Sources of increased N uptake in forest trees growing under elevated CO 2 : results of a large‐scale 15 N study

Abstract

Nitrogen availability in terrestrial ecosystems strongly influences plant productivity and nutrient cycling in response to increasing atmospheric carbon dioxide ( CO 2 ). Elevated CO 2 has consistently stimulated forest productivity at the Duke Forest free‐air CO 2 enrichment experiment throughout the decade‐long experiment. It remains unclear how the N cycle has changed with elevated CO 2 to support this increased productivity. Using natural‐abundance measures of N isotopes together with an ecosystem‐scale 15 N tracer experiment, we quantified the cycling of 15 N in plant and soil pools under ambient and elevated CO 2 over three growing seasons to determine how elevated CO 2 changed N cycling between plants, soil, and microorganisms. After measuring natural‐abundance 15 N differences in ambient and CO 2 ‐fumigated plots, we applied inorganic 15 N tracers and quantified the redistribution of 15 N for three subsequent growing seasons. The natural abundance of leaf litter was enriched under elevated compared to ambient CO 2 , consistent with deeper rooting and enhanced N mineralization. After tracer application, 15 N was initially retained in the organic and mineral soil horizons. Recovery of 15 N 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, 15 N recoveries in biomass and soil pools were not significantly different between CO 2 treatments, despite greater total N uptake under elevated CO 2 . After the third growing season, 15 N recovery in trees was significantly higher in elevated compared to ambient CO 2 . Natural‐abundance 15 N and tracer results, taken together, suggest that trees growing under elevated CO 2 acquired additional soil N resources to support increased plant growth. Our study provides an integrated understanding of elevated CO 2 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 CO 2 beyond the decadal time scale.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87073/1/gcb2465.pd