[A-CG36_29PM2] Science in the Arctic Region

[ACG36-13] Estimate of permafrost organic carbon balance in Alaskan boreal and tundra ecosystems using natural radiocarbon tracer

*Miyuki KONDO¹, Masao UCHIDA¹, Motoo UTSUMI², Go IWAHANA³, Kenji YOSHIKAWA³, Hiroki IWATA⁴, Yoshinobu HARAZONO³, Taro NAKAI⁵, Kiyoshi TANABE¹, Yasuyuki SHIBATA¹ (1.National Institute for Environmental Studies, 2.University of Tsukuba, 3.University of Alaska, Fairbanks, 4.Kyoto University, 5.Nagoya University)

The high-latitude regions, where a serious warming is expected, currently store large amounts of soil organic carbon in active-layer soils and permafrost, accounting for nearly half of the global belowground organic carbon pool. Despite the importance of these regions in the present carbon cycle, the soil C fluxes and budget are still only poorly known. Here, we use radiocarbon as the tool for quantifying the C balance of the inputs and decomposition in tundra and boreal soil. We evaluated the C inputs (I) and decomposition rates (k, inverse of turnover time) and net C accumulation (CA), using ¹⁴C approaches. Tundra and boreal soils show different patterns of depth distribution and C storage. Cumulative organic carbon stocks in boreal forest are 5.3 and 19.2 kgCm^-2, in surface organic layer (0-25 cm), and deep organic and mineral layers (25-70 cm), respectively. Large annual C input (0.25 kgCm^-2 yr^-1) and relatively slow decomposition (27 years) lead to rapid CA (0.05 kgCm^-2 yr^-1) in surface organic layer in boreal forest. Deep organic and mineral layers including near-surface permafrost show slower rate of input (0.03 kgCm^-2 yr^-1) and turnover (617 years) and CA about 20 times slower (0.003 kgCm^-2 yr^-1) than surface organic layer. Decomposition organic matter (Rh), which in accord with C losses from both surface and subsurface layers, was 0.23 kgCm^-2 yr^-1. This value agreed well with Rh (0.23 kgCm^-2 yr^-1) simulated by process-based models that simulate the biogeochemical and hydrologic cycle, where Rh averaged 45% of ecosystem respiration and 59% of soil respiration. In contrast, large amount of SOC (36.4 kg m^-2) have accumulated over millennia (turnover time: 4540 yrs) below the thin organic layer in tundra. The CA of mineral layer and permafrost is close to zero (0.003 kgCm^-2 yr^-1), and Rh is 0.008 kgCm^-2 yr^-1. Our radiocarbon data show that the most SOC in tundra soil was mode of stabilizing OC by permafrost and steady-state SOC stocks under current C balance.