Free atmospheric CO2 enrichment (FACE) increased labile and total carbon in the mineral soil of a short rotation Poplar plantation, Plant and Soil

The global net terrestrial carbon sink was estimated to range between 0.5 and 0.7 Pg C y)1 for the early 1990s. FACE (free atmospheric CO2 enrichment) studies conducted at the whole-tree and community scale indicate that there is a marked increase of 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. During the first rotation of the POP/EuroFACE experiment in a short-rotation Poplar plantation, total soil C content increased more under ambient CO2 treatment than under FACE, while under FACE more new C was incorporated than under ambient CO2. These unexpected and opposite effects may have been caused by a priming effect, where priming effect is defined as the stimulation of SOM decomposition caused by the addition of labile substrates. In order to gain insight into these processes affecting SOM decomposition, we obtained the labile, refractory and stable pools of soil C and N by chemical fractionation (acid hydrolysis) and measured rates of N-mineralization. Results of the first 2 years of the second rotation show a larger increase of total soil C % under FACE than under ambient CO2. In contrast to the first rotation, total C % is now increasing faster under FACE than under ambient CO2. Based on these observations we infer that the priming effect ceased during the second rotation. FAC