Depth profiles of radiocarbon and carbon isotopic compositions of organic matter and CO2 in a forest soil

Depth profiles of the specific activities of 14C and carbon isotopic compositions (Δ14C, δ13C) in soil organic matter and soil CO2 in a Japanese larch forest were determined. For investigating the transport of CO2 in soil, specific activities of 14C, Δ14C and δ13C in the organic layer, and atmospheric CO2 in the same forest area were also determined. The specific activity of 14C and Δ14C in the soil organic matter decreased with the increase in depth of 0–60 cm, while that of soil CO2 did not vary greatly at a soil depth of 13–73 cm and was more prevalent than that of atmospheric CO2. Peaks of specific activities of 14C appeared at the depth of 0–4 cm and Δ14C 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 δ13C in the soil organic matter increased at the depth of 0–55 cm, while that of soil CO2 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 Δ14C and δ13C of the respective components and using the Keeling plot approach it was made clear that the entering of atmospheric CO2 showed a large contribution to soil CO2 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 CO2 at the depth of 53–73 cm on 8 November 2004.