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.
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.