N₂ fixation in urbanization area rivers: spatial-temporal variations and influencing factors

While nitrogen (N₂) fixation is an important process in nitrogen (N) biogeochemical cycling, supplying a significant portion of the N in natural ecosystems, few quantitative constraints exist concerning its contribution to the N enrichment and export from river ecosystems. This study estimates the N₂ fixation rates of urban rivers in the Yangtze Estuary area using acetylene reduction. The results demonstrate that the prominent spatiotemporal variability of river N₂ fixation rates is driven by various environmental factors. River N₂ fixation rates are significantly higher in the summer (90.57 ± 14.60 ngN·L⁻¹·h⁻¹) than in the winter (57.98 ± 15.73 ngN·L⁻¹·h⁻¹). Spatially, rivers draining urban and suburban areas have higher N₂ fixation rates than those draining rural areas. The N₂ fixation rates are positively correlated with the N₂ fixing cyanobacteria density, water temperature, light, and the water phosphorus (P) concentration, but they are negatively correlated with the dissolved N concentration (NH₄ ⁺-N and NO₃ ⁻-N). The N₂ fixation rates annually range from 53.20 to 89.24 ngN·L⁻¹·h⁻¹ for all of the sampling rivers, which is equivalent to a depth integrated (0–0.6 m) N input of 0.163–0.274 gN·m⁻²·a⁻¹. The determined annual N input via N₂ fixation is generally higher than that of marine systems, but it is lower than that of eutrophic lakes. This study provides robust evidence that N₂ fixation can supply a substantial portion of the N input to human-impacted river ecosystems, which has not been sufficiently accounted for when determining the N mass balance of riverine ecosystems. A high N₂ fixation rate may increase the ratio of N to P input to river systems, and therefore render P the limiting factor in aquatic eutrophication.