FOLIAR NITROGEN DYNAMICS AND DECOMPOSITION OF YELLOW-POPLAR AND EASTERN WHITE-PINE DURING 4 SEASONS OF EXPOSURE TO ELEVATED OZONE AND CARBON-DIOXIDE

Yellow-poplar (Liriodendron tulipifera L.) and eastern white pine (Pin us strobus L.) seedlings growing in two plantations were fumigated fro m 1992 to 1995 in open-top chambers with charcoal-filtered air (CF), a mbient air (chamberless), one time ambient ozone (1X), twice ambient o zone (2X), or twice ambient ozone+twice ambient CO2 (2X+CO2). Across a ll treatments and years, concentrations of foliar nitrogen (N) in yell ow-poplar averaged 26.3 mg g(-1) in June and decreased to 13.8 mg g(-1 ) just prior to leaf senescence. While leaves from all treatments exhi bited similar seasonal reductions, actual N concentrations were greatl y affected by treatment. Ozone (O-3) alone did not significantly alter foliar N concentrations; however, 2X+CO2 decreased N concentrations b y 18-40% depending on time of sampling. After one season of fumigation , 2X+CO2-exposed leaves of yellow-poplar decayed significantly more sl owly than leaves from all other treatments. In contrast, white pine ne edles demonstrated few differences in N concentrations or decompositio n during the first 3 years of fumigation. By the forth season, 2X-CO2- air significantly reduced N concentrations of current year needles. In needles grown in CF air N concentrations ranged from 14.8 mg g(-1) in June to 17.2 mg g(-1) in October. 2X+CO2-air reduced N levels in whit e pine by 10-23% depending on time of sampling. For both species, sign ificant differences in N due to leaf age and canopy position must be t aken into consideration when evaluating the data. Our experiments indi cate that elevated CO2 in the presence of elevated O-3 can reduce foli ar N concentrations and reduce litter decay, thus affecting nutrient c ycling. (C) 1998 Elsevier Science B.V.