PARENT SESSION
Poster Session # 13: Biogeochemistry, Photosynthesis, and Respiration.

Tuesday, August 5 Presentation from 5:00 PM to 6:30 PM. SITCC Exhibit Hall B.

Turnover rates of particulate organic matter and chemical fractions of forest soils under elevated atmospheric CO₂.

Barron, Stacy^*,1, Long, Noah ¹, Lichter, John¹, Schlesinger, William², ¹ Biology Department and Environmental Studies Program, Brunswick, ME, 04011² Nicholas School of the Environment, Durham, NC, 27708

ABSTRACT- The Duke Forest Free Air CO₂ Enrichment experiment (FACE) offers a unique opportunity to quantify rates of soil carbon turnover in a deciduous forest under elevated concentrations of atmospheric CO₂. Because the CO₂ used in these experiments is derived from natural gas it is strongly depleted in ¹³C relative to ¹²C; this ¹³C depleted CO₂ is assimilated into plant biomass, thereby lowering the ¹³C:¹²C ratios of new plant tissue. As soils incorporate new plant tissue over time the turnover rate of different soil carbon pools can be estimated from rates of change in ¹³C:¹²C ratios. The estimated mean residence time (MRT) of soil organic matter in the bulk mineral soil between 0-15 cm depth was 22.0 years and for the bulk mineral soil between 15-30 cm depth, 62.9 years. To quantify turnover of various soil carbon pools we fractionated organic matter in the mineral soil by particle size and chemical methods. Coarse (> 0.250 mm) and sand-sized (0.053-0.0250 mm) fractions were wet-sieved while the silt and clay-sized (< 0.053 mm) fraction was centrifuged. In the upper mineral soil, the coarse, sand-sized, and silt/clay-sized fractions had MRTs of 12.3, 28.2, and 30.3 years, respectively. In the mineral soil between 15 and 30 cm depth, these same particulate organic matter (POM) fractions had MRTs of 41.8, 57.4, and 118.7 yr, respectively. A strong acid extraction was used to isolate clay-bound organic matter and a cold-water extraction was used to gather labile organic matter. The MRT of clay-bound organic matter was shorter than expected, 26.6 years, probably due to land use history, while that of labile organic matter was 14.0 years. These results are consistent with increasing stability of soil carbon with decreasing particle size, and with the recalcitrant nature of clay-bound organic matter.

Key words: MRT, FACE, carbon, POM