Soil carbon sequestration and turnover under elevated atmospheric CO2

Background/Question/Methods Anthropogenic CO₂ emissions have significantly altered the global C cycle and have been found to increase plant productivity and ecosystem C storage in experimental settings despite demonstrated nutrient limitation. These experiments have indicated that C storage may be stimulated or decreased by elevated atmospheric CO₂. Some evidence indicates that presumably stable soil C pools with low C:N ratios can be mined to meet the additional N demand under elevated CO₂. To better understand soil C sequestration and turnover under rising atmospheric CO₂ concentration, the Duke Forest Free-air CO2 enrichment experiment has been conducted over twelve years in an aggrading loblolly pine (Pinus taeda) plantation with clay loam soils. Samples were collected from eight experimental plots at a three-year interval and were separated into forest floor and two bulk mineral soil sections (0-15 cm and 15-30 cm depths) for %C, %N, 13C, and 15N analyses.

Results/Conclusions After the first six years of the Duke Forest experiment, an additional 52 + 16 gC m^-2 yr^-1 accumulated in the forest floor under elevated CO₂. The C and N contents of the forest floor increased linearly under both the treatment and the control before leveling off in the subsequent six years, apparently reaching a steady state with elevated CO₂ producing 34% and 31% more forest-floor C and N, respectively. Over the entire twelve-year period, an additional ~30 gC m^-2 yr^-1 was detected in the forest floor of the elevated CO₂ plots in response to increased litterfall and root turnover. No differences in C and N contents were detected in the mineral soil; however, a significant widening of the C:N ratio of the upper mineral soil under elevated CO₂ indicates that N is likely being mined from the soil and being transferred to plants, thereby supporting the additional productivity associated with elevated CO₂. SOM fractionations will be completed to determine from which SOM pools nitrogen was removed and to understand the long-term sustainability of this process.