[This dataset is embargoed until December 31, 2025]. The data set was generated at the University of Birmingham Free Air CO2 Enrichment (BIFoR-FACE) facility where the responses of nutrient transformation processes, litter decomposition, mycorrhizal biomass and turnover and enzyme functions, root exudation rates, root nitrogen uptake rates and preferences were evaluated during the 4th to 6th years of CO2 fumigation (2020-2022) The first hypothesis tested was that trees under elevated CO2 will allocate more carbon belowground (via exudation and symbiotic mycorrhizal fungi) for nutrient acquisition. The second hypothesis tested was the enhanced C allocation belowground will prime microbes for nutrient mineralisation via extracellular enzyme functions to meet enhanced nutrient demands. The third hypothesis tested was that trees would take more nitrogen and that the order preference for different available nitrogen forms is amino acids, ammonium, and nitrate. Coupled with this hypothesis the experimental work included both field and laboratory characterisation of selected variables and their responses to CO2 enrichment including gross N mineralisation, litter decomposition, extracullular enzyme activities, mycorrhizal (arubuscular and ectomycorhizae) characterisation, hyphael turnover, root C and N exudation and root uptake rates and preferences. The data on these variables is provided in CSV files together with metadata files detailing the experimental, analytical and quality control aspects of the data collection, curation and finalisation. These variables were assessed during the fourth to the sixth year of CO2 enrichment at BIFoR-FACE, thus creating a key data for the early responses of CO2 fumigation. Since BIFoR_FACE will continue the experiment into 2030s, thus this data provides the early responses in a long-term experiment of its kind in the UK and in northern temperate environments in the world. Full details about this dataset can be found at https://doi.org/10.5285/a05b9519-f0c8-48ef-a9c6-43d0326f590f