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

Abstract. Amazon forests play a significant role in the global C cycle by assimilating large amounts of CO₂ through photo- synthesis and storing C largely as biomass and soil organic matter. To evaluate the net budget of C in the Amazon, we must also consider the amplitude and timing of losses of C back to the atmosphere through respiration and biomass burning. One useful timescale metric that integrates such information in terrestrial ecosystems is the transit time of C, defined as the time elapsed between C entering and leaving the ecosystem; transit time is equivalent to the age of C exiting the ecosystem, which occurs mostly through respiration. We estimated the mean transit time of C for a central Amazon forest based on the C age in ecosystem respiration (ER), taking advantage of the large variations in CO₂ in the atmosphere below the forest canopy to estimate the radiocarbon signature of mean ER (∆¹⁴C_ER) using Keeling and Miller-Tans mixing models. To evaluate changes in the isotopic signature of the main ER sources, the δ¹³C_ER was estimated through Keeling plots using the same samples. 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). For the campaign of October 2019, the ∆¹⁴C_ER was 33.9 ± 7.7 ‰ using the Keeling plot method, and 31.6 ± 7.5 ‰ with the Miller-Tans method. In December 2021, ∆¹⁴C_ER was 77.0 ± 28.3 ‰ using the Keeling plot method, and 77.9 ± 24.0 ‰ with the Miller-Tans method. The δ¹³C_ER showed a smaller variation, being -27.8 ± 0.3 ‰ in October 2019 and -29.0 ± 0.5 ‰ in December 2021. Combining the ∆¹⁴C_ER estimates with the record of atmospheric radiocarbon from the bomb period, we obtained estimates of the mean transit time of 6 ± 2 years for 2019 and 18 ± 5 years for 2021. In contrast to steady-state carbon balance models that predict constant mean transit times, 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.