Nitrous oxide concentration and flux in Min River Basin of southeast China: Effects of land use, stream order and water variables

Estimates of riverine nitrous oxide (N₂O) emissions have great uncertainty partly due to the fact that the data is still sparse for stream sizes and land uses. Here we determined water dissolved N₂O concentration and water–air interface N₂O flux across the land uses and stream orders in Min River Basin of southeast China. N₂O concentrations ranged from 11.1 to 27.4 nmol/L, increasing from first- to eighth-order rivers. The first- and second-order streams showed negative N₂O flux (−0.33 and −0.05 µg m⁻²h⁻¹, respectively) compared to third- to eighth-order rivers (from 0.29 to 2.85 µg m⁻²h⁻¹). Average N₂O flux was significantly higher in urban (1.60 µg m⁻²h⁻¹) and cropland rivers (0.89 µg m⁻²h⁻¹) than in forest rivers (−0.47 µg m⁻²h⁻¹), which could be due to the main effect of land use was on the nitrogen load. Indirect N₂O emission factor varied between 0.035 and 0.14 % across the rivers, which decreased from the urban to forest rivers and were lower than the IPCC default value (0.26 %). N₂O concentration and flux were positively correlated with NO₃⁻, NH₄⁺, depth, velocity and temperature, suggesting the importance of stream surface hydromoprhology to N₂O emissions. Land use indirectly affected N₂O flux by affecting water NO₃⁻, NH₄⁺, DO, DOC, pH and temperature, which could further explain the land use dependence of N₂O emissions on water variables. High DOC:NO₃⁻ ratio (>6) was the main factor of N₂O sink in the small streams, which may be due to the enhanced N₂O consumption and low N₂O production. Therefore, riverine N₂O emissions are highly variable across stream sizes and land uses, which are not only increased by inorganic nitrogen load and are also dependent on stream hydraulics and morphology.