More new carbon in the mineral soil of a poplar plantation under Free Air Carbon Enrichment (POPFACE): Cause of increased priming effect?

[1] In order to establish suitability of forest ecosystems for long-term storage of C, it is necessary to characterize the effects of predicted increased atmospheric CO2 levels on the pools and fluxes of C within these systems. Since most C held in terrestrial ecosystems is in the soil, we assessed the influence of Free Air Carbon Enrichment (FACE) treatment on the total soil C content (C-total) and incorporation of litter derived C (C-new) into soil organic matter (SOM) in a fast growing poplar plantation. C-new was estimated by the C3/C4 stable isotope method. C-total contents increased under control and FACE respectively by 12 and 3%, i.e., 484 and 107 gC/m(2), while 704 and 926 gC/m(2) of new carbon was sequestered under control and FACE during the experiment. We conclude that FACE suppressed the increase of C-total and simultaneously increased C-new. We hypothesize that these opposite effects may be caused by a priming effect of the newly incorporated litter, where priming effect is defined as the stimulation of SOM decomposition caused by the addition of labile substrates.