Denitrification is an important net sink for NO 3 -in streams, but direct measurements are limited and in situ controlling factors are not well known. We measured denitrification at multiple scales over a range of flow conditions and NO 3 -concentrations in streams draining agricultural land in the upper Mississippi River basin. Comparisons of reach-scale measurements (in-stream mass transport and tracer tests) with local-scale in situ measurements (pore-water profiles, benthic chambers) and laboratory data (sediment core microcosms) gave evidence for heterogeneity in factors affecting benthic denitrification both temporally (e.g., seasonal variation in NO 3 -concentrations and loads, flood-related disruption and re-growth of benthic communities and organic deposits) and spatially (e.g., local stream morphology and sediment characteristics). When expressed as vertical denitrification flux per unit area of streambed (U denit , in), results of different methods for a given set of conditions commonly were in agreement within a factor of 2-3. At approximately constant temperature (*20 ± 4°C) and with minimal benthic disturbance, our aggregated data indicated an overall positive relation between U denit (*0-4,000 lmol N m -2 h -1 ) and stream NO 3 -concentration (*20-1,100 lmol L -1 ) representing seasonal variation from spring high flow (high NO 3 -) to late summer low flow (low NO 3 -). The temporal dependence of U denit on NO 3-was less than first-order and could be described about equally well with power-law or saturation equations (e.g., for the unweighted dataset, -008-9282-8 in m day -1 ) at seasonal and possibly event time scales; (2) although k1 denit was relatively large at low flow (low NO 3 -), its impact on annual loads was relatively small because higher concentrations and loads at high flow were not fully compensated by increases in U denit ; and (3) although NO 3 -assimilation and denitrification were linked through production of organic reactants, rates of NO 3 -loss by these processes may have been partially decoupled by changes in flow and sediment transport. Whereas k1 denit and v f,denit are linked implicitly with stream depth, NO 3 -concentration, and(or) NO 3 -load, estimates of U denit may be related more directly to field factors (including NO 3 -concentration) affecting denitrification rates in benthic sediments. Regional regressions and simulations of benthic denitrification in stream networks might be improved by including a non-linear relation between U denit and stream NO 3 -concentration and accounting for temporal variation.

Rates of benthic denitrification were measured using two techniques, membrane inlet mass spectrometry (MIMS) and isotope ratio mass spectrometry (IRMS), applied to sediment cores from two NO3(-)-rich streams draining agricultural land in the upper Mississippi River Basin. Denitrification was estimated simultaneously from measurements of N2:Ar (MIMS) and 15N[N2] (IRMS) after the addition of low-level 15NO3- tracer (15N:N = 0.03-0.08) in stream water overlying intact sediment cores. Denitrification rates ranged from about 0 to 4400 micromol N x m(-2) x h(-1) in Sugar Creek and from 0 to 1300 micromol N x m(-2) x h(-1) in Iroquois River, the latter of which possesses greater streamflow discharge and a more homogeneous streambed and water column. Within the uncertainties of the two techniques, there is good agreement between the MIMS and IRMS results, which indicates that the production of N2 by the coupled process of nitrification/denitrification was relatively unimportant and surface-water NO3- was the dominant source of NO3- for benthic denitrification in these streams. Variation in stream NO3- concentration (from about 20 micromol/L during low discharge to 1000 micromol/L during high discharge) was a significant control of benthic denitrification rates, judging from the more abundant MIMS data. The interpretation that NO3- concentration directly affects denitrification rate was corroborated by increased rates of denitrification in cores amended with NO3-. Denitrification in Sugar Creek removed < or = 11% per day of the instream NO3- in late spring and removed roughly 15-20% in late summer. The fraction of NO3- removed in Iroquois River was less than that of Sugar Creek. Although benthic denitrification rates were relatively high during periods of high stream flow, when NO3 concentrations were also high, the increase in benthic denitrification could not compensate for the much larger increase in stream NO3- fluxes during high flow. Consequently, fractional NO3- losses were relatively low during high flow.

Methane emissions were measured over a 15‐month interval for five lakes and five associated wetland sites in the Rockies of Colorado at elevations between 2800 and 3600 m. Three of the five lakes accumulated dissolved methane under ice; accumulation was as high as 53‐fold above ice‐free water column concentrations in the shallowest lake. The combination of high dissolved oxygen and low dissolved inorganic nitrogen concentrations within the water column during ice breakup suggests that methane emissions rather than methane oxidation led to the substantial loss of dissolved methane from the shallowest lake at the time of spring thaw. A pulsed release of methane to the atmosphere at the time of ice breakup within lakes may be widespread at high latitudes and may play a role in the observed increase in tropospheric methane concentrations in the northern hemisphere. During the ice‐free season, the mean emission rate was 1.6 mmol m−2 d−1 over open water for the five lakes, but the mean was much higher (13 mmol m−2 d−1) over the Nuphar lutea beds located in one of the lakes. Open water emissions occurred primarily through diffusion rather than bubbling. For wetlands near the lakes, average lake emissions ranged from negligible to almost 6 mmol m−2 d−1; the average across all sites was 2.1 mmol m−2 d−1 during the warm season. There was no measurable emission during the winter months. Surface dissolved methane explains 40% of the variation in emissions from the open water sites; the combination of soil organic C content and soil temperature explains 40% of variation in emissions from the wetland sites. The data from the Southern Rockies and information that has accumulated on other lake types over the last 15 years indicate that lakes may be a larger source strength of methane than reported estimates.

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