greenhouse gas exchange, fluxes, flux partitioning, stable isotopes
The most important greenhouse gases from agricultural areas are carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). The high share of agricultural emissions that contribute 72% and 63% to the total Swiss emissions of N2O and CH4, respectively, required a more detailed understanding of the processes governing those greenhouse gas fluxes. Particularly in grasslands, where N2O fluxes are known to be relevant, and where CH4 may be taken up via oxidation, or be released when soils are water saturated. The motivation for the proposed research had emerged during the record-breaking summer of 2003 which according to our preliminary understanding was not exceptional in many parts of the pre-Alps where grassland yields were rather good. This was in contrast to the situation experienced in the low altitudes of the Swiss Plateau where a significant drought affected agricultural yields dramatically, leading to severe economic losses for lowland farmers. This project was funded by the Swiss National Science Foundation, grant 200021-105949.
Our working hypothesis was thus that a changing climate of the type suggested by the summer of 2003 could potentially revive the economic importance of pre-Alpine and Alpine agriculture, namely in relation to lowland farming which revealed to be much more vulnerable to such extremes in climate. In order to understand the implications that such modifications of environmental conditions could have on the national greenhouse gas budget we suggested to quantify the N2O and CH4 fluxes over grassland in combination with continuous CO2 exchange measurements. We also hypothesized that if CH4 is oxidized by the grassland soils around cattle farms, then we should be able to find a close relation between the measured CH4 emissions from cattle and the CH4 uptake that we proposed to measure here.
We took benefit of the unique opportunities given by the three ETH Research Stations at 400, 1000, and >1900 m a.s.l., which provide a representative transect for the Swiss three-step system of Alpine summer grazing. We suggested to measure N2O and CH4 fluxes continuously with the eddy covariance method at the lowest elevation site. CO2 exchange and associated energy fluxes were continuously measured at the two lower stations. At all three sites we carried out additional stable isotope (13C and 18O) sampling campaigns to study the most relevant biogeochemical processes that are active during the growing season, and at the lowest site also in winter. These data enabled us to partition net CO2 fluxes obtained with eddy covariance into the process components assimilation and respiration. This allowed us to assess the carbon turnover of the grassland ecosystems.
Eugster W, Zeeman MJ (2006) Micrometeorological techniques to measure ecosystem-scale greenhouse gas fluxes for model validation and improvement. International Congress Series 1293:66-75
Eugster W, Zeyer K, Zeeman M, Michna P, Zingg A, Buchmann N, Emmenegger L (2007) Methodical study of nitrous oxide eddy covariance measurements using quantum cascade laser spectrometery over a Swiss forest. Biogeosciences 4(5):927-939
Fritsche J, Obrist D, Zeeman MJ, Conen F, Eugster W, Alewell C (2008) Elemental mercury fluxes over a sub-alpine grassland determined with two micrometeorological methods. Atmospheric Environment, doi: 10.1016/j.atmosenv.2007.12.055
Fritsche J, Wohlfahrt G, Ammann C, Zeeman M, Hammerle A, Obrist D, Alewell C (2008) Summertime elemental mercury exchange of temperate grasslands on an ecosystem-scale. Atmospheric Chemistry and Physics Discussions 8(1):1951-1979
Hiller R, Zeeman MJ, Eugster W (2008) Eddy covariance flux measurements in the complex terrain of an Alpine valley in Switzerland. Boundary-Layer Meteorology 129:449-467
Mohn J, Zeeman MJ, Werner RA, Eugster W, Emmenegger L (2008) Continuous field measurements of δ13CO2 and trace gases by FTIR spectroscopy. Isotopes in Environmental and Health Studies 44(3):241-251
Mohn J, Werner RA, Buchmann B, Emmenegger L (2007) High-precision δ13CO2 analysis by FTIR spectroscopy using a novel calibration strategy. Journal of Molecular Structure 834:95-101
Tuzson B, Zeeman MJ, Zahniser M, Emmenegger L (2008) Quantum cascade laser based spectrometer for in situ stable carbon dioxide isotope measurements. Infrared Physics & Technology 51:198-206, doi:10.1016/j.infrared.2007.05.006
Tuzson B, Mohn J, Zeeman MJ, Werner RA, Eugster W, Zahniser MS, Nelson DD, McManus JB, Emmenegger L (2008) High precision and continuous field measurements of δ13C and δ18O in carbon dioxide with a cryogenic free QCLAS. Applied Physics B. Lasers and Optics 92(3):451-458, doi:10.1007/s00340-008-3085-4
Zeeman MJ, Hiller R, Gilgen AK, Michna P, Plüss P, Buchmann N, Eugster W (2010) Management, not climate, controls net CO2 fluxes and carbon budgets of three grasslands along an elevational gradient in Switzerland. Agric. Forest Meteorology 150: 519–530
Zeeman MJ, Tuzson T, Emmenegger E, Knohl A, Buchmann N, Eugster W (2009) Conditional CO2 flux analysis of a managed grassland with the aid of stable isotopes. Biogeosciences Discussion 6: 3481-3910
Zeeman MJ, Werner RA, Eugster W, Siegwolf R, Wehrle G, Mohn J, Buchmann N (2008) Optimization of automated gas sample collection and isotope ratio mass spectrometric analysis of δ13C of CO2 in air. Rapid Communications in Mass Spectrometry 22:3883-3892, doi: 10.1002/rcm.3772
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