PARENT SESSION
Oral Session #40: Carbon sequestration and flux.
Presiding: G. Koch
Tuesday, August 6. 1:00 PM to 4:45 PM. Gila Meeting Room, TCC.

Sources of soil surface CO₂ efflux in a tropical plantation forest.

Giardina, Christian^*,1, Binkley, Dan², ¹ USDA Forest Service, Houghton, MI² Colorado State University, Fort Collins, CO

ABSTRACT- Soil surface CO₂ efflux includes: decomposing aboveground litter; root and mycorrhizal turnover, exudation and respiration; and decomposing soil organic carbon (SOC). Soil surface CO₂ efflux has been well characterized for many ecosystems and across wide gradients in temperature and moisture, but the size of the individual fluxes contributing to soil surface CO₂ efflux remain poorly quantified, particularly for tropical forests. Using a combination of mass balance and stable isotope techniques, we partitioned soil surface CO₂ efflux (15 Mg ha^-1 yr^-1) in a fast growing plantation forest in Hawaii into the following components: i) decomposing aboveground litter; ii) C release from roots and the rhizosphere, including root and mycorrhizal turnover and respiration; and iii) decomposing SOC. We found that over this 4-year study, 60% of soil surface CO₂ efflux was derived from C recently allocated belowground to roots and mycorrhizae (9 Mg ha^-1 yr^-1); 20% was derived from decomposing aboveground litterfall (3 Mg ha^-1 yr^-1); and, 20% was derived from the decomposition of SOC (3 Mg ha^-1 yr^-1). Because C₄ sugar cane agriculture had been practiced on the research site for 80 plus years, we were able to determine that half of the SOC released as CO₂ originated from sugar cane derived SOC, while the remaining 10% was derived from labile C leached from aboveground litter and new SOC originating from the forest plantation. These results indicate that less than 10% of the soil surface CO₂ efflux in this tropical forest originated from C pools older than one year.

KEY WORDS: Belowground carbon cycling, soil surface CO2 efflux, soil respiration, soil carbon