Diurnal cycle of carbon isotope ratio in soil CO2 in various ecosystems

Our investigations of diurnal variations of the C-13/C-12 ratio and CO2 content in soil air were carried out in three environments during periods of high biosphere activity. It has been observed that diurnal variation of CO2 concentration is negatively correlated delta(13)C. Particularly great variations occurred at shallow soil depths (10-30 cm) when the plant cover activity was high while the soil temperature was rather low. Under such conditions the delta(13)C variations had the magnitude of 4 parts per thousand, while the CO2 concentration varied more than doubly. The maximum of the C-13/C-12 ratio and the minimum of the CO2 concentration in a cultivated field with winter wheat took place in the afternoon, whereas in deciduous forest similar patterns were observed at dawn. In these cases soil temperatures at 10 cm depths varied less than 2 degrees C. Hence, under wheat the variation in root respiration rate seem to be the main reason of the recorded varations. In an uncultivated grass-field during the hottest period in summer we did not measure any distinct variations of CO2 properties in spite of the fact that soil temperature varied up to 5 degrees C. This might be due to dominant microbial respiration at the high soil temperature, which exceeded 20 degrees C.