Radiocarbon and stable carbon isotope compositions of chemically fractionated soil organic matter in a temperate-zone forest

Koarashi, Jun   ; Iida, Takao*; Asano, Tomohiro

To better understand the role of soil organic matter in terrestrial carbon cycle, carbon isotope compositions were measured for bulk, acid-insoluble and base-insoluble organic matter fractions separated by a chemical fractionation method. The measurements also made it possible to estimate indirectly radiocarbon (14C) abundances of chemically soluble organic matter fractions, through a mass balance of carbon among the fractions. The depth profiles of 14C abundances showed that bomb-derived 14C has been penetrated into the first 16 cm mineral soil at least, whereas a significant amount of it has been still preserved as the acid-soluble organic matter around litter-mineral soil boundary. In contrast, no or little bomb-derived 14C was observed for the base-insoluble fraction in all sampling depths, indicating that this recalcitrant fraction, accounting for approximately 15 % of total carbon in this temperate-zone forest soil, plays a role as a long-term sink in the carbon cycle. These results suggest that bulk soil organic matter can not provide a representative indicator as a source or a sink of carbon in soil, particularly on annual to decadal timescales.