Forests are potential carbon (C) sinks that partially offset anthropogenic carbon dioxide (CO2) emissions via enhanced C assimilation and productivity. However, the question remains whether mature trees will express sufficient plasticity in nutrient acquisition strategies to support enhanced growth under elevated CO2 (eCO2). Trees may sustain growth by investing C belowground to enhance nutrient acquisition, e.g., by increasing root absorptive surfaces for greater soil available resource exploration (a “do-it-yourself” strategy) or utilizing C exudation or mycorrhizal associations as priming mechanisms for nutrient acquisition (“outsourcing”). We show that 4 y of eCO2 (+140 ± 38 ppm; i.e., +35% above ambient) altered the relative belowground C investment strategies of mature oak (Quercus robur L.) in a 180-y-old temperate forest. Fine-root branching frequency increased 73% under eCO2. Specific root C exudation was enhanced under eCO2 (63%), particularly outside the peak growing season, and the exudate C to nitrogen (N) ratio was increased (28%). Ectomycorrhizal (ECM) biomass production increased during leaf fall (17%) while ECM turnover increased almost fourfold under eCO2. The exudate and root metabolome composition were considerably altered during the late growing season under eCO2. We find, therefore, that a broad suite of nutrient acquisition strategies are upregulated under eCO2, with dynamic shifting between different outsourcing and do-it-yourself elements at different times of the year. These belowground changes support the increase in net primary productivity observed in this forest, with implications for the role of mature temperate forests in the global carbon sink.