Younger carbon dominates global soil carbon efflux

Area of Science:

• Soil Science
• Biogeochemistry
• Environmental Science

Background:

• Soil carbon (C) comprises organic compounds of varying ages, influencing soil stability.
• Understanding the age of carbon efflux is crucial but challenging due to complex soil C pools.
• Distinct turnover behaviors of different carbon age groups remain poorly understood.

Purpose of the Study:

• To infer the age of carbon in soil efflux (a_efflux) globally.
• To compare efflux carbon age with radiocarbon-inferred ages of retained soil carbon (a_soil).
• To investigate the influence of soil depth and region on carbon age dynamics in efflux and soil.

Main Methods:

• Quantified soil carbon transit times to infer efflux carbon age.
• Analyzed seven sequential soil layers down to 2m depth globally.
• Compared efflux carbon age (a_efflux) with soil-retained carbon age (a_soil).

Main Results:

• Globally, mean efflux carbon age (a_efflux) is significantly younger than soil-retained carbon age (a_soil), with younger C dominating efflux.
• Both ages increase with soil depth, but their disparity narrows, indicating longer retention of younger C in deeper layers.
• Significant spatial variability exists; in deep cold regions, efflux carbon can be older than retained soil carbon.

Conclusions:

• Younger carbon inputs are disproportionately represented in soil carbon efflux, especially in upper soil layers.
• The majority of new carbon inputs may not contribute to long-term soil carbon storage.
• Older carbon efflux is limited to specific conditions, primarily in deep soil layers of cold regions.