They say “carbon!”, we say “…and nutrients!”: N-cycle biogeochemistry sustaining net productivity in a long-established temperate broadleaf forest under elevated CO2

Enhanced ‘woody growth’ (dry matter increments, specifically), averaging 10%, has been sustained in patches of long-established (180+ years old) oak forest through 9 years of treatment with elevated CO₂ (eCO₂; 150 ppm above ambient). Root exudation of carbon (C) into the rhizosphere increased by 63%, which primed the microbes for nutrient acquisition to meet enhanced tree N demands. A ‘faster-tighter’ nitrogen cycle accelerates the return of nitrogen via ammonification to plant-available forms and suppresses processes such as nitrification. This ecosystem-scale N conservation strategy supports increased net productivity by maintaining the nutritional balance of the trees in the C-rich atmosphere. The faster-tighter N-cycle makes an additional 25 kg N ha^-1 yr^-1 available to the trees under eCO₂. That is, the forest’s N-cycle adjusts to the increased C supply, but whether this capacity to adjust endures may be constrained by soil organic N stocks and anthropogenic N deposition. Further, when considering broader aspects of the forest under eCO₂, we find nutritional deficiencies producing a cascade of nascent ecosystem fragility in pollen, seeds, seedlings, and food webs. The clear policy implications are: (i) that enhanced net primary productivity does not, in itself, guarantee forest resilience; (ii) that both C and N emission pathways must be accounted for when forecasting 21^st-century C uptake into temperate forests; and (iii) that, when proposing forests as natural climate solutions, understanding C-nutrient interactions is of primary concern.