Although present at only trace levels in Earth’s atmosphere, nitrous oxide (N₂O) is a powerful greenhouse gas. Furthermore, its abundance in the atmosphere has steadily increased over the past several centuries – an increase that has been largely attributed to anthropogenic release of reactive nitrogen into the biosphere and hydrosphere. With its long residence time, a molecule of N₂O has been estimated to contribute over 300 times more to climate forcing than CO₂ on the timescale of a century. Despite its important contribution to the radiative budget of the Earth, however, our ability to predict atmospheric N₂O emissions remains quite poor, in part due to complex variations in the network of processes regulating its production and flux.

As coastal ecosystems are especially subject to elevated nitrogen, here we investigated controls on N₂O production mechanisms in intertidal sediments using novel isotopic tools and microsensors in flow-through sediment incubations. Sediment cores were collected from various intertidal zones (Sylt, Germany and Santa Catalina Island) and incubated under flow-through conditions. The continuous flow of overlying water was manipulated to specifically examine the influence of low O₂ and high nitrate conditions (known to influence N₂O production by nitrifying and denitrifying bacteria, respectively) on sediment biogeochemical reactions involved in N₂O cycling and flux.

While little change was observed in the N₂O flux and isotopes under the low O₂ incubations (with respect to our control conditions), we observed large increases in N₂O flux under the elevated nitrate loading conditions. This suggests that while low O₂ conditions may not impact relase of N₂O from coastal ecosystems, the continued and increasing release of nitrogen from human-based activities in watersheds will likely increase emission of this important greenhouse gas. Our microsensor measurements revealed remarkable heterogeneity in the location of zones of N₂O production in the upper 3-4 cm of the sediments. Based on our multi-compound and multi-isotope measurements of nitrate, nitrite, ammonium and N₂O, including a novel ¹⁷O isotopic approach, we were able to estimate the relative importance of four simultaneous N₂O cycling processes. Our results suggest that the increase in N₂O flux under high nitrate was explained by both direct bacterial activity, as well as an important contribution by fungi and/or chemical reactions with iron (‘chemodenitrification’). These findings shed new light on nitrogen cycling complexity in coastal environments and may help to explain the high variability of environmental N₂O fluxes, which may be driven by dynamic variations in the activity a range of N₂O production processes.

Last Modified: 06/30/2016
Modified by: Scott D Wankel