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 (N 2 O) to the atmosphere but recent evidence of N 2 O 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 N 2 O concentration and a Bayesian hierarchical model to predict summertime N 2 O concentration and emission rates in 465,896 waterbodies in the conterminous U.S. (CONUS). We found that N 2 O 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 N 2 O sinks. The model predicts dissolved N 2 O concentrations reasonably well based partly on interactions between nitrate concentration, waterbody surface area, and water temperature. Despite working with the largest aquatic N 2 O dataset to date, our national-scale estimate of summertime N 2 O emissions from CONUS lakes is poorly constrained, with a 95% credible interval ranging from net uptake to net emission (−282 − 482 metric tons N 2 O summer −1 ). Pervasive N 2 O undersaturation in CONUS waterbodies during the summer highlights the need to revisit N 2 O models which presume surface waters are a N 2 O source.