Diurnal variations and vertical distribution of d13 C, and concentration of atmospheric and soil CO2 in a meadow site, SE Poland

Abstract

We provide the results of 24-hour observations made in a meadow site located in a small river valley in Central Europe. Samples of atmospheric air were taken from three horizons: near the soil (0.05 m), in the grass (0.5 m) and above the meadow (2 m) at two-hour intervals. At the same time, samples of soil air were collected from two horizons: -0.1 and -0.5 m. We have found a variation of d13 C above the ground from -6 during the day to -20 late at night accompanied by variations in CO2 concentration from 270 ppm during the day, to various levels late at night at different heights above the ground. The maximum concentration was 1430 ppm at the ground level. The correlation coefficient between d13 C and reciprocal of concentration was the highest (R2 = 0.984) for the samples collected 2 m above the ground, the regression line clearly indicating CO2 mixing from the two sources: atmospheric and biogenic reservoirs. The intercept of the mixing line yields d13 C = -23.0 for the biogenic CO2. In contrast, the diurnal variations in the soil were relatively small, d13 C varied from -21.6 to -23.4, while CO2 concentration from 4300 to 8200 and from 24 700 to 34 500 ppm at depths of 0.1 m and 0.5 m respectively, which is less than 2-fold. Small diurnal variations are characteristic of dry soils, where d13 C is weakly correlated with CO2 concentration (in our case R2 was 0.30 and 0.54, respectively).