Partner und Internationale Organisationen
(Englisch)
|
A, B, CZ, DK, FIN, F, D, H, IS, IRL, I, LT, N, SI, E, CH, GB
|
Abstract
(Englisch)
|
The objective of this project is the partitioning of the Net Ecosystem Exchange (NEE) of CO2 in grassland plots, which were exposed to elevated CO2 (ca 600 ppm) and varying nitrogen supply. For a better understanding of the CO2 flux changes in the ecosystem due to the mentioned treatments, it is necessary to partition the NEE into its partial fluxes, which are the CO2 uptake by the vegetation (Assimilation), CO2 release form the plants and soil (respiration). The CO2 efflux from the soil, originates predominantly from the rhizosphere and the microorganisms. The NEE was measured with canopy gas exchange cuvettes, and the fluxes were partitioned by analyzing the stable 13C/12C and 18O/16O isotope ratio of the CO2 from the canopy, the atmosphere above canopy and the released CO2 from the soil and plant (respiration). The evapotranspiration was separated in transpiration and evaporation for all the treated sites.In this third year the last experiments were carried out. Further samples of the soil and soil air were collected and analyzed for their 13C/12C isotope ratio. This is of particular interest, because the CO2 exposure of the experimental sites was now terminated two years ago. The use of CO2, which was highly depleted in 13C left its finger-print in the soil, leading to a significantly lower d13C value in the soil of the CO2 exposed sites than on the control sites. With this last sampling we could quantify the change in the d13C of the soil organic matter, which is a measure for the trun over rate of the labile fraction of the soil. The continuous increase of the d13C values indicates a rapid change of the soil to the status as it was found before the CO2 exposure treatment. This is in agreement with other studies and indicates that a large part of the carbon deposition consists of labile carbon with a high turn over rate while only a minor amount is transformed into caltricent carbon. This is highly reflected in the organic material as well as in the soil CO2. Measurements of soil respiration showed no longer significant difference between all treatments. The heterogeneity of the results was within the statistical noise. The results will be published shortly. From early until late spring (April - May 2004) air sampling for the isotopic C and O measurements on CO2 was continued. NEE was measured with a cuvette system along with leaf, root and soil sampling. During this campaign we also measured the total ecosystem water loss and partitioned the evapotranspiration into evaporation as the component form the soil and plant transpiration. This is possible because the soil water has a distinctly different 18O signature from that of the leaf, which is highly enriched in H218O relative to the soil water. Therefore water vapor before and after the cuvette and leaves were sampled in two-hour intervalls. The leaf and soil water was extracted by cryodestillation and determined for the d18O in the laboratory. The sites with the highest nitrogen supply showed the highest evapotranspiration, while the sites with former CO2 exposure only were not different form the control sites, which were not subject to a high nitrogen regime. A first evaluation of the partitioning suggests that the higher evapotranspirative fluxes in the sites with high nitrogen treatment were due to a higher transpiration rate (plant water loss) relative to the control sites. In contrast to this result we could not find large differences in NEE between the different treatments. It could not be distinctively determined whether the differences were a result of the natural variability within an ecosystem or if the treatment effects were still visible. These results, however, could still have been biased from the previous year, where the severe draught prevented any production of grassland druing the usually most productive time between the beginning of June until the end of August. Such a loss in production has strong imnplications on the total belowground biomass production, in particular the formation of storad carbohydrates. The final analysis of the partial fluxes must still be completed, before a final conclusion can be drawn.
|
