DECOMPOSITION AND NITROGEN RELEASE FROM LEAVES OF 3 HARDWOOD SPECIES GROWN UNDER ELEVATED O-3 AND OR CO2/

Elevated concentrations of O-3 and CO2 have both been shown to affect structure, nutrient status, and deposition of secondary metabolites in leaves of forest trees. While such studies have produced robust model s of the effects of such air pollutants on tree ecophysiology and grow th, few have considered the potential for broader, ecosystem-level eff ects after these chemically and structurally altered leaves fall as le af litter and decay. To determine the effects of elevated O-3 and/or C O2 on the subsequent decomposition and nutrient release from the leave s grown in such altered atmospheres, we grew seedlings of three widesp read North American forest trees, black cherry (Prunus serotina) (BC), sugar maple (Acer saccharum) (SM), and yellow-poplar (Liriodendron tu lipifera) (YP) for two growing seasons in charcoal-filtered air (CF-ai r=approximately 25% ambient O-3), ambient O-3 (1X) or twice-ambient O- 3 (2X) in outdoor open-top chambers. We then assayed the loss of mass and N from the litter derived from those seedlings through one year li tterbag incubations in the forest floor of a neighboring forest stand. Mass loss followed linear functions and was not affected by the O-3 r egime in which the leaves were grown. Instantaneous decay rates (i.e. k values) averaged SM:-0.707 y(-1), BC:-0.613 y(-1), and YP:-0.859 y(- 1). N loss from ambient (1X) O-3-grown SM leaves was significantly gre ater than from CF-air leaves; N loss from BC leaves did not differ amo ng treatments. Significantly less N was released from CF-air-grown YP leaves than from 1X or 2X O-3-treated leaves. YP leaves from plants gr own in pots at 2X O-3 and 350 ppm supplemental CO2 in indoor pollutant fumigation chambers (CSTRs or Continuously Stirred Tank Reactors) los s 40% as much mass and 27% as much N over one year as did leaves from YP grown in CF-air or 2X O-3. Thus, for leaves from plants grown in po ts in controlled environment fumigation chambers, the concentrations o f both O-3 and CO2 can affect N release from litter incubated in the f ield whereas mass loss rate was affected only by CO2. Because both mas s loss and N release from leaves grown at elevated CO2 were reduced si gnificantly (at least for yellow-poplar), forests exposed to elevated CO2 may have significantly reduced N turnover rates, thereby resulting in increased N limitation of tree growth, especially in forests which are already N-limited.