?13C-variations of leaves in forests as an indication of reassimilated CO2 from the soil

Abstract

An attempt has been made to evaluate the contribution of soil respired CO₂ to the total assimilation of a forest tree, by heeding the ¹³C-concentrations of CO₂ from the free atmosphere and from mineralization processes within the soil respectively. An expression has been derived, according to which the assimilated fraction of CO₂ from the soil at a particular height of a tree is given by the δ¹³C-value of the corresponding leaves, δ¹³C of atmospheric CO₂, δ¹³C of soil respired CO₂ and the physiological state of the leaves expressed as the ratio of total respiration over gross photosynthesis and internal over external CO₂-concentration. In the particular case investigated, a δ¹³C-difference of 5‰ has been determined from bottom to top of a beech tree which results in a CO₂ contribution from the soil of about 22% for the lower forest strata, while the total contribution of soil respired CO₂ accounts for about 5% of the overall assimilation.