PARENT SESSION
Oral Session #5: Ecosystem processes: Decomposition and litter. Presiding: K. Lajtha.
Monday, August 6, 2001. 8:00 AM to 12:15 PM. Hall of Ideas E.

Can remote atmospheric ¹⁴C measurements tell us the age of ecosystem respiration?

Randerson, James¹, Enting, Ian ², Fung, Inez³, ^{1 2 3}

ABSTRACT- During the mid-1960s, large seasonal amplitudes were observed in surface measurements of ¹⁴C in the northern hemisphere. These oscillations are frequently attributed to stratosphere-troposphere exchange with the injection of bomb-¹⁴C into the troposphere during winter and spring mixing. Here we show that respiration fluxes from the terrestrial biosphere also contributed to the seasonal cycle of ¹⁴C during this period. In the Northern Hemisphere, respiration fluxes during summer and fall decreased atmospheric ¹⁴C near the surface because these fluxes were depleted by several hundred per mil relative to atmospheric levels. The terrestrial biosphere contribution diminished in the late 1960s because rapidly exchanging plant and litter carbon pools began to equilibrate with the post-bomb atmospheric ¹⁴C levels (with a corresponding decrease in their summer/fall atmosphere dilution potential). In more recent decades, we predict that the seasonal ¹⁴C forcing from terrestrial ecosystems has changed phase, with the amplitude and timing of the phase transition determined by the residence times of carbon within the footprint of the observation station. We describe how an expanded atmospheric measurement program at sites far from fossil fuel emissions might allow us to constrain the age of ecosystem respiration in northern ecosystems.

KEY WORDS: decomposition, biome, seasonal cycle, net ecosystem exchange