Chronic drought limits new carbon inputs and homogenizes surface soil carbon age and content in Mediterranean forests

Abstract. Increasing drought frequency and severity are expected to affect carbon cycling and storage in Mediterranean forests substantially. However, the temporal dynamics of carbon within ecosystem pools under long-term drought remain poorly understood. In this study, we present the first comprehensive assessment of the age of all major carbon pools and respired CO₂ in a Mediterranean forest, using ¹⁴C measurements complemented with soil carbon content analyses. Additionally, we fitted compartmental carbon cycle models to empirical data collected through the 25-year rainfall exclusion experiment. We found that carbon in drought plots was generally older than in controls, with consistent trends across most pools, except roots and foliage. Differences were most pronounced in ecosystem respiration and in the 0–5 cm soil layer. Drought homogenized upper soil layers in both carbon content and age, suggesting depletion of older reserves and reduced stabilization of recent inputs. Overall, our results show that long-term drought promotes the microbial use of older carbon substrates in the soil, limits the incorporation of new inputs into slow-cycling reservoirs, and increases the vulnerability of existing soil carbon stocks. These results underscore the need for forest management strategies that maintain sequestered carbon and enhance the stabilization of fresh CO₂ inputs under intensifying aridity.