Soil carbon accrual in particle-size fractions under Miscanthus x. giganteus cultivation

The energy crop Miscanthus x. giganteus is a deep rooting perennial rhizomatous C4 grass with great biomass production, even under temperate German climate conditions. Accordingly we hypothesized that this crop may accumulate great amounts of carbon in soil, particularly in deeper soil layers. We sampled several former C3-derived arable fields that had been cropped with Miscanthus for 0-19 years. We were able to trace the origin and turnover of soil organic C (SOC) on the basis of natural 13C/12C abundance measurements. The analysis was performed on bulk soil samples and on particle-size fractions that are known to comprise SOC of different availability for decay. Miscanthus-derived C accumulated at a rate of 1800 kg ha−1 y−1 down to a soil depth of 100 cm. Only about 50% of this C accrual occurred in the surface soil (0-10 cm). The C accumulation differed among size fractions. Miscanthus-derived C in the coarse-POM fraction increased rapidly during the first years of Miscanthus cultivation until a steady state was reached after approximately seven years. The stocks of Miscanthus-derived C associated with the clay fraction increased at a rate of 230 kg ha−1 y−1 in 0-5 cm, 45 kg ha−1 y−1 in 20-30 cm and 38 kg ha−1 y−1 in 50-75 cm. The C accumulation rate decreased with increasing soil depth. In particular, Miscanthus-derived C associated with the clay fraction led to increasing SOC stocks, even below the former Ap; that is, below a depth that would respond sensitively to a future land use change.