Faster-and-tighter nitrogen cycle supports mature forest productivity under elevated CO2.

Rising atmospheric carbon dioxide (CO2) 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 CO2 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 CO2 (eCO2). 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 eCO2, but in the longer term, this capacity may be constrained by soil-accessible organic N stocks and reductions in anthropogenic N deposition.