Tuesday, November 14, 2006

Sensitivity of Recalcitrant Soil Carbon Decomposition to Temperature.

Christian P. Giardina1, Michael G. Ryan2, Cinzia Fissore3, and John Vucetich3. (1) Institute of Pacific Islands Forestry, USDA Forest Service Pacific Southwest Research Station, Hilo, HI 96720, (2) USDA Forest Service, Rocky Mountain Research Station, 240 W Prospect St, Fort Collins, CO 80520, (3) Michigan Technological Univ, 1400 Townsend St, Houghton, MI 49931

Accelerated decomposition of the soil carbon (C) reservoir in response to global warming could transform mineral soils from a net sink of atmospheric CO2 to a net source. We examined the apparent sensitivity of old soil C decomposition in forests to temperature using field-based estimates of soil C decomposition across gradients in mean annual temperature (MAT) and laboratory-based estimates of soil C decomposition for soils sampled along a wide MAT gradient in North America. We found that the sensitivity of soil carbon decomposition rates in the field responded to changes in MAT with a Q10 of ~ 1.2. Using boot-strap statistical approach, we show that this Q10 estimate has a confidence interval of 1.0 to 1.4, ruling out a statistical explanation for the apparent lack of temperature sensitivity of soil C examined across our MAT gradient. Using a combination of long-term and short-term incubations, we also report that for old carbon (that C remaining after 300 days of incubation), apparent decomposition responses to temperature (Q10) were 20% to 60% lower depending on temperature than the apparent response of new carbon (newly incubated soils).  Taken together, our results suggest that decomposition of older soil C in upland temperate and tropical forests is unlikely to respond more strongly to rising global temperatures than labile C.