CO₂ exchange in an organic field growing barley or grass in eastern Finland

The CO 2 dynamics were measured in an organic soil in eastern Finland during the growing season and wintertime, and the annual CO 2 balance was calculated for plots where barley or grass was grown. During the summer, the CO 2 dynamics were measured by transparent and opaque chambers using a portable infrared gas analyser for the CO 2 analyses. During the winter, the CO 2 release was measured by opaque chambers analysing the samples in the laboratory with a gas chromatograph. Statistical response functions for CO 2 dynamics were constructed to evaluate the annual CO 2 exchange from the climatic data. The net CO 2 exchange was calculated for every hour in the snow‐free season. The carbon balance varied extensively depending on the weather conditions, and type and phenology of vegetation. During the growing season, the grassland was a net source while the barley field was a net sink for CO 2 . However, both soils were net sources for CO 2 when autumn, winter and spring were included also. The annual CO 2 emissions from the grassland and barley soil were 750 g CO 2 ‐C m −2 and 400 g CO 2 ‐C m −2 , respectively. The carbon accumulated in root and shoot biomass during the growing season was 330 g m −2 for grass and 520 g m −2 for barley. The C in the aboveground plant biomass ranged from 43 to 47% of the carbon fixed in photosynthesis ( P G ) and the proportion of C in the root biomass was 10% of the carbon fixed in photosynthesis. The bare soils had 10–60% higher net CO 2 emission than the vegetated soils. These results indicate that the carbon balance of organic soils is affected by the characteristics of the prevailing plant cover. The dry summer of 1997 may have limited the growth of grass in the late summer thus reducing photosynthesis, which could be one reason for the high CO 2 release from this grass field.