Nitrous oxide emission from denitrification in stream and river networks

Humans have more than doubled the availability of fixed nitrogen (N) in the biosphere, particularly through the production of N fertilizers and the cultivation of N-fixing crops (1). Increasing N availability is producing unintended environmental consequences including enhanced emissions of nitrous oxide (N₂O), a potent greenhouse gas (2) and an important cause of stratospheric ozone destruction (3). The Intergovernmental Panel on Climate Change (IPCC) estimates that the microbial conversion of agriculturally derived N to N₂O in soils and aquatic ecosystems is the largest source of anthropogenic N₂O to the atmosphere (2). The production of N₂O in agricultural soils has been the focus of intense investigation (i.e., >1,000 published studies) and is a relatively well constrained component of the N₂O budget (4). However, emissions of anthropogenic N₂O from streams, rivers, and estuaries have received much less attention and remain a major source of uncertainty in the global anthropogenic N₂O budget.

Microbial denitrification is a large source of N₂O emissions in terrestrial and aquatic ecosystems. Most microbial denitrification is a form of anaerobic respiration in which nitrate (NO₃⁻, the dominant form of inorganic N) is converted to dinitrogen (N₂) and N₂O gases (5). The proportion of denitrified NO₃⁻ that is converted to N₂O rather than N₂ (hereafter referred to as the N₂O yield and expressed as the mole ratio) partially controls how much N₂O is produced via denitrification (6), but few studies provide information on the N₂O yield in streams and rivers because of the difficulty of measuring N₂ and N₂O production in these systems. Here we report rates of N₂ and N₂O production via denitrification measured using whole-stream ¹⁵NO₃⁻-tracer experiments in 72 headwater streams draining different land-use types across the United States. This project, known as the second Lotic Intersite Nitrogen eXperiment (LINX II), provides unique whole-system measurements of the N₂O yield in streams.

Although N₂O emission rates have been reported for streams and rivers (7, 8), the N₂O yield has been studied mostly in lentic freshwater and marine ecosystems, where it generally ranges between 0.1 and 1.0%, although yields as high as 6% have been observed (9). These N₂O yields are low compared with observations in soils (0–100%) (10), which may be a result of the relatively lower oxygen (O₂) availability in the sediments of lakes and estuaries. However, dissolved O₂ in headwater streams is commonly near atmospheric equilibrium and benthic algal biofilms can produce O₂ at the sediment–water interface, resulting in strong redox gradients more akin to those in partially wetted soils. Thus, streams may have variable and often high N₂O yields, similar to those in soils (11). The N₂O yield in headwater streams is of particular interest because much of the NO₃⁻ input to rivers is derived from groundwater upwelling into headwater streams. Furthermore, headwater streams compose the majority of stream length within a drainage network and have high ratios of bioreactive benthic surface area to water volume (12).