We examined a new method to determine the aerosol yield of precursors of secondary organic aerosols in the presence of organic seed particles. To distinguish between the oxidation products of the compound in question and the organic seed, the compound was labeled with stable isotopes and aerosol samples were analyzed by isotope ratio mass spectrometry (IRMS). 13C labeled isoprene was obtained from isoprene emitting plants that were exposed to (13)CO2. The aerosol yield of isoprene was determined from the 13C/12C ratio measured in the aerosol. Measurements at organic aerosol mass concentrations as low as 10 microg m(-3) were performed. Three different methods of aerosolsampling procedureswere evaluated: impactor, filter, and electrostatic deposition. The excess-% 13C measured by the three sampling methods agreed well. The aerosol yield of isoprene derived from these measurements showed a strong dependence on further oxidation of first-generation products and is within the range of reported yield values (1-5%) obtained so far from pure isoprene experiments.

The fate of carbon (C) in grassland soils is of particular interest since the vast majority in grassland ecosystems is stored below ground and respiratory C-release from soils is a major component of the global C balance. The use of 13 C-depleted CO 2 in a 10-year free-air carbon dioxide enrichment (FACE) experiment, gave a unique opportunity to study the turnover of the C sequestered during this experiment. Soil organic matter (SOM), soil air and plant material were analysed for d 13 C and C contents in the last year of the FACE experiment (2002) and in the two following growing seasons. After 10 years of exposure to CO 2 enrichment at 600 ppmv, no significant differences in SOM C content could be detected between fumigated and non-fumigated plots. A 13 C depletion of 3.4& was found in SOM (0-12 cm) of the fumigated soils in comparison with the control soils and a rapid decrease of this difference was observed after the end of fumigation. Within 2 years, 49% of the C in this SOM (0-12 cm) was exchanged with fresh C, with the limitation that this exchange cannot be further dissected into respiratory decay of old C and freshly sequestered new C. By analysing the mechanistic effects of a drought on the plant-soil system it was shown that rhizosphere respiration is the dominant factor in soil respiration. Consideration of ecophysiological factors that drive plant activity is therefore important when soil respiration is to be investigated or modelled.

Short-term N uptake by Norway spruce roots was investigated after tracer application both in the field and in the laboratory. In the forest the influence of wood ash or liquid fertiliser treatments on the uptake was studied. A rapid uptake of the 15 NH 4 15 NO 3 tracer into the fine roots was observed. Within 1 day of application about 50% of the maximum δ 15 N value observed was measured and within 1 week as much as 70-90%. The 15 N enrichment in fine roots was not affected by wood ash and liquid fertiliser treatments applied in the previous year. δ 15 Ν increased continuously until 2 months after 15 N application and decreased to 60% of its maximum value within 1 year. Nine months after the tracer application, an analysis of δ 15 N distribution throughout the root system was conducted. The δ 15 N values were highest in roots of the topsoil compared to roots at deeper soil layers, and higher in fine roots than in larger roots. The 15 N was not translocated within the root system. The laboratory experiment, using Norway spruce seedlings, aimed at describing short-term 15 NO 3 uptake and assimilation into free amino acids with 2 mM or 20 mM nitrate application. The 15 N was detected within 4 h to 1 day in the amino acids Glu, Gln, Asp of roots, while in Asn, Ala and Ser and in the shoots, the 15 N was retrieved at the earliest after 1 day. The enrichment factor in the amino acids increased to a maximum within 3-7 days, depending on the nitrate concentration applied.

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