Rapid Increase in Soil Respiration and Reduction in Soil Nitrate Availability Following CO₂ Enrichment in a Mature Oak Forest

Kourmouli, Angeliki and Hamilton, R. Liz and Pihlblad, Johanna and Bartlett, Rebecca and MacKenzie, Angus Robert and Hartley, Iain P. and Ullah, Sami and Shi, Zongbo (2025) Rapid Increase in Soil Respiration and Reduction in Soil Nitrate Availability Following CO₂ Enrichment in a Mature Oak Forest. ACS Omega. ISSN 2470-1343

Abstract

In the future, with elevated atmospheric CO2 (eCO2), 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 eCO2. Soil respiration (Rs) and nutrient bioavailability are integrative ecosystem measures of below-ground dynamics. At Birmingham’s Institute of Forest Research Free Air CO2 Enrichment (BIFoR FACE) facility, we investigated the effects of eCO2 (+150 ppm above ambient) on a mature oak forest during the first year of exposure. We observed an annual Rs increase of ∼21.5%; 996 ± 398 g C m–2 year–1 (ambient) to 1210 ± 483 g C m–2 year–1 (eCO2). The eCO2 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.