Depth profiles of radiocarbon and carbon isotopic compositions of organic matter and CO_2 in a forest soil

摘要

Depth profiles of the specific activities of ~(14)C and carbon isotopic compositions (Δ~(14)C, δ~(13)C) in soil organic matter and soil CO_2 in a Japanese larch forest were determined. For investigating the transport of CO_2 in soil, specific activities of ~(14)C, Δ~(14)C and δ~(13)C in the organic layer, and atmospheric CO_2 in the same forest area were also determined. The specific activity of ~(14)C and Δ~(14)C in the soil organic matter decreased with the increase in depth of 0-60 cm, while that of soil CO_2 did not vary greatly at a soil depth of 13-73 cm and was more prevalent than that of atmospheric CO_2. Peaks of specific activities of ~(14)C appeared at the depth of 0-4 cm and Δ~(14)C values were positive in the depth range from 0 to 15 cm. These results suggest that the present soil at a depth of 0-4 cm had been produced from the mid-1950s up until 1963, and the bomb C had reached the depth of 15 cm in the objective soil area. The δ~(13)C in the soil organic matter increased at the depth of 0-55 cm, while that of soil CO_2 collected on 8 November 2004 decreased rapidly at the depth of 0-13 cm and only slightly at the depth of 53-73 cm. By combining the Δ~(14)C and δ~(13)C of the respective components and using the Keeling plot approach it was made clear that the entering of atmospheric CO_2 showed a large contribution to soil CO_2 at the depth of 0-13 cm and a negligible contribution at the depth of 53-73 cm for soil air collected on 8 November 2004. Respiration of live roots was presumed to be the main source of soil CO_2 at the depth of 53-73 cm on 8 November 2004.