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

Nitrous oxide (NO) emissions show large variability among dam reservoirs, which makes it difficult to estimate NO 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 NO emissions from dam reservoirs. To test this hypothesis, we investigated NO fluxes and related parameters in the cascade reservoirs along the Wujiang River in Southwest China. The NO fluxes showed obvious temporal and spatial variations, ranging from -7.86 to 337.22 μmol m d, with an average of 12.76 μmol m d. Nitrification was the main pathway of NO production in these reservoirs, and seasonal dissolved oxygen (DO) stratification played an important role in regulating the NO production. The reservoir NO flux had a significant negative logarithmic relationship with the hydraulic load, suggesting its control of the NO 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 NO emissions to global GHGs.