Drivers of increased soil respiration in a poplar coppice exposed to elevated CO2

The response of soil respiration (SR) to elevated CO2 is driven by a number of processes and feedbacks. This work aims to i) detect the effect of elevated CO2 on soil respiration during the second rotation of a short rotation forest, at two levels of N availability; and ii) identify the main drivers behind any changes in soil respiration. A poplar plantation (POP-EUROFACE) was grown for two rotations of 3 years under elevated CO2 maintained by a FACE (Free Air CO2 Enrichment) technique. Root biomass, litter production and soil respiration were followed for two consecutive years after coppice. In the plantation, the stimulation of fine root and litter production under elevated CO2 observed at the beginning of the rotation declined over time. Soil respiration (SR) was continuously stimulated by elevated CO2, with a much larger enhancement during the growing (up to 111 %) than in the dormant season (40 %). The SR increase at first appeared to be due to the increase in fine root biomass, but at the end of the 2nd rotation was supported by litter decomposition and the availability of labile C. Soil respiration increase under elevated CO2 was not affected by N availability. The stimulation of SR by elevated CO2 was sustained by the decomposition of above and belowground litter and by the greater availability of easily decomposable substrates into the soil. In the final year as elevated CO2 did not increase C allocation to roots, the higher SR suggests greater C losses from the soil, thus reducing the potential for C accumulation.