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Estuaries sit at the land‐ocean boundary and act as natural reactors for nitrogen (N) transformations among numerous forms. Superimposing onto physical mixing, biological processes lead to nonconservative N behavior in estuaries. Under the coinfluences of biological alteration and multiple end‐members, however, some N species can exhibit apparent conservation. To explore N dynamics and potential mechanisms that modulate the mixing behaviors, we measured concentrations and natural isotopic compositions of multiple N species (nitrate: NO₃⁻, nitrite: NO₂⁻, ammonium: NH₄⁺, and particulate nitrogen) in a typical eutrophic estuary in southern China. Additionally, N uptake and oxidation rates were measured by using isotope labelling techniques to evaluate processes potentially offsetting the conservative mixing of specific N pools. We found that NO₃⁻ followed conservative two end‐member mixing with …