J. L. Tank S. K. Hamilton W. M. Wollheim R. O. Hall Jr. P. J. Mulholland B. J. Peterson L. R. Ashkenas L. W. Cooper Clifford N. Dahm W. K. Dodds N. B. Grimm S. L. Johnson W. H. McDowell G. C. Poole Valett H. Maurice C. P. Arango M. J. Bernot A. J. Burgin C. L. Crenshaw A. M. Helton L. T. Johnson J. M. O’Brien J. D. Potter R.W. Sheibley D. J. Sobota S. M. Thomas J. J. Beaulieu 2011 <div id="abstract-1" class="section abstract"><div id="abstract-1" class="section abstract"><p id="p-6">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<span>&nbsp;</span>(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<span>&nbsp;</span>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 &lt;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⁻<span>&nbsp;</span>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<span>&nbsp;</span>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.</p></div><p id="p-7">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 (<a id="xref-ref-1-1" class="xref-bibr" href="https://www.pnas.org/content/108/1/214#ref-1" data-mce-href="https://www.pnas.org/content/108/1/214#ref-1">1</a>). Increasing N availability is producing unintended environmental consequences including enhanced emissions of nitrous oxide (N₂O), a potent greenhouse gas (<a id="xref-ref-2-1" class="xref-bibr" href="https://www.pnas.org/content/108/1/214#ref-2" data-mce-href="https://www.pnas.org/content/108/1/214#ref-2">2</a>) and an important cause of stratospheric ozone destruction (<a id="xref-ref-3-1" class="xref-bibr" href="https://www.pnas.org/content/108/1/214#ref-3" data-mce-href="https://www.pnas.org/content/108/1/214#ref-3">3</a>). 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 (<a id="xref-ref-2-2" class="xref-bibr" href="https://www.pnas.org/content/108/1/214#ref-2" data-mce-href="https://www.pnas.org/content/108/1/214#ref-2">2</a>). The production of N₂O in agricultural soils has been the focus of intense investigation (i.e., &gt;1,000 published studies) and is a relatively well constrained component of the N₂O budget (<a id="xref-ref-4-1" class="xref-bibr" href="https://www.pnas.org/content/108/1/214#ref-4" data-mce-href="https://www.pnas.org/content/108/1/214#ref-4">4</a>). 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.</p><p id="p-8">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 (<a id="xref-ref-5-1" class="xref-bibr" href="https://www.pnas.org/content/108/1/214#ref-5" data-mce-href="https://www.pnas.org/content/108/1/214#ref-5">5</a>). The proportion of denitrified NO₃⁻<span>&nbsp;</span>that is converted to N₂O rather than N₂<span>&nbsp;</span>(hereafter referred to as the N₂O yield and expressed as the mole ratio) partially controls how much N₂O is produced via denitrification (<a id="xref-ref-6-1" class="xref-bibr" href="https://www.pnas.org/content/108/1/214#ref-6" data-mce-href="https://www.pnas.org/content/108/1/214#ref-6">6</a>), but few studies provide information on the N₂O yield in streams and rivers because of the difficulty of measuring N₂<span>&nbsp;</span>and N₂O production in these systems. Here we report rates of N₂<span>&nbsp;</span>and N₂O production via denitrification measured using whole-stream<span>&nbsp;</span>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.</p><p id="p-9">Although N₂O emission rates have been reported for streams and rivers (<a id="xref-ref-7-1" class="xref-bibr" href="https://www.pnas.org/content/108/1/214#ref-7" data-mce-href="https://www.pnas.org/content/108/1/214#ref-7">7</a>,<span>&nbsp;</span><a id="xref-ref-8-1" class="xref-bibr" href="https://www.pnas.org/content/108/1/214#ref-8" data-mce-href="https://www.pnas.org/content/108/1/214#ref-8">8</a>), 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 (<a id="xref-ref-9-1" class="xref-bibr" href="https://www.pnas.org/content/108/1/214#ref-9" data-mce-href="https://www.pnas.org/content/108/1/214#ref-9">9</a>). These N₂O yields are low compared with observations in soils (0–100%) (<a id="xref-ref-10-1" class="xref-bibr" href="https://www.pnas.org/content/108/1/214#ref-10" data-mce-href="https://www.pnas.org/content/108/1/214#ref-10">10</a>), which may be a result of the relatively lower oxygen (O₂) availability in the sediments of lakes and estuaries. However, dissolved O₂<span>&nbsp;</span>in headwater streams is commonly near atmospheric equilibrium and benthic algal biofilms can produce O₂<span>&nbsp;</span>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 (<a id="xref-ref-11-1" class="xref-bibr" href="https://www.pnas.org/content/108/1/214#ref-11" data-mce-href="https://www.pnas.org/content/108/1/214#ref-11">11</a>). The N₂O yield in headwater streams is of particular interest because much of the NO₃⁻<span>&nbsp;</span>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 (<a id="xref-ref-12-1" class="xref-bibr article-ref-popup hasTooltip" href="https://www.pnas.org/content/108/1/214#ref-12" data-hasqtip="4" data-mce-href="https://www.pnas.org/content/108/1/214#ref-12">12</a>).</p></div> application/pdf 10.1073/pnas.1011464108 en National Academy of Sciences of the United States of America Nitrous oxide emission from denitrification in stream and river networks article