In the context of the EURoot project, a large maize phenotyping field experiment was conducted in order to assess root architecture and functional traits amongst a panel of 23 maize genotypes across a gradient of P fertility (long term P fertilizer trial of INRA Auzeville near Toulouse SW France). Four P levels were tested, from P0 (treatment without P fertilizer since 1969) to P4 (treatment with rates of P fertilizer application between 3- and 4-fold the offtake by crops). Rhizosphere (soil adhering to roots) and bulk soil were destructively sampled at the 6-8 leaves stage when P deficiency was clearly visible in many genotypes in the P0 treatment. We measured the substrate-Induced respiration with the MicroRespTM technique (3 substrates and a proxy of microbial biomass) and basal respiration of the 350 soil samples from the topsoils of the two extreme P treatments (P0 and P4). Our first data analysis shows a positive P effect on microbial biomass and a significant interaction of phosphorus/type of soil effect (bulk/rhizosphere). No maize genotype effect was evident from the analysis. In general, P fertilization (P4 versus P0) seemed to have increased the differences in microbial parameters between rhizosphere and bulk soils. Acknowledgement: This research was part of the EURoot project, which received funding from the European Community Seventh Framework Programme FP7-KBBE-2011-5 under grant agreement no.289300.