Main content area

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

Ishizuka, Shigehiro, Kawamuro, Kimiyasu, Imaya, Akihiro, Torii, Atsushi, Morisada, Kazuhito
Biogeochemistry 2014 v.118 no.1-3 pp. 339-355
C3 plants, C4 plants, carbon, climate, color, forest stands, forests, grasses, grasslands, humus, land use, latitude, organic matter, pollen, polluted soils, soil organic carbon, soil profiles, stable isotopes, temperature, tephra, Japan
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 C₃vegetation. Excluding these soils, the average contribution ratio of C₄grass in Black Soils was estimated to be ~44.6 % of SOC by mass balance calculation from the δ¹³C of SOC. The proportion of C₄plants supplying soil carbon was smaller at higher latitudes, this indicating that the δ¹³C values of SOC were affected by the competitiveness of C₄grass and C₃plants 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 C₄grass 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.