Global Change Abstracts The Swiss Contribution - SCNAT

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154 Global Change Abstracts – The Swiss Contribution | Coupled Systems and Cycles 08.1-303 Effects of increased soil water availability on grassland ecosystem carbon dioxide fluxes Risch A C, Frank D A USA, Switzerland Agriculture, Soil Sciences , Ecology , Plant Sciences There is considerable interest in how ecosystems will respond to changes in precipitation. Alterations in rain and snowfall are expected to influence the spatio-temporal patterns of plant and soil processes that are controlled by soil moisture, and potentially, the amount of carbon (C) exchanged between the atmosphere and ecosystems. Because grasslands cover over one third of the terrestrial landscape, understanding controls on grassland C processes will be important to forecast how changes in precipitation regimes will influence the global C cycle. In this study we examined how irrigation affects carbon dioxide (CO 2) fluxes in five widely variable grasslands of Yellowstone National Park during a year of approximately average growing season precipitation. We irrigated plots every 2 weeks with 25% of the monthly 30-year average of precipitation resulting in plots receiving approximately 150% of the usual growing season water in the form of rain and supplemented irrigation. Ecosystem CO 2 fluxes were measured with a closed chamber-system once a month from May- September on irrigated and unirrigated plots in each grassland. Soil moisture was closely associated with CO 2 fluxes and shoot biomass, and was between 1.6% and 11.5% higher at the irrigated plots (values from wettest to driest grassland) during times of measurements. When examining the effect of irrigation throughout the growing season (May-September) across sites, we found that water additions increased ecosystem CO 2 fluxes at the two driest and the wettest sites, suggesting that these sites were water-limited during the climatically average precipitation conditions of the 2005 growing season. In contrast, no consistent responses to irrigation were detected at the two sites with intermediate soil moisture. Thus, the ecosystem CO 2 fluxes at those sites were not water-limited, when considering their responses to supplemental water throughout the whole season. In contrast, when we explored how the effect of irrigation varied temporally, we found that irrigation increased ecosystem CO 2 fluxes at all the sites late in the growing season (September). The spatial differences in the response of ecosystem CO 2 fluxes to irrigation likely can be explained by site specific differences in soil and vegetation properties. The temporal effects likely were due to delayed plant senescence that promoted plant and soil activity later into the year. Our results suggest that in Yellowstone National Park, above-normal amounts of soil moisture will only stimulate CO 2 fluxes across a portion of the ecosystem. Thus, depending on the topographic location, grassland CO 2 fluxes can be water-limited or not. Such information is important to accurately predict how changes in precipitation/soil moisture will affect CO 2 dynamics and how they may feed back to the global C cycle. Biogeochemistry, 2007, V86, N1, OCT, pp 91-103. 08.1-304 Chemistry, transport and dry deposition of trace gases in the boundary layer over the tropical Atlantic Ocean and the Guyanas during the GABRIEL field campaign Stickler A, Fischer H, Bozem H, Gurk C, Schiller C, Martinez Harder M, Kubistin D, Harder H, Williams J, Eerdekens G, Yassaa N, Ganzeveld L, Sander R, Lelieveld J Switzerland, Germany, Canada Meteorology & Atmospheric Sciences , Modelling We present a comparison of different Lagrangian and chemical box model calculations with measurement data obtained during the GABRIEL campaign over the tropical Atlantic Ocean and the Amazon rainforest in the Guyanas, October 2005. Lagrangian modelling of boundary layer (BL) air constrained by measurements is used to derive a horizontal gradient (approximate to 5.6 pmol/mol km(-1)) of CO from the ocean to the rainforest (east to west). This is significantly smaller than that derived from the measurements (16-48 pmol/mol km(-1)), indicating that photochemical production from organic precursors alone cannot explain the observed strong gradient. It appears that HCHO is overestimated by the Lagrangian and chemical box models, which include dry deposition but not exchange with the free troposphere (FT). The relatively short lifetime of HCHO implies substantial BL-FT exchange. The mixing-in of FT air affected by African and South American biomass burning at an estimated rate of 0.12 h(-1) increases the CO and decreases the HCHO mixing ratios, improving agreement with measurements. A mean deposition velocity of 1.35 cm/s for H 2O 2 over the ocean as well as over the rainforest is deduced assuming BL-FT exchange adequate to the results for CO. The measured increase of the organic peroxides from the ocean to the rainforest (approximate to 0.66 nmol/mol d(-1)) is significantly overestimated by the Lagrangian model, even when using high values for the deposition velocity and the entrainment rate. Our results point at either heterogeneous loss of organic peroxides and/or their radical precursors, underestimated photodissociation
Global Change Abstracts – The Swiss Contribution | Coupled Systems and Cycles or missing reaction paths of peroxy radicals not forming peroxides in isoprene chemistry. We calculate a mean integrated daytime net ozone production (NOP) in the BL of (0.2+/-5.9) nmol/mol (ocean) and (2.4+ /-2.1) nmol/mol (rainforest). The NOP strongly correlates with NO and has a positive tendency in the boundary layer over the rainforest. Atmospheric Chemistry and Physics, 2007, V7, N14, pp 3933-3956. 08.1-305 Challenges in quantifying biosphere-atmosphere exchange of nitrogen species Sutton M A, Nemitz E, Erisman J W, Beier C, Butterbach Bahl K, Cellier P, de Vries W, Cotrufo F, Skiba U, Di Marco C, Jones S, Laville P, Soussana J F, Loubet B, Twigg M, Famulari D, Whitehead J, Gallagher M W, Neftel A, Flechard C R, Herrmann B, Calanca P, Schjoerring J K, Daemmgen U, Horvath L, Tang Y S, Emmett B A, Tietema A, Penuelas J, Kesik M, Brueggemann N, Pilegaard K, Vesala T, Campbell C L, Olesen J E, Dragosits U, Theobald M R, Levy P, Mobbs D C, Milne R, Viovy N, Vuichard N, Smith J U, Smith P, Bergamaschi P, Fowler D, Reis S Scotland, Netherlands, Denmark, Germany, France, Italy, England, Switzerland, Hungary, Wales, Spain, Finland Modelling , Meteorology & Atmospheric Sciences , Agriculture, Soil Sciences Recent research in nitrogen exchange with the atmosphere has separated research communities according to N form. The integrated perspective needed to quantify the net effect of N on greenhouse-gas balance is being addressed by the Nitro- Europe Integrated Project (NEU). Recent advances have depended on improved methodologies, while ongoing challenges include gas-aerosol interactions, organic nitrogen and N-2 fluxes. The NEU strategy applies a 3-tier Flux Network together with a Manipulation Network of global-change experiments, linked by common protocols to facilitate model application. Substantial progress has been made in modelling N fluxes, especially for N 2O, NO and bi-directional NH 3 exchange. Landscape analysis represents an emerging challenge to address the spatial interactions between farms, fields, ecosystems, catchments and air dispersion /deposition. European up-scaling of N fluxes is highly uncertain and a key priority is for better data on agricultural practices. Finally, attention is needed to develop N flux verification procedures to assess compliance with international protocols. Environmental Pollution, 2007, V150, N1, NOV, pp 125-139. 155 08.1-306 Sensitivity of carbon cycling in the European Alps to changes of climate and land cover Zierl B, Bugmann H Switzerland Forestry , Modelling , Plant Sciences , Ecology Assessments of the impacts of global change on carbon stocks in mountain regions have received little attention to date, in spite of the considerable role of these areas for the global carbon cycle. We used the regional hydro-ecological simulation system RHESSys in five case study catchments from different climatic zones in the European Alps to investigate the behavior of the carbon cycle under changing climatic and land cover conditions derived from the SRES scenarios of the IPCC. The focus of this study was on analyzing the differences in carbon cycling across various climatic zones of the Alps, and to explore the differences between the impacts of various SRES scenarios (A1FI, A2, B1, B2), and between several global circulation models (GCMs, i.e., HadCM3, CGCM2, CSIRO2, PCM). The simulation results indicate that the warming trend generally enhances carbon sequestration in these catchments over the first half of the twenty- first century, particularly in forests just below treeline. Thereafter, forests at low elevations increasingly release carbon as a consequence of the changed balance between growth and respiration processes, resulting in a net carbon source at the catchment scale. Land cover changes have a strong modifying effect on these climate- induced patterns. While the simulated temporal pattern of carbon cycling is qualitatively similar across the five catchments, quantitative differences exist due to the regional differences of the climate and land cover scenarios, with land cover exerting a stronger influence. The differences in the simulations with scenarios derived from several GCMs under one SRES scenario are of the same magnitude as the differences between various SRES scenarios derived from one single GCM, suggesting that the uncertainty in climate model projections needs to be narrowed before accurate impact assessments under the various SRES scenarios can be made at the local to regional scale. We conclude that the carbon balance of the European Alps is likely to shift strongly in the future, driven mainly by land cover changes, but also by changes of the climate. We recommend that assessments of carbon cycling at regional to continental scales should make sure to adequately include sub-regional differences of changes in climate and land cover, particularly in areas with a complex topography. Climatic Change, 2007, V85, N1-2, NOV, pp 195-212.
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