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
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).
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 aAgricultural 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–aquatic linkages, hydrologic exchanges between the channel, floodplain/riparian complex, and subsurface waters, and interactions between coupled biogeochemical cycles.
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/10.1890/08021103833nas a2200493 4500008004100000245007100041210006900112300001500181490000700196520248100203100001702684700001702701700001502718700002102733700001802754700001802772700001502790700001902805700001702824700001502841700001802856700001602874700001902890700001802909700001902927700001602946700001702962700001602979700001702995700001703012700001903029700001703048700001703065700001703082700001603099700001603115700002303131700001803154700001703172700001903189700001703208700001503225856009903240 2010 eng d00aInter-regional comparison of land-use effects on stream metabolism0 aInterregional comparison of landuse effects on stream metabolism a1874 -18900 v553 a1. Rates of whole-system metabolism (production and respiration) are fundamental indicators of ecosystem structure and function. Although first-order, proximal controls are well understood, assessments of the interactions between proximal controls and distal controls, such as land use and geographic region, are lacking. Thus, the influence of land use on stream metabolism across geographic regions is unknown. Further, there is limited understanding of how land use may alter variability in ecosystem metabolism across regions. 2. Stream metabolism was measured in nine streams in each of eight regions (n = 72) across the United States and Puerto Rico. In each region, three streams were selected from a range of three land uses: agriculturally influenced, urban-influenced, and reference streams. Stream metabolism was estimated from diel changes in dissolved oxygen concentrations in each stream reach with correction for reaeration and groundwater input. 3. Gross primary production (GPP) was highest in regions with little riparian vegetation (sagebrush steppe in Wyoming, desert shrub in Arizona/New Mexico) and lowest in forested regions (North Carolina, Oregon). In contrast, ecosystem respiration (ER) varied both within and among regions. Reference streams had significantly lower rates of GPP than urban or agriculturally influenced streams. 4. GPP was positively correlated with photosynthetically active radiation and autotrophic biomass. Multiple regression models compared using Akaike’s information criterion (AIC) indicated GPP increased with water column ammonium and the fraction of the catchment in urban and reference land-use categories. Multiple regression models also identified velocity, temperature, nitrate, ammonium, dissolved organic carbon, GPP, coarse benthic organic matter, fine benthic organic matter and the fraction of all land-use categories in the catchment as regulators of ER. 5. Structural equation modelling indicated significant distal as well as proximal control pathways including a direct effect of land-use on GPP as well as SRP, DIN, and PAR effects on GPP; GPP effects on autotrophic biomass, organic matter, and ER; and organic matter effects on ER. 6. Overall, consideration of the data separated by land-use categories showed reduced inter-regional variability in rates of metabolism, indicating that the influence of agricultural and urban land use can obscure regional differences in stream metabolism.1 aBernot, M.J.1 aSobota, D.J.1 aHall, R.O.1 aMulholland, P.J.1 aDodds, W., K.1 aWebster, J.R.1 aTank, J.L.1 aAshkenas, L.R.1 aCooper, L.W.1 aDahm, C.N.1 aGregory, S.V.1 aGrimm, N.B.1 aHamilton, S.K.1 aJohnson, S.L.1 aMcDowell, W.H.1 aMeyer, J.L.1 aPeterson, B.1 aPoole, G.C.1 aValett, H.M.1 aArango, C.P.1 aBeaulieu, J.J.1 aBurgin, A.J.1 aCrenshaw, C.1 aHelton, A.M.1 aJohnson, L.1 aMerriam, J.1 aNiederlehner, B.R.1 aO'Brien, J.M.1 aPotter, J.D.1 aSheibley, R.W.1 aThomas, S.M.1 aWilson, K. uhttp://lter.konza.ksu.edu/content/inter-regional-comparison-land-use-effects-stream-metabolism03146nas a2200469 4500008004100000245009200041210006900133300001300202490000700215520184400222100001502066700001502081700001702096700002102113700001802134700001802152700001802170700001702188700001602205700001902221700001602240700001902256700001802275700001902293700001602312700001802328700001502346700001702361700001902378700001702397700001902414700001702433700002302450700001802473700001702491700001902508700001702527700001702544700001902561700001702580856007902597 2009 eng d00aNitrate removal in stream ecosystems measured by 15N addition experiments: Total uptake0 aNitrate removal in stream ecosystems measured by 15N addition ex a653 -6650 v543 aWe measured uptake length of 15NO3− in 72 streams in eight regions across the United States and Puerto Rico to develop quantitative predictive models on controls of NO3− uptake length. As part of the Lotic Intersite Nitrogen eXperiment II project, we chose nine streams in each region corresponding to natural (reference), suburban-urban, and agricultural land uses. Study streams spanned a range of human land use to maximize variation in NO3− concentration, geomorphology, and metabolism. We tested a causal model predicting controls on NO3− uptake length using structural equation modeling. The model included concomitant measurements of ecosystem metabolism, hydraulic parameters, and nitrogen concentration. We compared this structural equation model to multiple regression models which included additional biotic, catchment, and riparian variables. The structural equation model explained 79% of the variation in log uptake length (SWtot). Uptake length increased with specific discharge (Q/w) and increasing NO3− concentrations, showing a loss in removal efficiency in streams with high NO3− concentration. Uptake lengths shortened with increasing gross primary production, suggesting autotrophic assimilation dominated NO3− removal. The fraction of catchment area as agriculture and suburban-urban land use weakly predicted NO3− uptake in bivariate regression, and did improve prediction in a set of multiple regression models. Adding land use to the structural equation model showed that land use indirectly affected NO3− uptake lengths via directly increasing both gross primary production and NO3− concentration. Gross primary production shortened SWtot, while increasing NO3− lengthened SWtot resulting in no net effect of land use on NO3− removal.
1 aHall, R.O.1 aTank, J.L.1 aSobota, D.J.1 aMulholland, P.J.1 aO'Brien, J.M.1 aDodds, W., K.1 aWebster, J.R.1 aValett, H.M.1 aPoole, G.C.1 aPeterson, B.J.1 aMeyer, J.L.1 aMcDowell, W.H.1 aJohnson, S.L.1 aHamilton, S.K.1 aGrimm, N.B.1 aGregory, S.V.1 aDahm, C.N.1 aCooper, L.W.1 aAshkenas, L.R.1 aThomas, S.M.1 aSheibley, R.W.1 aPotter, J.D.1 aNiederlehner, B.R.1 aJohnson, L.T.1 aHelton, A.M.1 aCrenshaw, C.M.1 aBurgin, A.J.1 aBernot, M.J.1 aBeaulieu, J.J.1 aArango, C.P. uhttps://aslopubs.onlinelibrary.wiley.com/doi/abs/10.4319/lo.2009.54.3.065303136nas a2200481 4500008004100000245009500041210006900136300001300205490000700218520179900225100002102024700001502045700001702060700001802077700002002095700001602115700001902131700001902150700001802169700001502187700001902202700001702221700001502238700001802253700001802271700001602289700001902305700001602324700001702340700001802357700001702375700001902392700001702411700001702428700001902445700001702464700001802481700002302499700001702522700001902539700001702558856007902575 2009 eng d00aNitrate removal in stream ecosystems measured by 15N addition experiments: Denitrification0 aNitrate removal in stream ecosystems measured by 15N addition ex a666 -6800 v543 aWe measured denitrification rates using a field 15NO3− tracer-addition approach in a large, cross-site study of nitrate uptake in reference, agricultural, and suburban-urban streams. We measured denitrification rates in 49 of 72 streams studied. Uptake length due to denitrification (SWdenn) ranged from 89 m to 184 km (median of 9050 m) and there were no significant differences among regions or land-use categories, likely because of the wide range of conditions within each region and land use. N2 production rates far exceeded N2O production rates in all streams. The fraction of total NO3− removal from water due to denitrification ranged from 0.5% to 100% among streams (median of 16%), and was related to NH4+ concentration and ecosystem respiration rate (ER). Multivariate approaches showed that the most important factors controlling SWden were specific discharge (discharge / width) and NO3− concentration (positive effects), and ER and transient storage zones (negative effects). The relationship between areal denitrification rate (Uden) and NO3− concentration indicated a partial saturation effect. A power function with an exponent of 0.5 described this relationship better than a Michaelis-Menten equation. Although Uden increased with increasing NO3− concentration, the efficiency of NO3− removal from water via denitrification declined, resulting in a smaller proportion of streamwater NO3− load removed over a given length of stream. Regional differences in stream denitrification rates were small relative to the proximate factors of NO3− concentration and ecosystem respiration rate, and land use was an important but indirect control on denitrification in streams, primarily via its effect on NO3− concentration.
1 aMulholland, P.J.1 aHall, R.O.1 aSobota, D.J.1 aDodds, W., K.1 aFindlay, S.E.G.1 aGrimm, N.B.1 aHamilton, S.K.1 aMcDowell, W.H.1 aO'Brien, J.M.1 aTank, J.L.1 aAshkenas, L.R.1 aCooper, L.W.1 aDahm, C.N.1 aGregory, S.V.1 aJohnson, S.L.1 aMeyer, J.L.1 aPeterson, B.J.1 aPoole, G.C.1 aValett, H.M.1 aWebster, J.R.1 aArango, C.P.1 aBeaulieu, J.J.1 aBernot, M.J.1 aBurgin, A.J.1 aCrenshaw, C.L.1 aHelton, A.M.1 aJohnson, L.T.1 aNiederlehner, B.R.1 aPotter, J.D.1 aSheibley, R.W.1 aThomas, S.M. uhttps://aslopubs.onlinelibrary.wiley.com/doi/abs/10.4319/lo.2009.54.3.066602900nas a2200481 4500008004100000245009100041210006900132300001300201490000800214520160100222100002101823700001701844700001601861700001501877700001901892700001901911700001501930700001901945700001701964700001501981700001801996700002002014700001802034700001602052700001802068700001902086700001602105700001702121700001802138700001702156700001902173700001702192700001702209700001702226700001602243700002302259700001802282700001702300700001902317700001702336700001702353856004802370 2008 eng d00aStream denitrification across biomes and its response to anthropogenic nitrate loading0 aStream denitrification across biomes and its response to anthrop a202 -2070 v4523 aAnthropogenic addition of bioavailable nitrogen to the biosphere is increasing1, 2 and terrestrial ecosystems are becoming increasingly nitrogen-saturated3, causing more bioavailable nitrogen to enter groundwater and surface waters4, 5, 6. Large-scale nitrogen budgets show that an average of about 20–25 per cent of the nitrogen added to the biosphere is exported from rivers to the ocean or inland basins7, 8, indicating that substantial sinks for nitrogen must exist in the landscape9. Streams and rivers may themselves be important sinks for bioavailable nitrogen owing to their hydrological connections with terrestrial systems, high rates of biological activity, and streambed sediment environments that favour microbial denitrification6, 10, 11. Here we present data from nitrogen stable isotope tracer experiments across 72 streams and 8 regions representing several biomes. We show that total biotic uptake and denitrification of nitrate increase with stream nitrate concentration, but that the efficiency of biotic uptake and denitrification declines as concentration increases, reducing the proportion of in-stream nitrate that is removed from transport. Our data suggest that the total uptake of nitrate is related to ecosystem photosynthesis and that denitrification is related to ecosystem respiration. In addition, we use a stream network model to demonstrate that excess nitrate in streams elicits a disproportionate increase in the fraction of nitrate that is exported to receiving waters and reduces the relative role of small versus large streams as nitrate sinks.
1 aMulholland, P.J.1 aHelton, A.M.1 aPoole, G.C.1 aHall, R.O.1 aHamilton, S.K.1 aPeterson, B.J.1 aTank, J.L.1 aAshkenas, L.R.1 aCooper, L.W.1 aDahm, C.N.1 aDodds, W., K.1 aFindlay, S.E.G.1 aGregory, S.V.1 aGrimm, N.B.1 aJohnson, S.L.1 aMcDowell, W.H.1 aMeyer, J.L.1 aValett, H.M.1 aWebster, J.R.1 aArango, C.P.1 aBeaulieu, J.J.1 aBernot, M.J.1 aBurgin, A.J.1 aCrenshaw, C.1 aJohnson, L.1 aNiederlehner, B.R.1 aO'Brien, J.M.1 aPotter, J.D.1 aSheibley, R.W.1 aSobota, D.J.1 aThomas, S.M. uhttps://www.nature.com/articles/nature06686