Climate change and growing anthropogenic perturbation have increased riverine exports of ammonium (NH₄⁺) and particles from the continents to the coasts over the last decades, profoundly impacting coastal nitrogen cycling and water quality. NH₄⁺ consumption in estuarine waters has become an environmental issue of great concern; however, how it changes under the influence of increasing particle export in eutrophic estuaries remains unclear. Based on the ¹⁵N-labeling experiments, we quantified ammonia oxidation (AO) and ammonium uptake (AU) in response to particle addition along the salinity gradient in the Jiulong River Estuary, a highly human-impacted river of China. Results showed that in situ particle addition only stimulated the AO upstream; in contrast, it enhanced both the AO and AU downstream and more significantly for the AU. Furthermore, the downstream AO and AU were less stimulated by adding riverine particles collected from the upstream. Our findings suggest that different responses of NH₄⁺ consumption pathways to particle addition along the eutrophic estuarine salinity gradient are regulated by ambient substrate, particle biodegradation, and the communities of ammonia-utilizing microorganisms and their interactions. This study provides a mechanistic understanding of the effects of increasing particles on estuarine NH₄⁺ dynamics, and has implications for predicting estuarine nitrogen cycling and water quality under global change.

coastal nitrogen cycle, particle abundance, ammonia oxidation, ammonium uptake, subtropical estuary