Latitudinal gradient of C4 grass contribution to Black Soil organic carbon and correlation between δ13C and the melanic index in Japanese forest stands

Black Soil in Japanese forests is believed to have formed under grasslands, on the basis of pollen and stable carbon isotopic analyses. Carbon stable isotope ratios (delta C-13) have indicated that the delta C-13 value widely ranged from -25 to -17 aEuro degrees in various land-use soils. We measured the delta C-13 of soil organic carbon (SOC) in Black Soil in forests from northern (43A degrees N) to southern (31A degrees N) Japan. The delta C-13 values in topmost soils were contaminated by carbon from current C-3 vegetation. Excluding these soils, the average contribution ratio of C-4 grass in Black Soils was estimated to be similar to 44.6 % of SOC by mass balance calculation from the delta C-13 of SOC. The proportion of C-4 plants supplying soil carbon was smaller at higher latitudes, this indicating that the delta C-13 values of SOC were affected by the competitiveness of C-4 grass and C-3 plants which might depend on the temperature. The melanic index, which is an index of humus properties and divides the humus into "Type A" (a parts per thousand currency sign1.7) and other humus (> 1.7), correlates negatively with delta C-13 values. This result indicates that C-4 grass played an important role in generating the dark-colored organic matter in Japanese Black Soils. The delta C-13 values of soil profiles with key tephra are therefore potentially useful for the study of past climate dynamics and vegetation responses.