Sources of increased N uptake in forest trees growing under elevated CO2: results of a large‐scale 15N study

Nitrogen availability in terrestrial ecosystems strongly influences plant productivity and nutrient cycling in response to increasing atmospheric carbon dioxide (CO₂). Elevated CO₂ has consistently stimulated forest productivity at the Duke Forest free‐air CO₂ enrichment experiment throughout the decade‐long experiment. It remains unclear how the N cycle has changed with elevated CO₂ to support this increased productivity. Using natural‐abundance measures of N isotopes together with an ecosystem‐scale ¹⁵N tracer experiment, we quantified the cycling of ¹⁵N in plant and soil pools under ambient and elevated CO₂ over three growing seasons to determine how elevated CO₂ changed N cycling between plants, soil, and microorganisms. After measuring natural‐abundance ¹⁵N differences in ambient and CO₂‐fumigated plots, we applied inorganic ¹⁵N tracers and quantified the redistribution of ¹⁵N for three subsequent growing seasons. The natural abundance of leaf litter was enriched under elevated compared to ambient CO₂, consistent with deeper rooting and enhanced N mineralization. After tracer application, ¹⁵N was initially retained in the organic and mineral soil horizons. Recovery of ¹⁵N in plant biomass was 3.5 ± 0.5% in the canopy, 1.7 ± 0.2% in roots and 1.7 ± 0.2% in branches. After two growing seasons, ¹⁵N recoveries in biomass and soil pools were not significantly different between CO₂ treatments, despite greater total N uptake under elevated CO₂. After the third growing season, ¹⁵N recovery in trees was significantly higher in elevated compared to ambient CO₂. Natural‐abundance ¹⁵N and tracer results, taken together, suggest that trees growing under elevated CO₂ acquired additional soil N resources to support increased plant growth. Our study provides an integrated understanding of elevated CO₂ effects on N cycling in the Duke Forest and provides a basis for inferring how C and N cycling in this forest may respond to elevated CO₂ beyond the decadal time scale.