How long does carbon stay in a near-pristine central Amazon forest? An empirical estimate with radiocarbon

Amazon forests play a significant role in the global C cycle by assimilating large amounts of CO₂ through photosynthesis and storing C largely as biomass and soil organic matter. However, for the net budget of C, we must also consider the amplitude and timing of losses of C back to the atmosphere through respiration and biomass burning. A time metric integrating this information in terrestrial ecosystems is the transit time of C (TT), defined as the time elapsed between C entering and leaving the ecosystem. We estimated the mean TT for a central Amazon forest based on the C age in ecosystem respiration (ER), taking advantage of the large variations in sub-canopy CO₂ to estimate ∆¹⁴CER using Keeling and Miller-Tans (M-T) mixing models. We collected air samples in vertical profiles in October 2019 and December 2021 at the Amazon Tall Tower Observatory (ATTO) in the central Amazon. Air samples were collected in a diel cycle from two heights below and one above the canopy (4, 24, and 79 m agl, respectively). Estimates of ∆¹⁴C_ER were similar among mixing models. For the campaign of October 2019, ∆¹⁴C_ER was 31.6 ± 7.5 ‰ with the M-T method. In December 2021, it was 77.9 ± 24.0 ‰. Combining the ∆¹⁴C_ER estimates with the record of atmospheric radiocarbon from the bomb period, we obtained estimates of the mean TT of 6 ± 2 years for 2019 and 18 ± 5 years for 2021. In contrast to steady-state carbon balance models that predict constant mean TT, these results suggest an important level of variation in mean transit times. Nevertheless, new carbon fixed in this tropical forest is respired, on average, in one or two decades, which means that only a fraction of the assimilated C can act as a sink for decades or longer.