Document: ADR-3-63-29

The distribution of N in plants, soils and microbes following three years of free-air carbon dioxide enrichment (FACE).

FINZI, A.C.* ¹, A.S.ALLEN ², E.H.DELUCIA ³ and W.H.SCHLESINGER ⁴

Boston University, Boston, MA 02215 USA ¹
UCSB, Santa Barbara, CA 93106 USA ²
Univerisity of Illinois, Urbana, IL 61801 USA ³
Duke University, Durham NC 27708 ⁴

Abstract:
In prior research, we found that elevated carbon dioxide increased forest productivity and the flux of nitrogen (N) from vegetation to soils via litterfall. We hypothesized that the loss of N in litterfall would constrain ecosystem NPP under elevated CO₂ unless the N loss was offset by a sustained increase in forest N-use efficiency (NUE) or N uptake from soils. To determine (1) the source of N lost from forest vegetation; (2) how N availability will change over time; and (3) whether high NUE is a mechanism capable of maintaining high NPP in a N limited ecosystem, we studied the pools and fluxes of N within a loblolly pine - southern hardwood forest exposed to three years of free-air carbon dioxide enrichment (FACE). Specifically, we mass balanced the ecosystem with respect to N pools present in vegetation (loblolly pine and deciduous species), soils and microbes and the fluxes between these pools. We also measured N input through net canopy exchange. The mass balance suggested that the N lost in litterfall was derived from plant uptake of soil N (including N deposition) and redistribution from storage. Microbial biomass and activity (¹⁵N pool dilution) in these soils has not changed under elevated CO₂ despite significant increases in belowground C-allocation. Net rates of N mineralization have remained constant in mineral soil but have increased in the forest floor due to the increase in forest floor mass. However, NUE increased through time. These results suggest that plant uptake of N is exceeding the rate of N supply. Thus, in the long-term, N limitation will very likely constrain forest productivity under FACE.

Keywords: Forest; Nitrogen; Litterfall; NPP; Elevated CO2

This abstract is being presented at: 3:30 PM in session:
Poster Session #15: Nutrient Cycling.