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.