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 (∼20 to 60 cm depth) from the incorporation of ‘bomb’ radiocarbon ( 14 C) 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 ∼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 13 C‐NMR spectroscopy. The 14 C content of HF‐C increased rapidly from 1992 to 2009, indicating a significant time lag (>20 years) for the arrival of ‘bomb’ 14 C 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 14 C 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.