Dynamics of decadally cycling carbon in subsurface soils

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 (similar to 20 to 60 cm depth) from the incorporation of 'bomb' radiocarbon (C-14) using samples taken over 50 years from grassland and forest soils in the Sierra Nevada Mountains, California. The radiocarbon content of all organic matter fractions (roots, low-density (LF), high-density (HF), and non-oxidizable HF) increased from the pre- to post-bomb samples, indicating similar to 1-6 kgC m(-2), or about half of the subsoil C, consists of C fixed since 1963. Low-density (LF-C) represented <24% (grassland) to 40-55% (forest) of the subsurface C and represented a mixture of post-bomb C and varying amounts of pre-1950 charcoal, identified using C-13-NMR spectroscopy. The C-14 content of HF-C increased rapidly from 1992 to 2009, indicating a significant time lag (>20 years) for the arrival of 'bomb' C-14 to this fraction. A two-pool (fast-cycling and passive) model including >20 year time lag showed that 28-73% of the subsoil mineral-associated C had turnover times of 10-95 years. Microbially respired C was enriched in bomb C-14 compared to both LF and HF fractions in 2009. Overall, we estimate that C fluxes through decadally cycling pools in the subsurface are equivalent to 1-9% (grassland) to 10-54% (forest) of the surface litterfall at these sites. Our results demonstrate the importance of decadally cycling C for ecosystem C balance, and that a lagged response of the large subsurface C stores to changes in environmental conditions is possible.