Control of the Hydraulic Load on Nitrous Oxide Emissions from Cascade Reservoirs

Nitrous oxide (N₂O) emissions show large variability among dam reservoirs, which makes it difficult to estimate N₂O contributions to global greenhouse gases (GHGs). Because river damming alters hydraulic residence time and water depth, the hydraulic load (i.e., the ratio of the mean water depth to the residence time) was hypothesized to control N₂O emissions from dam reservoirs. To test this hypothesis, we investigated N₂O fluxes and related parameters in the cascade reservoirs along the Wujiang River in Southwest China. The N₂O fluxes showed obvious temporal and spatial variations, ranging from-7.86 to 337.22 μmol m^-2 d^-1, with an average of 12.76 μmol m^-2 d^-1. Nitrification was the main pathway of N₂O production in these reservoirs, and seasonal dissolved oxygen (DO) stratification played an important role in regulating the N₂O production. The reservoir N₂O flux had a significant negative logarithmic relationship with the hydraulic load, suggesting its control of the N₂O emission. This was because the hydraulic load was a prerequisite for regulating the nitrification-denitrification and the DO stratification in the dam reservoirs. This empirical relationship will help to estimate the contribution of reservoir N₂O emissions to global GHGs.