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In the future, with elevated atmospheric CO 2 (eCO 2 ), forests are expected to increase woody biomass to capture more carbon (C), though this is dependent on soil nutrient availability. While young forests may access unused nutrients by growing into an unexplored soil environment, it is unclear how or if mature forests can adapt belowground under eCO 2 . Soil respiration ( R s ) and nutrient bioavailability are integrative ecosystem measures of below-ground dynamics. At Birmingham's Institute of Forest Research Free Air CO 2 Enrichment (BIFoR FACE) facility, we investigated the effects of eCO 2 (+150 ppm above ambient) on a mature oak forest during the first year of exposure. We observed an annual R s increase of ∼21.5%; 996 ± 398 g C m -2 year -1 (ambient) to 1210 ± 483 g C m -2 year -1 (eCO 2 ). The eCO 2 impact was greater on belowground nutrient cycling, with monthly nitrate availability decreasing by up to 36%. These results show that high C uptake resulted in higher soil respiration with a concomitant decrease in the level of soil nitrate during the first year. These belowground responses and their long-term dynamics will have implications for the carbon budget of mature forest ecosystems in changing climate., Competing Interests: The authors declare no competing financial interest., (© 2025 The Authors. Published by American Chemical Society.)