Lesley K. Smith

Methane emissions from the Orinoco River floodplain, Venezuela

Methane emissions were measured over a 17-monthinterval at 21 locations on the Orinoco fringingfloodplain and upper delta (total area,14,000 km2). Emissions totaled 0.17 Tgyr−1, or 7.1 mmol d−1 (114 mg d−1;standard deviation, ±18%) per m2 of watersurface. Ebullition accounted for 65% of emissions. Emission rates were about five times as high forfloodplain forest as for open water or macrophytemats. Emission rates were positively correlated withcarbon content of sediment and amount of methane inthe water column, and negatively correlated withdissolved oxygen, but the correlations were weak. Emission from floodplain soils occurred only when thewater content of soil exceeded 25%, which occurredwithin 20 m of standing water during floodplaindrainage (3 months/yr). Bare soils emitted 60mmol/day per m of shoreline length; soils covered bystranded macrophyte beds emitted five times thisamount. Total emissions were accounted for primarilyby flooded forest (94%); macrophyte mats, open water,and exposed soils made only small contributions. Theflux-weighted mean δ13C for the floodplainwas −62 ± 8‰; for δD the mean was −271 ± 27‰. The δ13C and δD were negativelycorrelated. Overall emission rates were notably lowerthan for the Amazon. The depth and duration offlooding are considerably less for the Orinoco thanfor the Amazon floodplain; oxygen over sediments isthe rule for the Orinoco but not for the Amazon. TheOrinoco data illustrate the difficulty of generalizingemission rates. Current information for tropicalAmerica, including revised estimates for inundatedarea along the Amazon, indicate that methane emissionsfrom tropical floodplains have been overestimated.

Denitrification in nitrate-rich streams: Application of N2:Ar and 15N-tracer methods in intact cores

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.

Investigation of denitrification rates in an ammonia-dominated constructed wastewater treatment wetland

Denitrification measurements were made under simulated field conditions using sediment cores and water collected from the Hemet/San Jacinto Multipurpose Demonstration Wetland (Riverside Country, California, USA). The 9.9 ha constructed wetland is used to both polish ammonia-dominated secondary municipal effluent and provide migratory bird habitat. The wetland was originally constructed as a marshpond-marsh system in 1994. Over the period from January through March 1998, measured denitrification rates averaged 20.9±20.9 μmol N m−2 h−1 within the emergent marsh portions of the wetland and 646±353 μmol N m−2 h−1 in open water areas. The mean areal denitrification removal rate for this period was 0.70 kg N ha−1 d−1, which accounted for about 8% of the total N removed by the wetland. Internal retention was the main N-removal mechanism. Synoptic water quality surveys indicated that denitrification was limited by a lack of nitrification within the wetland. Between April 1998 and January 1999, the wetland was reconfigured as a hemi-marsh system, having equal areas of interspersed emergent marsh and deep open water. In May 1999, measured denitrification rates averaged 1414±298 μmol N m−2 h−1 within the emergent marsh zones and 682±218 μmol N m−2 h−1 in the open water areas. The mean areal denitrification removal rate was 3.58 kg N ha−1 d−1, which accounted for 40% of the total N removed by the wetland. A synoptic water quality survey indicated that nitrification within the wetland had been enhanced by the reduction of emergent macrophyte biomass and the increase in the area of interspersed deep open water. The modifications to the wetland shifted the nitrogen balance from a large internal storage component and a small denitrification component in 1998 to a more denitrifying system in 1999.

Landscape patterns of CH4 fluxes in an alpine tundra ecosystem

We measured CH4 fluxes from three major plant communities characteristic of alpine tundra in the Colorado Front Range. Plant communities in this ecosystem are determined by soil moisture regimes induced by winter snowpack distribution. Spatial patterns of CH4 flux during the snow-free season corresponded roughly with these plant communities. In Carex-dominated meadows, which receive the most moisture from snowmelt, net CH4 production occurred. However, CH4 production in one Carex site (seasonal mean = +8.45 mg CH4 m-2 d-1) was significantly larger than in the other Carex sites (seasonal means = –0.06 and +0.05 mg CH4 m-2 d-1). This high CH4 flux may have resulted from shallower snowpack during the winter. In Acomastylis meadows, which have an intermediate moisture regime, CH4 oxidation dominated (seasonal mean = –0.43 mg CH4 m-2 d-1). In the windswept Kobresia meadow plant community, which receive the least amount of moisture from snowmelt, only CH4 oxidation was observed (seasonal mean = –0.77 mg CH4 m-2 d-1). Methane fluxes correlated with a different set of environmental factors within each plant community. In the Carex plant community, CH4 emission was limited by soil temperature. In the Acomastylis meadows, CH4 oxidation rates correlated positively with soil temperature and negatively with soil moisture. In the Kobresia community, CH4 oxidation was stimulated by precipitation. Thus, both snow-free season CH4 fluxes and the controls on those CH4 fluxes were related to the plant communities determined by winter snowpack.

Lens on Climate Change: Making Climate Meaningful Through Student-Produced Videos

Learning about climate change is tangible when it addresses impacts that can be observed close to home. In this program, sixty-four diverse middle and high school students produced videos about locally relevant climate change topics. Graduate and undergraduate students provided mentorship. The program engaged students in research and learning about climate change, and sparked their interest in science careers. Evaluation results showed that students were highly motivated by the experience, developed a genuine interest in their science topic, learned about the scientific process, and developed twenty-first century skills. The program provided a unique and authentic approach to science learning and communication.

Helping Scientists Become Effective Partners in Education and Outreach

How does a scientist find herself standing before a group of lively third-graders? She may be personally motivated—seeking to improve public understanding of scientific issues and the nature of science, or to see her own children receive a good science education—or perhaps she simply enjoys this kind of work [Andrews et al., 20057semi; Kim and Fortner, 2008]. In addition to internal motivating factors, federal funding agencies have begun to encourage scientists to participate in education and outreach (E/O) related to their research, through NASA program requirements for such activities (see “Implementing the Office of Space Science Education/Public Outreach Strategy,” at http://spacescience.nasa.gov/admin/pubs/edu/imp_plan .htm) and the U.S. National Science Foundations increased emphasis on “broader impacts” in merit review of research proposals (see http://www.nsf.gov/pubs/2003/nsf032/bicexamples.pdf).

Seasonal Variations in the Isotopic Composition of Methane Associated with Aquatic Macrophytes

Samples of methane bubbles were collected from sediments populated by emergent aquatic macrophytes during the early rising water period on the Amazon River floodplain. Methane δ13C was distinctly lighter (-61.3 ± 0.3‰, n = 3) in bubbles stirred from sediments populated by the rooted C-3 plant Oryza perinnis than it was in bubbles from sediments containing the rooted C-4 plants Paspalum repens S (-52.0± 1.1‰, n = 6) or Echinochloa polystachya (-48.3± 0.2‰, n = 4). Leaf carbon δ 13C was -28.2‰ in Oryza and -11.4‰ in Paspalum. In August, just after the peak flood stage of the river, the Paspalum, now a detached floating mat, was sampled again. Methane bubbles trapped within the floating meadow had an isotopic composition of -43.5 ± 0.1‰(n = 3), suggesting an increase in the importance of microbial methane oxidation as the emergent aquatic macrophytes progressed through their annual life cycle from a rooted plant to a floating meadow. The life cycle of these macrophytes may be partly responsible for the observed 10‰ seasonal variation in the isotopic composition of methane emitted from the Central Amazon floodplain.