Nitrogen input modulates the effects of coastal acidification on nitrification and associated N₂O emission

Acidification of coastal waters, synergistically driven by increasing atmospheric carbon dioxide (CO₂) and intensive land-derived nutrient inputs, exerts significant stresses on the biogeochemical cycles of coastal ecosystem. However, the combined effects of anthropogenic nitrogen (N) inputs and aquatic acidification on nitrification, a critical process of N cycling, remains unclear in estuarine and coastal ecosystems. Here, we showed that increased loading of ammonium (NH₄⁺) in estuarine and coastal waters alleviated the inhibitory effect of acidification on nitrification rates but intensified the production of the potent greenhouse gas nitrous oxide (N₂O), thus accelerating global climate change. Metatranscriptomes and natural N₂O isotopic signatures further suggested that the enhanced emission of N₂O may mainly source from hydroxylamine (NH₂OH) oxidation rather than from nitrite (NO₂^-) reduction pathway of nitrifying microbes. This study elucidates how anthropogenic N inputs regulate the effects of coastal acidification on nitrification and associated N₂O emissions, thereby enhancing our ability to predict the feedbacks of estuarine and coastal ecosystems to climate change and human perturbations.