Earthquake regulation of nitrogen dynamics in subduction zone

Hadal trenches at subduction margins represent hotspots where episodic tectonic events can reorganize sedimentation and biogeochemical processing. In particular, earthquake-triggered depositional pulses may redistribute and rapidly bury matter including nitrogen. Despite this potential importance, nitrogen burial in the deepest trench environments is still poorly constrained, and how earthquakes modulate nitrogen inputs, internal cycling, and delivery toward subduction remains unclear. Here, we measured elemental and isotopic geochemical parameters from high-resolution long sediment cores drilled at the Japan Trench during IODP Expedition 386. By integrating high-resolution event stratigraphy with robust chronological constraints, we quantified nitrogen inputs, transformations, and burial under two contrasting depositional regimes: background sedimentation and earthquake-triggered event deposition. We find that earthquake event layers deliver nitrogen-bearing material and are associated with substantially enhanced nitrogen burial relative to background intervals. Estimated nitrogen burial fluxes rise from ~24 to ~300 Tg N yr^-1 and the event-driven flux exceeds published deep-sea mean burial estimates by ~1.5×10². In addition, earthquake-triggered event deposition markedly increases the relative proportion of bioavailable nitrogen in trench sediments, from 82.6% in background intervals to 91.4%. Collectively, these changes in the bioavailable nitrogen supply may diversify nitrogen transformation pathways, manifesting as a more complex nitrogen-cycling signal in hadal trench sediments. We propose a "seismic nitrogen pump" in which earthquakes transiently accelerate nitrogen cycling through organic matter activation and mass transport, enhancing sedimentary retention and potentially reshaping the subducting nitrogen reservoir. Our findings challenge the view of trenches as static nitrogen repositories, identifying tectonic forcing as a key driver of subduction-zone biogeochemistry and underscoring the need to incorporate episodic perturbations into global nitrogen budgets and deep carbon–nitrogen coupling frameworks.