Estimating the contribution of leaf litter decomposition to soil CO 2 efflux in a beech forest using 13 C‐depleted litter

The contribution of leaf litter decomposition to total soil CO 2 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 CO 2 from soil covered with either control (−30.32‰) or 13 C‐depleted leaf litter (−49.96‰). The δ 13 C of respired soil CO 2 ranged from −25.50‰ to −22.60‰ and from −24.95‰ to −20.77‰, 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 .