mass-balance calculation, we propose that such δ¹⁵N decrease is driven by both increased atmospheric nitrogen deposition and enhanced N-fixation. Specifically, atmospheric deposition delivers iron alongside nitrogen to the open ocean, which stimulates N-fixation rather than suppresses it. Consequently, through both direct deposition and the stimulation of N-fixation, anthropogenic emissions contribute substantially to an increasing oceanic N input.