Beyond carbon: reactive nitrogen emissions from ammonia as a marine fuel and implications for ocean ecosystems

Maritime shipping, responsible for ~3% of global carbon dioxide (CO2) emissions, must rapidly decarbonise to meet the International Maritime Organization’s target of net-zero greenhouse gas (GHG) emissions by 2050. Renewable-based ammonia has emerged as a leading near-zero-carbon fuel candidate; however, its reactive nitrogen (Nr) footprint remains poorly characterised. Across the ammonia marine fuel (AMF) value chain, Nr emissions arise as nitrogen oxides (NOx) and nitrous oxide (N2O) from combustion, and ammonia (NH3) losses to air (i.e., fugitive leaks and operational emissions) during production, storage, bunkering, and shipboard use, alongside episodic releases to water from spills and nitrogen-bearing effluent. This review synthesises current evidence on Nr emissions across the AMF value chain, examines plausible pathways through which their downstream transport, deposition, and fate may impact marine ecosystems and climate, and highlights key knowledge gaps. We show that resulting impacts—including altered marine productivity and community structure, eutrophication, biodiversity loss, and indirect N2O formation—are geographically dependent. Identical emissions can produce markedly different outcomes depending on whether deposition and other inputs occur in oligotrophic gyres, coral reefs, oxygen minimum zones, or already-stressed coastal and marginal seas. We also highlight potentially vulnerable regions where major shipping corridors overlap with nitrogen-sensitive marine ecosystems and Nr inputs may generate disproportionate ecological or climate impacts. Finally, we review mitigation measures and governance gaps across the AMF lifecycle. Realizing the full climate and environmental benefits of AMF will depend on integrated lifecycle accounting, stringent emissions controls, and policies that address nitrogen-cycle impacts and protect marine ecosystems alongside carbon reductions.