Dynamics of decadally cycling carbon in subsurface soils

Koarashi, Jun   ; Hockaday, W.*; Masiello, C.*; Trumbore, S.*

Subsurface horizons contain more than half of the global soil carbon (C), yet the dynamics of this C remains poorly understood. We estimated the amount of decadally-cycling subsurface C from the incorporation of "bomb' radiocarbon (C) using samples taken over 50 years from grassland and forest soils in the Sierra Nevada Mountains, California. The C content of all organic matter fractions increased from the pre- to post-bomb samples, indicating about half of the subsoil C consists of C fixed since 1963. The C content of mineral-associated C increased rapidly from 1992-2009, indicating a significant time lag (20 years) for the arrival of "bomb" C to this fraction. Our results demonstrate that a lagged response of the large subsurface C stores to changes in environmental conditions is possible.