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Nitrous oxide (N 2 O) 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 N 2 O via microbial denitrification that converts N to N 2 O and dinitrogen (N 2 ). The fraction of denitrified N that escapes as N 2 O rather than N 2 (i.e., the N 2 O yield) is an important determinant of how much N 2 O is produced by river networks, but little is known about the N 2 O yield in flowing waters. Here, we present the results of whole-stream 15 N-tracer additions conducted in 72 headwater streams draining multiple land-use types across the United States. We found that stream denitrification produces N 2 O at rates that increase with stream water nitrate (NO 3 − ) concentrations, but that <1% of denitrified N is converted to N 2 O. Unlike some previous studies, we found no relationship between the N 2 O yield and stream water NO 3 − . We suggest that increased stream NO 3 − loading stimulates denitrification and concomitant N 2 O production, but does not increase the N 2 O yield. In our study, most streams were sources of N 2 O 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 N 2 O in river networks, equivalent to 10% of the global anthropogenic N 2 O emission rate. This estimate of stream and river N 2 O emissions is three times greater than estimated by the Intergovernmental Panel on Climate Change.H umans 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 (N 2 O), 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 N 2 O in soils and aquatic ecosystems is the largest source of anthropogenic N 2 O to the atmosphere (2). The production of N 2 O 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 N 2 O budget (4). However, emissions of anthropogenic N 2 O from streams, rivers, and estuaries have received much less attention and remain a major source of uncertainty in the global anthropogenic N 2 O budget.Microbial denitrification is a large source of N 2 O emissions in terrestrial and aquatic ecosystems. Most microbial denitrification is a form of anaerobic respiration in which nitrate (NO 3 − , the dominant form of inorganic N) is converted to dinitrogen (N 2 ) and N 2 O gases (5). The proportion of denitrified NO 3 − that is converted to N 2 O rather than N 2 (h...

The temperature of stream water is an important control of many in-stream processes. To better understand the processes and consequences of solar energy inputs to streams, stream temperature dynamics were examined before, during, and after experimental shading of a 150-m reach of a second-order stream in the Oregon Cascade Range. Maximum water temperatures declined significantly in the shaded reach, but minimum and mean temperatures were not modified. Heat budget calculations before shading show the dominance of solar energy as an influence of stream temperature. The influence of substrate type on stream temperature was examined separately where the water flowed first over bedrock and then through alluvial substrates. Maximum temperatures in the upstream bedrock reach were up to 8.6°C higher and 3.4°C lower than downstream in the alluvial reach. Better understanding of factors that influence not only maximum but minimum temperatures as well as diurnal temperature variation will highlight types of reaches in which stream temperature would be most responsive to changes in shading. Many apparent discrepancies in stream temperature literature can be explained by considering variation in the relative importance of different stream temperature drivers within and among streams and over time.Résumé : Dans les cours d'eau, la température de l'eau est un important facteur de contrôle de plusieurs processus internes. Afin de mieux comprendre les processus reliés à l'apport d'énergie solaire dans les cours d'eau et d'en éva-luer les conséquences, nous avons examiné la dynamique thermique avant, pendant et après une expérience dans laquelle nous avons ombragé expérimentalement une section de 150 m d'un ruisseau d'ordre deux dans la chaîne des monts Cascades en Oregon. Les températures maximales de l'eau ont décru significativement dans la section ombragée, mais les températures minimales et moyennes sont restées inchangées. Les calculs de bilans thermiques avant l'expérience ont montré que l'énergie solaire a une influence dominante sur la température du cours d'eau. L'effet du type de substrat sur la température du cours d'eau a pu être examiné séparément là où l'eau coule d'abord sur la roche-mère pour ensuite traverser des substrats alluviaux. Les températures maximales dans la section de roche-mère d'amont sont jusqu'à 8,6°C supérieures et 3,4°C inférieures à celles de la section alluviale d'aval. Une meilleure compréhension des facteurs qui influencent non seulement les températures maximales et minimales, mais aussi la variation journalière de la température, permettra d'identifier les sections dans lesquelles la température du cours d'eau est plus susceptible d'être affectée par les changements d'ombrage. Plusieurs contradictions apparentes dans la littéra-ture scientifique concernant la température des cours d'eau peuvent s'expliquer en considérant les variations dans le temps, ainsi que dans un même cours d'eau et d'un cours d'eau à un autre, de l'importance relative des différents facteurs qui régissent la températu...

1. 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. . 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.

Summary 1. 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.

Spatial models of under-canopy temperature show that old-growth forests are cooler in spring months than mature forest plantations.

[1] The hydrologic and biogeochemical responses of forested watersheds to inputs of rainfall and snowmelt can be an indicator of internal watershed function. In this study, we assess how the quantity and quality, both chemical and spectroscopic, of stream water DOC changes in response to a 6-day storm event during the wet season of 2003 in three small (<1 km 2 ) basins in the H. J. Andrews Experimental Forest, Oregon. The watersheds included one old-growth watershed (WS02) and two previously logged watersheds (WS01 and WS10). Prestorm concentrations of DOC ranged from 1.5 to 2.2 mg C L À1 in the three watersheds and increased approximately threefold on the ascending limb of the storm hydrograph. Concentrations of DOC were both highest in the unharvested, old-growth watershed. The specific UV absorbance (SUVA, 254 nm) of DOC in the three watersheds increased by 9 to 36% during the storm, suggesting that DOC mobilized from catchment soils during storms is more aromatic than DOC entering the stream during baseflow. The increase in SUVA was most pronounced in the previously harvested catchments. Chromatographic fractionation of DOC showed that the percentage of DOC composed of non-humic material decreasing by 9 to 22% during the storm. Shifts in the fluorescence properties of DOC suggest that there was not a pronounced change in the relative proportion of stream water DOC derived from allochthonous versus autochthonous precursor material. Taken together, these results suggest that spectroscopic and chemical characterization of DOC can be used as tools to investigate changing sources of DOC and water within forested watersheds.Citation: Hood, E., M. N. Gooseff, and S. L. Johnson (2006), Changes in the character of stream water dissolved organic carbon during flushing in three small watersheds, Oregon,

Stream temperature controls the rates of many biotic and abiotic processes and is influenced by changes in streamside land use practices. We compiled historic stream temperature data and reestablished study sites in three small basins in the H.J. Andrews Experimental Forest in the western Cascades, Oregon, to reexamine the effects on and recovery of stream temperatures following removal of riparian vegetation. Maximum stream temperatures increased 7°C and occurred earlier in the summer after clear-cutting and burning in one basin and after debris flows and patch-cutting in another. Diurnal fluctuations in June increased from approximately 2 to 8°C. Stream temperatures in both basins gradually returned to preharvest levels after 15 years. The influence of the primary factor controlling stream temperatures, shortwave solar radiation, was amplified following removal of riparian vegetation, and conduction between stream water and nearby soils or substrates also appeared to be an important factor. Shifts in the timing of summer maxima and greater increases in early summer stream temperatures could impact sensitive stages of aquatic biota.Résumé : La température des cours d'eau régit le rythme de nombreux processus biotiques et abiotiques, et se trouve sous l'influence des changements dans les pratiques d'aménagement du territoire sur les rives. Nous avons compilé les données historiques sur la température des cours d'eau et rétabli des stations d'étude dans trois petits bassins de la forêt expérimentale H.J. Andrews, dans l'ouest des Cascades (Oregon), pour réexaminer les effets de l'enlèvement de la végétation riveraine sur la température des cours d'eau, et le rétablissement subséquent. Les températures maximales des cours d'eau ont augmenté de 7°C, et les augmentations se sont produites plus tôt dans l'été, après la coupe à blanc et le brûlage dans un bassin, et après des apports de débris et du jardinage par bouquets dans un autre. Les fluctuations diurnes en juin ont augmenté d'environ 2 à 8°C. Dans les deux bassins, les températures des cours d'eau sont graduellement revenues aux niveaux pré-exploitation au bout de 15 ans. L'influence du principal facteur régissant la température des cours d'eau, le rayonnement solaire à ondes courtes, a été amplifiée par suite de l'élimination de la végétation riveraine, et la conduction entre l'eau de la rivière et les sols ou les substrats adjacents semblait aussi un facteur important. Les changements dans l'occurrence des maximums d'été, et les augmentations plus fortes des températures des cours d'eau au début de l'été, pourraient avoir un impact sur les stades vulnérables du biote aquatique.[Traduit par la Rédaction] Johnson and Jones 39