Elevated concentrations of atmospheric CO₂ stimulate photosynthesis in most terrestrial ecosystems. Whether this "CO₂ fertilization effect" will result in long-term carbon sequestration depends largely on whether the additional photosynthesized C accumulates in soils. At the free-air CO₂ enrichment (FACE) experiment at Oak Ridge National Laboratory in Oak Ridge, Tennessee, USA, sweetgum (Liquidambar styraciflua L.) trees in the elevated-CO₂ plots were grown between 1998 and 2009 in an atmospheric CO₂ concentration 150 ppm greater than that of plots with current ambient CO₂ concentrations.

Results/Conclusions

There was significant stimulation of plant net primary productivity (NPP) in the elevated-CO₂ plots, and inputs of new C to the upper 20 cm of soil via fine roots and leaf litter were consequently greater in the elevated-CO₂ plots than in the ambient-CO₂ plots. However, direct measurements of soil organic C (SOC) still do not reveal statistically significant SOC accumulation that is attributable to elevated CO₂ in the top 20 cm of soil after 11 years, probably because SOC is spatially heterogeneous and SOC accumulation occurs slowly. The magnitude of future C accumulation in the soil depends on the mean residence time (MRT) of the SOC. We quantified the MRT using two methods: (1) a ¹³C tracer from the ¹³C-depleted CO2 in the elevated plots, and (2) a difference equation with observed SOC and C inputs. Using these methods, we calculated a one-pool MRT of about 20 y for SOC in the top 20 cm of soil. Multiple soil carbon pools with different exponential decay rates are often used to capture the experimentally-observed decay behavior of soil C. However, a one-pool model adequately explained our observations of soil C for the first five years of our measurements. Two-pool dynamics became apparent only after a decade of observation. Our observations suggest a large, relatively recalcitrant SOC pool and a small, relatively labile SOC pool.