N 2 fixation in urbanization area rivers: spatial-temporal variations and influencing factors.

While nitrogen (N ₂ ) 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 N ₂ fixation rates of urban rivers in the Yangtze Estuary area using acetylene reduction. The results demonstrate that the prominent spatiotemporal variability of river N ₂ fixation rates is driven by various environmental factors. River N ₂ 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 N ₂ fixation rates than those draining rural areas. The N ₂ fixation rates are positively correlated with the N ₂ fixing cyanobacteria density, water temperature, light, and the water phosphorus (P) concentration, but they are negatively correlated with the dissolved N concentration (NH ₄ ⁺ -N and NO ₃ ^- -N). The N ₂ 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 N ₂ fixation is generally higher than that of marine systems, but it is lower than that of eutrophic lakes. This study provides robust evidence that N ₂ 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 N ₂ fixation rate may increase the ratio of N to P input to river systems, and therefore render P the limiting factor in aquatic eutrophication.