Soil Carbon Responses to Elevated Atmospheric CO2
Julie Jastrow, Argonne contact (jdjastrow@anl.gov)
Determining the potential carbon sink strength of terrestrial ecosystems requires better understanding and improved quantitation of processes involved in soil carbon storage and turnover. Relatively labile carbon can be physically protected from decomposition by incorporation into soil aggregates or chemically protected by association with soil minerals. The processes involved in soil aggregate formation and turnover form the theoretical basis for isolating measurable carbon pools with functionally meaningful relationships to soil carbon dynamics. We use physicochemical fractionation techniques, stable carbon isotopes, long-term incubations, and compound-specific isotope analysis of biopolymer structures to evaluate the dynamics, sources, and stability of functional soil carbon pools and their responses to atmospheric CO2 enrichment. Studies carried out at open-top chamber and free air CO2 enrichment (FACE) facilities use repeated measurements over time and the isotopic tracers available at these sites to investigate fundamental questions regarding potential saturation of soil carbon protection mechanisms, the effects of changes in input availability and source, the stability and longevity of accrued carbon, and the influence of species-specific responses and edaphic properties on soil carbon dynamics in ecosystems exposed to atmospheric CO2 enrichment. An important goal of this research is to contribute data and process knowledge to help parameterize and validate soil organic matter simulation models, thereby allowing extrapolation of results to the broader scales needed to predict the role of terrestrial ecosystems in continental and global carbon cycles.
Detecting changes in soil carbon against the large and variable background of existing soil organic matter is difficult, particularly over relatively short time periods. We used meta-analysis techniques to evaluate the collective responses of 35 experimental observations from diverse temperate ecosystems. The results indicated that CO2 enrichment increased soil carbon by an average of 5.6% over 2-9 years, at a median rate of 0.19 Mg C ha‑1 y‑1. We also observed increases in soil carbon, at rates exceeding 0.4 Mg C ha‑1 y‑1 for 5-8 years, in Tennessee sweetgum forest and Kansas prairie exposed to elevated CO2. Carbon accrual in both systems was measurable because the vegetation responded to CO2 enrichment with large increases in the production of root litter.
 Over half of the soil carbon accrued in both experiments was incorporated into microaggregates, which can protect carbon from rapid decomposition and increase its potential residence time in soil. In prairie soil, the proportion of accrued carbon incorporated into microaggregates varied with depth. In the surface 5 cm, where inputs and native soil organic matter were greatest, the capacity of microaggregates to protect additional carbon appeared saturated, and carbon accumulated in more labile non-microaggregated pools — mostly as particulate organic matter. Below 5 cm, however, most of the accrued carbon was protected in microaggregates. In long-term laboratory incubations, 55% of the incremental increase in soil carbon at 0–5 cm was mineralized, whereas only 16% was mineralized at 5–15 cm, confirming that carbon accrued at 5–15 cm was indeed better protected. In contrast, in the sweetgum forest the proportion of carbon in microaggregated soil averaged 58% in both elevated-CO2 and ambient plots and was unchanged over time. These data suggest that additional inputs derived from CO2 enrichment at the sweetgum site are being processed and protected much as in ambient soil, with little apparent saturation of protection mechanisms during the initial 5 years of the study. |
We also monitored root carbon turnover for 5 years in the Tennessee sweetgum and North Carolina loblolly pine FACE facilities. In the elevated-CO2 treatment, the tree stands were continuously fumigated with extra atmospheric CO2 depleted in 13CO2, and we followed the transfer of 13C incorporated into root tissues. We found fine-root carbon turnover times of 1.25–9 years, depending on root diameter and forest type. The pine forest had the slowest root carbon turnover, and thus a decreased short-term potential to accumulate carbon in soil. In the sweetgum forest, faster root carbon turnover and increases in root production led to a rapid and significant increase in soil carbon. Our findings demonstrate that the rate of root carbon turnover is highly important for predicting the transfer of carbon to soil organic matter.
Collaborators
Thomas W. Boutton, Texas A&M University, http://rangeland.tamu.edu/people/boutton/
Timothy R. Filley, Purdue University, http://www.purdue.edu/eas/biogeochem/
Miquel A. Gonzalez-Meler, University of Illinois at Chicago, http://www.uic.edu/labs/meler/index.htm
Links
Sweetgum FACE: http://www.esd.ornl.gov/facilities/ORNL-FACE/
Loblolly pine FACE: http://face.env.duke.edu/main.cfm
AspenFACE: http://aspenface.mtu.edu/
SoyFACE: http://www.soyface.uiuc.edu/
Prairie Elevated CO2 Experiment: http://spuds.agron.ksu.edu/