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AbstTactWater vapour and CO2 fluxes were measured using the eddy correlation method above and below the overstorey of a 21-m tall aspen stand in the boreal forest of central Saskatchewan as part of the Boreal Ecosystem-Atmosphere Study (BOREAS). Measurements were made at the 39.5-m and 4-m heights using 3-dimensional sonic anemometers (Kaijo-Denki and Solent, respectively) and closed-path gas analysers (LI-COR 6262) with 6-m and 4.7-m long heated sampling tubing, respectively. Continuous measurements were made from early October to mid-November 1993 and from early February to lateSeptember 1994. Soil CO2 flux (respiration) was measured using a LI-COR 6000-09 soil chamber and soil evaporation was measured using lysimetry.The leaf area index of the aspen and hazelnut understorey reached 1.8 and 3.3, respectively. The maximum daily evapotranspiration (£) rate was 5-6 mm d^^ Following leaf-out the hazelnut and soil accounted for 22% of the forest £. The estimated total £ was 403 mm for 1994. About 88% of the precipitation in 1994 was lost as evapotranspiration.During the growing season, the magnitude of half-hourly eddy fluxes of CO2 from the atmosphere into the forest reached 1.2 mg CO2 m'^ s^* (33 |imol C m~^ s"^) during the daytime. Downward eddy fluxes at the 4-m height were observed when the hazelnut was growing rapidly in June and July. Under well-ventilated night-time conditions, the eddy fluxes of CO2 above the aspen and hazelnut, corrected for canopy storage, increased exponentially with soil temperature at the 2-cm depth. Estimates of daytime respiration rates using these relationships agreed well with soil chamber measurements. During the 1994 growing season, the cumulative net ecosystem exchange (N££) was -3.5 t C ha"^ y"^ (a net gain by the system). For 1994, cumulative NEE, ecosystem respiration (K) and gross ecosystem photosynthesis (GEP = R-NEE) were estimated to be -1.3, 8.9 and 10.2 t C ha"^ y~^, respectively. Gross photosynthesis of the hazelnut was 32% of GEP.

[1] Nitrous oxide (N 2 O) is a greenhouse gas with a large global warming potential and is a major cause of stratospheric ozone depletion. Croplands are the dominant source of N 2 O, but mitigation strategies have been limited by the large uncertainties in both direct and indirect emission factors (EFs) implemented in "bottom-up" emission inventories. The Intergovernmental Panel on Climate Change (IPCC) recommends EFs ranging from 0.75% to 2% of the anthropogenic nitrogen (N) input for the various N 2 O pathways in croplands. Consideration of the global N budget yields a much higher EF ranging between 3.8% and 5.1% of the anthropogenic N input. Here we use 2 years of hourly high-precision N 2 O concentration measurements on a very tall tower to evaluate the IPCC bottom-up and global "top-down" EFs for a large representative subsection of the United States Corn Belt, a vast region spanning the U.S. Midwest that is dominated by intensive N inputs to support corn cultivation. Scaling up these results indicates that agricultural sources in the Corn Belt released 420˙50 Gg N (mean˙1 standard deviation; 1 Gg = 10 9 g) in 2010, in close agreement with the top-down estimate of 350˙50 Gg N and 80% larger than the bottom-up estimate based on the IPCC EFs (230˙180 Gg N). The large difference between the tall tower measurement and the bottom-up estimate implies the existence of N 2 O emission hot spots or missing sources within the landscape that are not fully accounted for in the IPCC and other bottom-up emission inventories. Reconciling these differences is an important step toward developing a practical mitigation strategy for N 2 O.

Subtropical lakes are important source of atmospheric methane (CH4). This study aims to investigate spatial variations of CH4 flux in Lake Taihu, a large (area 2400 km2) and shallow (mean depth 1.9 m) eutrophic lake in Eastern China. The lake exhibited high spatial variations in pollution level, macrophyte vegetation abundance, and algal growth. We measured the diffusion CH4 flux via the transfer coefficient method across the whole lake. In addition, data obtained with the flux gradient and the eddy covariance methods were used in conjunction with the data on the diffusion flux to estimate the contribution by ebullition. Results from 3 years' measurements indicated high spatial variabilities in the diffusion CH4 flux. The spatial pattern of the diffusion CH4 emission was correlated with water clarity, dissolved oxygen concentration, and the spatial distributions of algal and submerged vegetation. In comparison to the transfer coefficient method, the eddy covariance and the flux gradient method observed a lake CH4 flux that was 3.39 ± 0.58 (mean ± 1 standard deviation) and 1.95 ± 0.36 times higher in an open‐water eutrophic zone and in a habitat of submerged macrophytes, respectively. The result implied an average of 71% and 49% ebullition contribution to the total CH4 flux in the two zones. The annual mean diffusion CH4 flux of the whole lake was 0.54 ± 0.30 g m−2 yr−1. Our CH4 emission data suggest that the average CH4 emission reported previously for lakes in Eastern China was overestimated.

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The oxygen isotope composition of evapotranspiration (δ F ) represents an important tracer in the study of biosphere-atmosphere interactions, hydrology, paleoclimate, and carbon cycling. Here, we demonstrate direct measurement of δ F based on the eddy-covariance and tunable diode laser spectroscopy (EC-TDL) techniques. Results are presented from laboratory experiments and field measurements in agricultural ecosystems. The field measurements were obtained during the growing seasons of 2008 and 2009. Water vapour mixing ratios (χ w ) and fluxes (F) were compared using EC-TDL and traditional eddy-covariance and infrared gas analyser techniques over a soybean canopy in 2008. The results indicate that χ w and F agreed to within 1 and 6%, respectively. Measurements of δ F above a corn canopy in 2009 revealed a diurnal pattern with an expected progressive 18 O enrichment through the day ranging from about −20 before sunrise to about −5 in late afternoon. The isotopic composition of evapotranspiration was similar to the xylem water isotope composition (δ x = −7.2 ) for short periods of time during 1400-1800 LST, indicating near steady-state conditions. Finally, the isotopic forcing values (I F ) revealed a diurnal 123 308 T. J. Griffis et al.pattern with mean maximum values of 0.09 m s −1 at midday. The I F values could be described as an exponential relation of relative humidity confirming previous model calculations and measurements over a soybean canopy in 2006. These patterns and comparisons indicate that long-term continuous isotopic water vapour flux measurements based on the eddy-covariance technique are feasible and can provide new insights related to the oxygen isotope fractionation processes at the canopy scale.

[1] Quantifying isotopic CO 2 exchange between the biosphere and atmosphere presents a significant measurement challenge, but has the potential to provide important constraints on local, regional, and global carbon cycling. Past approaches have indirectly estimated isotopic CO 2 exchange using relaxed eddy accumulation, the flask-based isoflux method, and flux-gradient techniques. Eddy covariance (EC) is an attractive method because it has the fewest theoretical assumptions and the potential to give a direct measure of isotopic CO 2 flux, but it requires a highly sensitive and relatively fast response instrument. To date, no such field measurements have been reported. Here we describe the use of a closed-path tunable diode laser absorption spectroscopy and eddy covariance (EC-TDL) system for isotopic (C O transport showed that d N x was relatively independent of eddy scale for this ecosystem type. Flux loss, therefore, did not significantly bias d N x . There was excellent agreement between isofluxes (F d x ) measured using the flux-gradient and eddy covariance methods. Application of the EC-TDL technique over rougher surfaces or below canopy, where the flux-gradient approach is difficult to apply, appears promising for obtaining continuous long-term measurements of isotopic CO 2 exchange.

The U.S. Corn Belt is one of the most intensive agricultural regions of the world and is drained by the Upper Mississippi River (UMR), which forms one of the largest drainage basins in the U.S. While the effects of agricultural nitrate (NO3−) on water quality in the UMR have been well documented, its impact on the production of nitrous oxide (N2O) has not been reported. Using a novel equilibration technique, we present the largest data set of freshwater dissolved N2O concentrations (0.7 to 6 times saturation) and examine the controls on its variability over a 350 km reach of the UMR. Driven by a supersaturated water column, the UMR was an important atmospheric N2O source (+68 mg N2O N m−2 yr−1) that varies nonlinearly with the NO3− concentration. Our analyses indicated that a projected doubling of the NO3− concentration by 2050 would cause dissolved N2O concentrations and emissions to increase by about 40%.