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

Abstract

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.