Inland Waters can Act as Nitrous Oxide Sinks: Observation and Modeling Reveal that Nitrous Oxide Undersaturation May Partially Offset Emissions

Inland waters are significant, yet highly uncertain, natural sources of nitrous oxide (N2O). Many emission models assume that N2O is only emitted from freshwaters, and that N2O sink behavior is negligible. However, observational studies have reported N2O undersaturation, suggesting that inland waters can act as N2O sinks due to net N2O consumption. This study leverages data from the National Ecological Observatory Network (NEON) and an existing global emission model to examine the prevalence of and controls on N2O undersaturation in streams, rivers, and lakes across scales and biomes. We find that N2O undersaturation is prevalent in the NEON data set (14%-30% of samples) and process-based model outputs (38%), occurring across biomes and spatial scales. Failing to account for undersaturation in the NEON data set could result in an 100% overestimation of N2O emissions. These results show that consideration of N2O sink behavior is needed for accurate emission estimates. Nitrous oxide (N2O) is a climate-relevant greenhouse gas that is increasing in the atmosphere due to human activities. It also contributes to stratospheric ozone depletion. Inland waters (e.g., streams, rivers, and lakes) are significant, yet uncertain, sources of N2O to the atmosphere. In general, emission models assume that N2O is produced in and emitted from inland waters. However, field observations have shown that inland waters can also act as sinks of atmospheric N2O, potentially offsetting emissions. Understanding when and where inland waters act as N2O sinks is needed for accurate emission estimates. This study uses a large observational data set and an existing global emission model to explore when, where, and to what extent inland waters act as N2O sinks. The results show that N2O sink behavior is common and may significantly offset emissions. Nitrous oxide (N2O) undersaturation in inland waters is prevalent, indicating that inland waters periodically act as sinks of atmospheric N2OPeriodic N2O sink behavior occurs at most monitored sites and the frequency of N2O sink behavior is invariant to ecosystem size or biomeN2O sink behavior has the strong potential to partially offset N2O emissions from inland waters