Soil carbon sequestration in a pine forest after 9 years of atmospheric CO2 enrichment

The impact of anthropogenic CO2 emissions on climate change may be mitigated in part by C sequestration in terrestrial ecosystems as rising atmospheric CO2 concentrations stimulate primary productivity and ecosystem C storage. Carbon will be sequestered in forest soils if organic matter inputs to soil profiles increase without a matching increase in decomposition or leaching losses from the soil profile, or if the rate of decomposition decreases because of increased production of resistant humic substances or greater physical protection of organic matter in soil aggregates. To examine the response of a forest ecosystem to elevated atmospheric CO2 concentrations, the Duke Forest Free-Air CO2 Enrichment (FACE) experiment in North Carolina, USA, has maintained atmospheric CO2 concentrations 200 mu L L-1 above ambient in an aggrading loblolly pine (Pinus taeda) plantation over a 9-year period (1996-2005). During the first 6 years of the experiment, forest-floor C and N pools increased linearly under both elevated and ambient CO2 conditions, with significantly greater accumulations under the elevated CO2 treatment. Between the sixth and ninth year, forest-floor organic matter accumulation stabilized and C and N pools appeared to reach their respective steady states. An additional C sink of similar to 30 g C m(-2) yr(-1) was sequestered in the forest floor of the elevated CO2 treatment plots relative to the control plots maintained at ambient CO2 owing to increased litterfall and root turnover during the first 9 years of the study. Because we did not detect any significant elevated CO2 effects on the rate of decomposition or on the chemical composition of forest-floor organic matter, this additional C sink was likely related to enhanced litterfall C inputs. We also failed to detect any statistically significant treatment effects on the C and N pools of surface and deep mineral soil horizons. However, a significant widening of the C : N ratio of soil organic matter (SOM) in the upper mineral soil under both elevated and ambient CO2 suggests that N is being transferred from soil to plants in this aggrading forest. A significant treatment x time interaction indicates that N is being transferred at a higher rate under elevated CO2 (P=0.037), suggesting that enhanced rates of SOM decomposition are increasing mineralization and uptake to provide the extra N required to support the observed increase in primary productivity under elevated CO2.