Tuesday, November 14, 2006

Elevated CO2 Enhances Soil Organic Matter Decomposition via Soil Microbial Composition and Activity.

J. Patrick Megonigal1, Karen M. Carney2, Bruce A. Hungate3, and Bert G. Drake1. (1) Smithsonian Environmental Research Center, Smithsonian Environmental Research Center, PO Box 28 647 Contees Wharf Rd., Edgewater, MD 21037-0028, (2) US Agency for Int'l Development, EGAT/NRM/F, Ronald Reagan Bldg (3.8.81B), Washington, DC 20523, (3) Biological Sciences Dept, Northern Arizona Univ, PO Box 5640, Flagstaff, AZ 86011

Soil carbon pools reflect a long-term balance between carbon inputs and outputs, driven primarily by plant production and microbial decomposition. It is important to understand plant-microbe feedbacks on carbon cycling in order to predict how the balance between these processes will change in a future atmosphere of elevated CO2 (E). We examined the influence of E on microbial community composition, microbial activity, and soil carbon content in a Florida scrub oak ecosystem. The field experiment consisted of replicate (n = 8) open-top chambers exposed to ambient or twice-ambient atmospheric CO2 concentrations for 6 years. E soils had higher fungi:bacteria ratios (p=0.04) and polyphenoloxidase activity (p=0.090) than ambient CO2 (A) soils. Polyphenoloxidase degrades recalcitrant organic substrates. We also conducted a laboratory-based experiment in which soil organic matter (SOM) and leaf litter amendments had distinct isotopic signatures. Adding litter stimulated microbial respiration rates more in E soils than A soils (p = 0.055), suggesting a potential stimulation or ‘priming’ effect on SOM mineralization. We present two lines of evidence to argue that E has stimulated SOM mineralization in the field study. First, E soils consistently lost SOM relative to A soils between 1998 and 2002 (r2=0.98, p=0.009). Second, the size of the laboratory-derived priming effect was negatively related to the size of the in situ SOM pool (r2=-0.49, p=0.003). These data suggest that E enhanced soil organic matter degradation through changes in microbial community composition and activity; this mechanism may be an important constraint on how much of the ‘extra’ carbon assimilated by plants in an E atmosphere enters long-term storage in soil carbon pools.