Nitrous oxide emission from denitrification in stream and river networks

TitleNitrous oxide emission from denitrification in stream and river networks
Publication TypeJournal Article
Year of Publication2011
AuthorsBeaulieu, JK, Tank, JL, Hamilton, SK, Wollheim, WM, Hall, RO, Mulholland, PJ, Peterson, BJ, Ashkenas, LR, Cooper, LW, Dahm, CN, Dodds, WK, Grimm, NB, Johnson, SL, McDowell, WH, Poole, GC, Valett, HM, Arango, CP, Bernot, MJ, Burgin, AJ, Crenshaw, C, Helton, AM, Johnson, L, O'Brien, JM, Potter, JD, Sheibley, RW, Sobota, DJ, Thomas, SM
JournalProceedings of the National Academy of Sciences of the United States of America
Pagination214 -219
Accession NumberKNZ001360

Nitrous oxide (N2O) is a potent greenhouse gas that contributes to climate change and stratospheric ozone destruction. Anthropogenic nitrogen (N) loading to river networks is a potentially important source of N2O via microbial denitrification that converts N to N2O and dinitrogen (N2). The fraction of denitrified N that escapes as N2O rather than N2 (i.e., the N2O yield) is an important determinant of how much N2O is produced by river networks, but little is known about the N2O yield in flowing waters. Here, we present the results of whole-stream 15N-tracer additions conducted in 72 headwater streams draining multiple land-use types across the United States. We found that stream denitrification produces N2O at rates that increase with stream water nitrate (NO3−) concentrations, but that <1% of denitrified N is converted to N2O. Unlike some previous studies, we found no relationship between the N2O yield and stream water NO3−. We suggest that increased stream NO3− loading stimulates denitrification and concomitant N2O production, but does not increase the N2O yield. In our study, most streams were sources of N2O to the atmosphere and the highest emission rates were observed in streams draining urban basins. Using a global river network model, we estimate that microbial N transformations (e.g., denitrification and nitrification) convert at least 0.68 Tg·y−1 of anthropogenic N inputs to N2O in river networks, equivalent to 10% of the global anthropogenic N2O emission rate. This estimate of stream and river N2O emissions is three times greater than estimated by the Intergovernmental Panel on Climate Change. 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).