HR: 11:05h
AN: B12C-04    [Abstracts]
TI: Comparing three methods of NEE-flux partitioning from the same grassland ecosystem: the 13C, 18O isotope approach and using simulated Ecosystem respiration
AU: * Siegwolf, R
EM: rolf.siegwolf@psi.ch
AF: Lab of Atmospheric Chemistry, Paul Scherrer Institut, Villigen-PSI, 5232, Switzerland
AU: Bantelmann, E
AF: Lab of Atmospheric Chemistry, Paul Scherrer Institut, Villigen-PSI, 5232, Switzerland
AU: Saurer, M
AF: Lab of Atmospheric Chemistry, Paul Scherrer Institut, Villigen-PSI, 5232, Switzerland
AU: Eugster, W
AF: Institute of Plant Science, Swiss Federal University, Zürich, CH-8092, Switzerland
AU: Buchmann, N
AF: Institute of Plant Science, Swiss Federal University, Zürich, CH-8092, Switzerland
AB: As a change in the global climate occurs with increasing temperatures, the Carbon exchange processes of terrestrial ecosystems will change as well. However, it is difficult to quantify the degree to what ecosystem respiration will change relative to the CO2 uptake by photosynthesis. To estimate the carbon sequestration potential of terrestrial vegetation cover it is essential to know both fluxes: ecosystem respiration and the carbon uptake by the vegetation cover. Therefore the net ecosystem exchange of CO2 (NEE) was measured with the eddy covariance method and separated into assimilation and respiration flux. We applied three different approaches, 1) the conventional method, applying the nighttime relationship between soil temperature and NEE for calculating the respiration flux during the day, 2) the use of stable carbon and 3) oxygen isotopes. We compared the results of the three partitioning exercises for a temperate grassland ecosystem in the pre-Alps of Switzerland for four days in June 2004. The assimilation flux derived with the conventional NEE partitioning approach, was best represented at low PAR and low temperatures, in the morning between 5 and 9 am. With increasing temperature and PAR the assimilation for the whole canopy was underestimated. For partitioning NEE via 18O approach, correlations of temperature and radiation with assimilation and respiration flux were significantly higher for the partitioning approach with 18O than for the 13C NEE partitioning. A sensitivity analysis showed the importance of an accurate determination of the equilibrium term θ between CO2 and leaf water δ18O for the NEE partitioning with 18O. For using 13C to partition NEE, the correct magnitude of the 13C fractionation and for the respiration term is essential. The analysis of the data showed that for low light and low morning temperatures the conventional method delivers reasonably good results. When the temperatures exceeded 21°C the isotope approach provided the more realistic results, particularly the use of the oxygen isotopes. These results represent the situation for this particular grassland in the Swiss Alps while in other ecosystems the three partitioning approaches could show different results with regard to the quality and precision of the flux separation. In the presentation the potential reasons for the variation of the three approaches will be discussed.
DE: 0315 Biosphere/atmosphere interactions (0426, 1610)
DE: 0414 Biogeochemical cycles, processes, and modeling (0412, 0793, 1615, 4805, 4912)
SC: Biogeosciences [B]
MN: 2007 Fall Meeting