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

Abstract

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 (δ13C) have indicated that the δ13C value widely ranged from −25 to −17 ‰ in various land-use soils. We measured the δ13C of soil organic carbon (SOC) in Black Soil in forests from northern (43°N) to southern (31°N) Japan. The δ13C values in topmost soils were contaminated by carbon from current C3 vegetation. Excluding these soils, the average contribution ratio of C4 grass in Black Soils was estimated to be ~44.6 % of SOC by mass balance calculation from the δ13C of SOC. The proportion of C4 plants supplying soil carbon was smaller at higher latitudes, this indicating that the δ13C values of SOC were affected by the competitiveness of C4 grass and C3 plants which might depend on the temperature. The melanic index, which is an index of humus properties and divides the humus into “Type A” (≤1.7) and other humus (>1.7), correlates negatively with δ13C values. This result indicates that C4 grass played an important role in generating the dark-colored organic matter in Japanese Black Soils. The δ13C values of soil profiles with key tephra are therefore potentially useful for the study of past climate dynamics and vegetation responses.