06005nas a2200433   4500008004100000245007700041210006900118300001300187490000800200520484900208100001905057700001505076700001905091700001905110700001505129700002105144700001905165700001905184700001705203700001505220700001805235700001605253700001805269700001905287700001605306700001705322700001705339700001705356700001705373700001705390700001705407700001605424700001805440700001705458700001905475700001705494700001705511856004305528       2011                        eng d00aNitrous oxide emission from denitrification in stream and river networks0 aNitrous oxide emission from denitrification in stream and river   a214 -2190 v1083 a<p>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&minus;) 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&minus;. We suggest that increased stream NO3&minus; 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&middot;y&minus;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., &gt;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&minus;, the dominant form of inorganic N) is converted to dinitrogen (N2) and N2O gases (5). The proportion of denitrified NO3&minus; 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&minus;-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&ndash;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&ndash;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&minus; 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).</p>1 aBeaulieu, J.K.1 aTank, J.L.1 aHamilton, S.K.1 aWollheim, W.M.1 aHall, R.O.1 aMulholland, P.J.1 aPeterson, B.J.1 aAshkenas, L.R.1 aCooper, L.W.1 aDahm, C.N.1 aDodds, W., K.1 aGrimm, N.B.1 aJohnson, S.L.1 aMcDowell, W.H.1 aPoole, G.C.1 aValett, H.M.1 aArango, C.P.1 aBernot, M.J.1 aBurgin, A.J.1 aCrenshaw, C.1 aHelton, A.M.1 aJohnson, L.1 aO'Brien, J.M.1 aPotter, J.D.1 aSheibley, R.W.1 aSobota, D.J.1 aThomas, S.M.  uhttps://www.pnas.org/content/108/1/21402203nas a2200385   4500008004100000245009000041210006900131300001300200490000600213520111200219100001701331700001601348700001601364700001901380700001901399700002101418700002001439700001901459700001701478700001901495700001701514700001801531700001801549700001701567700001901584700001801603700001901621700001701640700001501657700001701672700001701689700001801706700002201724856007101746       2011                        eng d00aThinking outside the channel: modeling nitrogen cycling in networked river ecosystems0 aThinking outside the channel modeling nitrogen cycling in networ  a229 -2380 v93 a<p>Agricultural and urban development alters nitrogen and other biogeochemical cycles in rivers worldwide. Because such biogeochemical processes cannot be measured empirically across whole river networks, simulation models are critical tools for understanding river-network biogeochemistry. However, limitations inherent in current models restrict our ability to simulate biogeochemical dynamics among diverse river networks. We illustrate these limitations using a river-network model to scale up in situ measures of nitrogen cycling in eight catchments spanning various geophysical and land-use conditions. Our model results provide evidence that catchment characteristics typically excluded from models may control river-network biogeochemistry. Based on our findings, we identify important components of a revised strategy for simulating biogeochemical dynamics in river networks, including approaches to modeling terrestrial&ndash;aquatic linkages, hydrologic exchanges between the channel, floodplain/riparian complex, and subsurface waters, and interactions between coupled biogeochemical cycles.</p>1 aHelton, A.M.1 aPoole, G.C.1 aMeyer, J.L.1 aWollheim, W.M.1 aPeterson, B.J.1 aMulholland, P.J.1 aBernhardt, E.S.1 aStanford, J.A.1 aArango, C.P.1 aAshkenas, L.R.1 aCooper, L.W.1 aDodds, W., K.1 aGregory, S.V.1 aO’Hall, R.1 aHamilton, S.K.1 aJohnson, S.L.1 aMcDowell, W.H.1 aPotter, J.D.1 aTank, J.L.1 aThomas, S.M.1 aValett, H.M.1 aWebster, J.R.1 aZeglin, Lydia, H.  uhttps://esajournals.onlinelibrary.wiley.com/doi/abs/10.1890/08021102610nas a2200337   4500008004100000245007800041210006900119300001500188490000700203520162900210100001801839700001901857700001501876700001701891700001801908700001901926700001701945700001501962700002001977700001801997700001602015700001902031700001802050700001402068700001902082700001602101700001802117700001702135700001902152856010102171       2003                        eng d00aFactors affecting ammonium uptake in streams - an inter-biome perspective0 aFactors affecting ammonium uptake in streams an interbiome persp  a1329 -13520 v483 a1. The Lotic Intersite Nitrogen eXperiment (LINX) was a coordinated study of the relationships between North American biomes and factors governing ammonium uptake in streams. Our objective was to relate inter-biome variability of ammonium uptake to physical, chemical and biological processes.

2. Data were collected from 11 streams ranging from arctic to tropical and from desert to rainforest. Measurements at each site included physical, hydraulic and chemical characteristics, biological parameters, whole-stream metabolism and ammonium uptake. Ammonium uptake was measured by injection of 15N-ammonium and downstream measurements of 15N-ammonium concentration.

3. We found no general, statistically significant relationships that explained the variability in ammonium uptake among sites. However, this approach does not account for the multiple mechanisms of ammonium uptake in streams. When we estimated biological demand for inorganic nitrogen based on our measurements of in-stream metabolism, we found good correspondence between calculated nitrogen demand and measured assimilative nitrogen uptake.

4. Nitrogen uptake varied little among sites, reflecting metabolic compensation in streams in a variety of distinctly different biomes (autotrophic production is high where allochthonous inputs are relatively low and vice versa).

5. Both autotrophic and heterotrophic metabolism require nitrogen and these biotic processes dominate inorganic nitrogen retention in streams. Factors that affect the relative balance of autotrophic and heterotrophic metabolism indirectly control inorganic nitrogen uptake.1 aWebster, J.R.1 aMulholland, P.1 aTank, J.L.1 aValett, H.M.1 aDodds, W., K.1 aPeterson, B.J.1 aBowden, W.B.1 aDahm, C.N.1 aFindlay, S.E.G.1 aGregory, S.V.1 aGrimm, N.B.1 aHamilton, S.K.1 aJohnson, S.L.1 aMarti, E.1 aMcDowell, W.H.1 aMeyer, J.L.1 aMorrall, D.D.1 aThomas, S.A.1 aWollheim, W.M.  uhttp://knz.lternet.edu/content/factors-affecting-ammonium-uptake-streams-inter-biome-perspective02845nas a2200373   4500008004100000245012600041210006900167300001300236490000700249520170000256653001301956653002401969653002101993653002302014653001102037653001802048100002102066700001502087700001802102700001702120700001802137700001802155700001602173700001902189700001802208700001402226700001902240700001602259700001602275700001902291700001702310700001902327856012502346       2002                        eng d00aCan uptake length in streams be determined by nutrient addition experiments? Results from an inter-biome comparison study0 aCan uptake length in streams be determined by nutrient addition   a544 -5600 v213 aNutrient uptake length is an important parameter for quantifying nutrient cycling in streams. Although nutrient tracer additions are the preferred method for measuring uptake length under ambient nutrient concentrations, short-term nutrient addition experiments have more frequently been used to estimate uptake length in streams. Theoretical analysis of the relationship between uptake length determined by nutrient addition experiments (SW′) and uptake length determined by tracer additions (SW) predicted that SW′ should be consistently longer than SW, and that the overestimate of uptake length by SW′ should be related to the level of nutrient addition above ambient concentrations and the degree of nutrient limitation. To test these predictions, we used data from an interbiome study of NH4+ uptake length in which 15NH4+ tracer and short-term NH4+ addition experiments were performed in 10 streams using a uniform experimental approach. The experimental results largely confirmed the theoretical predictions: SW′ was consistently longer than SW and SW′:SW ratios were directly related to the level of NH4+ addition and to indicators of N limitation. The experimentally derived SW′:SW ratios were used with the theoretical results to infer the N limitation status of each stream. Together, the theoretical and experimental results showed that tracer experiments should be used whenever possible to determine nutrient uptake length in streams. Nutrient addition experiments may be useful for comparing uptake lengths between different streams or different times in the same stream, however, provided that nutrient additions are kept as low as possible and of similar magnitude.10aammonium10anitrogen limitation10anutrient cycling10anutrient spiraling10astream10auptake length1 aMulholland, P.J.1 aTank, J.L.1 aWebster, J.R.1 aBowden, W.B.1 aDodds, W., K.1 aGregory, S.V.1 aGrimm, N.B.1 aHamilton, S.K.1 aJohnson, S.L.1 aMarti, E.1 aMcDowell, W.H.1 aMerriam, J.1 aMeyer, J.L.1 aPeterson, B.J.1 aValett, H.M.1 aWollheim, W.M.  uhttp://knz.lternet.edu/content/can-uptake-length-streams-be-determined-nutrient-addition-experiments-results-inter-biome01724nas a2200289   4500008004100000245006800041210006800109300001100177490000800188520088600196100001901082700001901101700002101120700001801141700001601159700001501175700001401190700001701204700001701221700001801238700001901256700001801275700001901293700001601312700001801328856008801346       2001                        eng d00aControl of nitrogen export from watersheds by headwater streams0 aControl of nitrogen export from watersheds by headwater streams  a86 -900 v2923 aA comparative 15N-tracer study of nitrogen dynamics in headwater streams from biomes throughout North America demonstrates that streams exert control over nutrient exports to rivers, lakes, and estuaries. The most rapid uptake and transformation of inorganic nitrogen occurred in the smallest streams. Ammonium entering these streams was removed from the water within a few tens to hundreds of meters. Nitrate was also removed from stream water but traveled a distance 5 to 10 times as long, on average, as ammonium. Despite low ammonium concentration in stream water, nitrification rates were high, indicating that small streams are potentially important sources of atmospheric nitrous oxide. During seasons of high biological activity, the reaches of headwater streams typically export downstream less than half of the input of dissolved inorganic nitrogen from their watersheds.1 aPeterson, B.J.1 aWollheim, W.M.1 aMulholland, P.J.1 aWebster, J.R.1 aMeyer, J.L.1 aTank, J.L.1 aMarti, E.1 aBowden, W.B.1 aValett, H.M.1 aHershey, A.E.1 aMcDowell, W.H.1 aDodds, W., K.1 aHamilton, S.K.1 aGregory, S.1 aMorrall, D.D.  uhttp://knz.lternet.edu/content/control-nitrogen-export-watersheds-headwater-streams