Background <p>Leaf litter-derived organic matter (OM) serves as a major source for carbon burial and a key driver for reactive nitrogen (Nr) biogeochemical cycles in coastal wetlands. However, its impacts on sediment Nr removal and associated nitrous oxide (N₂O) production remain unexplored, particularly in the context of biological invasion.</p> Methods <p>We applied a ¹⁵N-isotope tracing technique combined with slurry incubations to investigate how leaf litter from native mangrove (<i>Kandelia candel</i>, KC) and invasive cordgrass (<i>Spartina alterniflora</i>, SA) modulates potential rates of denitrification, anammox and N₂O production across distinct estuarine and wetland sediments.</p> Results <p>Higher background potential denitrification but significantly lower potential anammox rates were observed in mangrove sediments compared to those in estuarine sediments. Litter addition from both native KC and invasive SA enhanced potential denitrification rates in estuarine sediments but significantly suppressed potential denitrification in wetland sediments. Simultaneously, both types of litter generally suppressed potential anammox regardless of the source and sampling region. Notably, the addition of either litter type resulted in up to 90% of the denitrification products being released as N₂O. This decoupling of denitrification steps may directly trigger pulse-like N₂O emissions in both wetland and estuarine sediments.</p> Conclusion <p>Overall, our findings demonstrate that although the effects of native and invasive litter on total sediment nitrogen removal are highly environment-dependent, both may substantially increase the potential risk of N₂O release from coastal sediments. These findings provide new mechanistic insights into how litter-driven biogeochemical shifts regulate the trade-off between nitrogen removal and greenhouse gas production in coastal ecosystems.</p>