More than half of the Earth's surface is covered by abyssal plains, yet nitrogen cycling in these vast sedimentary environments remains poorly constrained. In this study, we investigated benthic nitrogen cycling in abyssal plain sediments from the Kuroshio Extension region of the Northwest Pacific. We measured porewater nutrients, nitrate nitrogen and oxygen isotopes (δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-), sedimentary total nitrogen and organic carbon contents, and their isotope compositions (δ¹⁵N-TN and δ¹³C-TOC). These geochemical data were further interpreted using a one-dimensional diagenetic model to constrain nitrogen transformation rates and isotope fractionation during nitrate reduction.

Our results reveal a clear vertical partitioning of nitrogen cycling processes in the studied sediments. The oxic layer is mainly characterized by nitrification, while the upper nitrate reduction (NAR) layer is dominated by denitrification, with possible contributions from anammox and nitrification. In contrast, the lower NAR layer shows stronger evidence for anammox-dominated nitrogen transformation. This vertical partitioning is reflected not only in porewater DIN profiles, but also in the dual-isotope behavior of nitrate. In particular, the decoupling between δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^- provides evidence for anammox involvement in nitrate reduction. Model-derived isotope effects further indicate that nitrate reduction in these sediments is not uniform with depth. The net fractionation factors varied between layers, with relatively high ¹⁵ε_NAR and lower ¹⁸ε_NAR where anammox contributed more strongly. Across the studied sites, ¹⁵ε_NAR ranged from 15 to 56‰, whereas ¹⁸ε_NAR ranged from 12 to 32‰. These patterns suggest that labile organic matter input regulates the relative importance of denitrification and anammox, thereby controlling the isotopic response of nitrate reduction. Overall, this study provides new constraints on benthic nitrogen cycling and isotope dynamics in abyssal plain sediments, and highlights the importance of anammox in deep-sea nitrogen removal.