Soil microbial indices as bioindicators of environmental changes in a poplar plantation

An understanding of microbial biomass and microbial activity as part of belowground processes as affected by elevated CO2 is crucial in order to predict the long-term response of ecosystems to climatic changes. The ratio of biomass C to soil organic C (Cmic:Corg), the metabolic quotient (the specific soil respiration of the microbial biomass, qCO2), the C mineralization quotient (the fraction of total organic C mineralized throughout the incubation, qM), the microbial biomass change rate quotient (qC) and soil inorganic nitrogen content were determined on soil samples taken during 3 years (Fall 2000-Fall 2003) in a poplar plantation exposed to increased atmospheric CO2 by means of FACE (Free Air CO2 Enrichment) technique and nitrogen fertilization. A competition for nitrogen between plants and microrganisms, stronger in FACE plots, induced a stress condition within microbial community. FACE treatment provided C for microbial growth (Cmic:Corg), but reducing nitrogen availability, led to a higher microbial loss over time (qC). Nitrogen fertilization decreased microbial mortality lowering energetic maintenance requirements (qCO2) and induced a short-term shift in favour of microrganisms more rapid in the use of the resources. The C mineralization quotient (qM) was not affected by either FACE nor fertilization treatment meaning that the fraction of total organic carbon mineralized during the incubation period did not vary significantly.