Beyond ‘emissions factors’: Aquatic ecosystems as sources and sinks of N₂O

Abstract

Despite decades of effort to establish direct, quantitative relationships between NO₃¯ pollution and N₂O emissions from aquatic ecosystems, we struggle to predict whether waterways are N₂O sources or sinks. Hydrology (e.g., residence time, stream order) is now shown to play a key role in regulating N₂O emissions from rivers and estuaries. Yet N₂O cycling in the biologically productive shallow streams, ponds, and wetlands that punctuate most landscapes remains difficult to predict. Data from a range of coastal wetlands and streams shows that sediments in these systems tend uptake, rather than emit N₂O. However, daily and seasonal variations in light and water level can trigger pulses of N₂O emission. It is proposed that productivity regimens, rather than NO₃¯ loads, determine N₂O emissions from small aquatic ecosystems. This idea is explored by combining continuous stream metabolism (O₂, light, and temperature) measurements and with monthly N₂O and NO₃¯ measurements from nine first order sub-tropical streams. Dynamic trace gas production and reduction in these ubiquitous landscape features highlights the importance of working across the terrestrial-aquatic interface in order to resolve ecosystem nitrogen budgets.