Soil Carbon Source and Accumulation Over 12,000 Years in a Semi-Natural Miscanthus Sinensis Grassland in Southern Japan.

Miscanthus, a promising feedstock for the production of lignocellulosic ethanol, will likely see a considerable increase in cultivation and production in the U.S. and Europe in the coming decades. To better predict the effect of this change on land use, we examined a Miscanthus sinensis-dominated grassland in its native range in southern Japan. Soil organic carbon age, accumulated carbon stock, and carbon source were determined on pyrolysis volatile (Py-V) and pyrolysis residue (Py-R) fractions of soil organic matter. Stable carbon isotopic composition of soil organic matter was also determined. Values of total sequestered C summed from two soil profiles (0 – 1.5 m) were 515 and 559 Mg C ha^-1. Stable isotope analysis showed that about 66.8% of soil C, ranging from 52.0 to 84.8%, was derived from C₄ plants. Variation of soil age indicated by ¹⁴C dates of Py-V and Py-R fractions was correlated linearly with soil depths with a R² value ranging from 0.98 – 0.99. Phytolith data showed an increase in abundance of Miscanthus sinensis and Pleioblastus sect. Nezasa following a pyroclastic eruption 7 kyr ago, and a decrease in Sasa sect. Crassinodi from 12 to 7 kyr ago. Organic carbon flux ranged from 0.85 to 1.63 Mg C ha^-1 yr^-1 through 1.5-m soil profiles, and Miscanthus-derived carbon flux was distributed in a similar pattern to the total C throughout the soil profiles. Over a 12,000 year history of land use at the Aso caldera, Japan, Miscanthus sinensis was found to be a major contributor to soil carbon and a sink for atmospheric CO₂, which has implications for the potential sequestration of soil carbon under Miscanthus in cultivated conditions.