Direct measurements of dissolved N2 and N2O highlight the strong nitrogen (N) removal potential of riverine wetlands in a headwater stream

Increasing levels of nitrogen (N) in aquatic ecosystems due to intensified human activities is focusing attention on N removal mechanisms as a means to mitigate environmental damage. Important N removal processes such as denitrification can resolve this issue by converting N to gaseous emissions. Here, the spatiotemporal variability of N removal rates in China's Zhongtian River, a headwater stream that contains wetlands, was investigated by quantifying gaseous emissions of the main end products, N₂ and N₂O, using the water-air exchange model. Excess concentrations of these gases relative to their saturations in the water column generally varied within 1.4–8.7 μmol L⁻¹ and 8.7–20.3 nmol L⁻¹, with mean values of 4.5 μmol L⁻¹ and 13.7 nmol L⁻¹, respectively, demonstrating significant N removal in the river. The reach with wetlands was characterized by higher in-stream N₂ production than the non-wetland reach, especially in July, when aquatic vegetation is most abundant. High N₂O emissions during the same period in the non-wetland reach indicate that environmental conditions associated with vegetation are conducive to N₂ production and likely constrain N₂O emission. Changes in dissolved oxygen, pH, temperature, and carbon to nitrogen ratios are correlated with the observed spatiotemporal variabilities in gaseous N production. The mean N removal rate in the wetland reach was roughly twice that in the non-wetland reach, i.e., 22.4 vs. 10.3 mmol N m⁻² d⁻¹, while the corresponding efficiency was about five times as high, i.e., 15 % vs. 3 %. This study reveals the spatiotemporal patterns of in-stream N removal in a headwater stream and highlights the efficacy of wetlands in N removal. The data provide a strong rationale for constructing artificial wetlands as a means to mitigate N pollution and thereby optimize riverine environmental conditions.