Microorganisms are the regulators of decomposition processes occurring in soil, they also constitute a labile fraction of potentially available N. Microbial mineralization and nutrient cycling could be affected through altered plant inputs at elevated CO 2. An understanding of microbial biomass and microbial activity in response to belowground processes induced by elevated CO 2 is thus crucial in order to predict the long-term response of ecosystems to climatic changes. Microbial biomass, microbial respiration, inorganic N, extractable P and six enzymatic activities related to C, N, P and S cycling (β-glucosidase, cellulase, chitinase, protease, acid phosphatase and arylsulphatase) were investigated in soils of a poplar plantation exposed to elevated CO 2. Clones of Populus alba, Populus nigra and Populus x euramericana were grown in six 314m 2 plots treated either with atmospheric (control) or enriched (550μmolmol −1 CO 2 ) CO 2 concentration with FACE technology (free-air CO 2 enrichment). Chemical and biochemical parameters were monitored throughout a year in soil samples collected at five sampling dates starting from Autumn 2000 to Autumn 2001.The aim of the present work was: (1) to determine if CO 2 enrichment induces modifications to soil microbial pool size and metabolism, (2) to test how the seasonal fluctuations of soil biochemical properties and CO 2 level interact, (3) to evaluate if microbial nutrient acquisition activity is changed under elevated CO 2 .CO 2 enrichment significantly affected soil nutrient content and three enzyme activities: acid phosphatase, chitinase and arylsulphatase, indicators of nutrient acquisition activity. Microbial biomass increased by a 16% under elevated CO 2 . All soil biochemical properties were significantly affected by the temporal variability and the interaction between time and CO 2 level significantly influenced β-glucosidase activity and microbial respiration. Data on arylsulphatase and chitinase activity suggest a possible shift of microbial population in favour of fungi induced by the FACE treatment.