Estimating the contribution of leaf litter decomposition to soil CO2 efflux in a beech forest using 13C-depleted litter

The contribution of leaf litter decomposition to total soil CO2 efflux (F-L/F) was evaluated in a beech (Fagus sylvatica L.) forest in eastern France. The Keeling-plot approach was applied to estimate the isotopic composition of respired soil CO2 from soil covered with either control (-30.32 parts per thousand) or C-13-depleted leaf litter (-49.96 parts per thousand). The delta C-13 of respired soil CO2 ranged from -25.50 parts per thousand to -22.60 parts per thousand and from -24.95 parts per thousand to -20.77 parts per thousand, respectively, with depleted or control litter above the soil. The F-L/F ratio was calculated by a single isotope linear mixing model based on mass conservation equations. It showed seasonal variations, increasing from 2.8% in early spring to about 11.4% in mid summer, and decreasing to 4.2% just after leaf fall. Between December 2001 and December 2002, cumulated F and F-L reached 0.98 and 0.08 kg(C) m(-2), respectively. On an annual basis, decomposition of fresh leaf litter accounted for 8% of soil respiration and 80% of total C loss from fresh leaf litter. The other fraction of carbon loss during leaf litter decomposition that is assumed to have entered the soil organic matter pool (i.e. 20%) represents only 0.02 kg(C) m(-2).