δ13C-variations of leaves in forests as an indication of reassimilated CO2 from the soil

An attempt has been made to evaluate the contribution of soil respired CO2 to the total assimilation of a forest tree, by heeding the C-13-concentrations of CO2 from the free atmosphere and from mineralization processes within the soil respectively. An expression hag been derived, according to which the assimilated fraction of CO2 from the soil at a particular height of a tree is given by the delta C-13-value of the corresponding leaves, delta C-13 of atmospheric CO2, delta C-13 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 delta C-13-difference of 5 parts per thousand 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.