5:30 PM - 5:45 PM
[ACG36-13] Estimate of permafrost organic carbon balance in Alaskan boreal and tundra ecosystems using natural radiocarbon tracer
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.