Free atmospheric CO2 enrichment (FACE) increased respiration and humification in the mineral soil of a poplar plantation

Free atmospheric CO2 enrichment (FACE) studies conducted at the whole-tree and ecosystem scale indicate that there is a marked increase in primary production, mainly allocated into below-ground biomass. The enhanced carbon transfer to the root system may result in enhanced rhizodeposition and subsequent transfer to soil C pools. However, the impact of elevated CO2 on soil C contents has yielded variable results. The fate and function of this extra C into the soil in response to elevated CO2 are not clear. The POPFACE experiment was initiated early 1999 with the objective to determine the functional responses of a short-rotation poplar plantation to actual and future atmospheric CO2 concentrations. During the first 2 years of the second rotation (2002–2003), the increase of total soil C% was larger under FACE than under ambient CO2. Chemical fractionation revealed the presence of more labile soil C under FACE, which is in agreement with the larger input of plant litter and root exudates under FACE. In order to gain insight into the fate and function of this extra C into the soil and the dynamics of soil C, we incubated soil samples, measured respiration rates and used a simple soil C model to interpret the results. FACE increased the accumulated 28-day CO2 production and the initial Cslow pool content (metabolizable plant remains and partly decomposed SOM). FACE also increased the decomposition rates of the metabolizable C pools (Cfast + Cslow) in the top soil, while for the subsoil the opposite effect was observed. The modeled formation of humified SOM was also enhanced by FACE. Our results support the terrestrial feedback hypothesis, i.e. an increase of the long-term terrestrial C sink in response to increasing atmospheric CO2 concentrations.