Faster-and-tighter nitrogen cycle supports mature forest productivity under elevated CO 2
- Manon Rumeau
- Carolina Mayoral
- Fotis Sgouridis
- Michaela K. Reay
- Yolima Carrillo
- Richard J. Norby
- Liz Hamilton
- Iain P. Hartley
- Gianni Micucci
- Emma Sayer
- A. Rob Mackenzie
- Sami Ullah
2026-07-29
Rising atmospheric carbon dioxide (CO 2 ) concentration could enhance forest carbon (C) uptake, but nitrogen (N) is thought to progressively constrain this effect. Yet, significant biomass gains were demonstrated in a mature oak forest after 6 years of Free Air CO 2 Enrichment. This study investigates whether changes in soil N fluxes explain these gains. We show that enhanced N mineralization and ecosystem N conservation support increased productivity under elevated CO 2 (eCO 2 ). In situ net and gross N ammonification was enhanced by ∼30%, with a pronounced effect at oak budburst. Higher N ammonification rates were associated with increases in both fine root biomass and soil respiration. Gross nitrification was reduced, revealing a faster-yet-tighter cycle facilitated by plant-soil interactions. Hence, forest N cycling can adapt to support C uptake under eCO 2 , but in the longer term, this capacity may be constrained by soil-accessible organic N stocks and reductions in anthropogenic N deposition.