Free Atmospheric CO2 Enrichment (FACE) Increased Labile and Total Carbon in the Mineral Soil of a Short Rotation Poplar Plantation

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. FACE treatment increased the labile C fraction at 0–10 cm depth, which is in agreement with the larger input of plant litter and root exudates under FACE. N-mineralization rates were not affected by FACE. We infer that the system switched from a state where extra labile C and sufficient N-availability (due to the former agricultural use of the soil) caused a priming effect (first rotation), to a state where extra C input is accumulating due to limited N-availability (second rotation). Our results on N-mineralization (second rotation) are in agreement with observations made at three forest FACE sites (Duke Forest, Oak Ridge, and Rhinelander), but our finding of increasing mineral soil C content contrasted with results at the Duke Forest where no significant increase in C content of the mineral soil occurred. However, the FACE induced increase in total C content occurred within the fraction with the shortest turnover time, i.e. the labile fraction. The refractory and stable fractions were not affected. The question remains whether the currently observed larger increase of total soil C and the increase of labile C under FACE will eventually result in long-term C storage in refractory and stable organic matter fractions.