Pervasive summertime nitrous oxide undersaturation in U.S. lakes and reservoirs.

Abstract

Lakes and reservoirs are estimated to be globally important sources of nitrous oxide (N₂O) to the atmosphere but recent evidence of N₂O uptake across a broad range of lakes have called the accuracy of emission estimates into question. Here, we use a new national-scale dataset of dissolved N₂O concentration and a Bayesian hierarchical model to predict summertime N₂O concentration and emission rates in 465,896 waterbodies in the conterminous U.S. (CONUS). We found that N₂O undersaturation was pervasive throughout the CONUS during the summer of 2017, with an estimated 72.9% (95% credible interval: 68.9-76.6%) of lakes functioning as N₂O sinks. The model predicts dissolved N₂O concentrations reasonably well based partly on interactions between nitrate concentration, waterbody surface area, and water temperature. Despite working with the largest aquatic N₂O dataset to date, our national-scale estimate of summertime N₂O emissions from CONUS lakes is poorly constrained, with a 95% credible interval ranging from net uptake to net emission (-282 - 482 metric tons N₂O summer^-1). Pervasive N₂O undersaturation in CONUS waterbodies during the summer highlights the need to revisit N₂O models which presume surface waters are a N₂O source.