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

Authors: Hofmockel KS, A Gallet-Budynek, HR McCarthy, WS Currie, RB Jackson, A Finzi

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 (FACE) experiment throughout the decade-long experiment. However it remains unclear how the N cycle has changed with elevated atmospheric 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 atmospheric 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 in the 7th year of CO2 fumigation and quantified the partitioning and redistribution of 15N for three subsequent growing seasons. The natural abundance of leaf litter was enriched under elevated compared to ambient CO2, consistent with enhanced N mineralization rates. After tracer application, the 15N tracer was initially retained in the organic and mineral soil horizons. Following the first two growing seasons after tracer application, 15N recoveries in tree biomass and soil pools were not significantly different between ambient and elevated CO2 treatments, even though total N uptake was greater under elevated CO2. By the end of the third growing season, 15N recovery in trees was significantly higher in elevated compared to ambient CO2. Recovery of 15N in plant biomass was predominantly in the canopy (3.5 ± 0.5%) followed by roots (1.7 ± 0.2%) and branches (1.7 ± 0.2%). Our natural-abundance 15N and 15N tracer results, taken together, clearly indicate that trees growing under elevated CO2 were able to acquire additional surface soil N resources to support increased plant growth. This study provides an integrated understanding of the effects of elevated CO2 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.

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