2.2.2. Soil respiration
Gas samples from the pots without plants were taken on days 24, 31 and 35 of the experiment. For this purpose, a closed but ventilated chamber was placed on top of each pot and kept there for three minutes. After that time, a sample was taken with a 50 ml syringe. 30 ml of this volume were used to flush a 12ml exetainer in which the remainder of the air sample was then collected. The gas sample was analysed for CO2 concentration and d13C with an isotope ratio mass spectrometer (DeltaPlusXL, Thermo Finnigan,Bremen, Germany) coupled with an upstream gas chromatographic
system (Thermo Finnigan, Bremen, Germany). The values represent repeated measurements with an analytical precision of better than ±0.2‰ and were calculated versus V-PDB standard using CO2 reference gas. CO2 originating from soil
respiration was calculated as the difference between the atmospheric background and the sample. A two component isotopic mixing model was applied to calculate the contribution of C originating from the soil additives (C4 signal, cchar) and the soil itself (C3 signal, csoil):