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INTERACTIVE EFFECTS OF CO2 AND O3 ON A PONDEROSA PINE PLANT/LITTER/SOIL MESOCOSM
Olszyk, D M., M G. Johnson, D L. Phillips, R J. Seidler, D T. Tingey, AND L S. Watrud. INTERACTIVE EFFECTS OF CO2 AND O3 ON A PONDEROSA PINE PLANT/LITTER/SOIL MESOCOSM. ENVIRONMENTAL POLLUTION. Elsevier Science Ltd, New York, NY, 115:447-462.
To study individual and combined impacts of two important atmospheric trace gases, CO2 and O3, on C and N cycling in forest ecosystems; a four-year experiment using a small-scale ponderosa pine (Pinus ponderosa Laws.) seedling/soil/litter system was initiated in April, 1998. The experiment was conducted in outdoor, sun-lit chambers where aboveground and belowground ecological processes can be studied in detail. This paper describes the approach and experimental methodology used in the experiment, and presents preliminary data on responses after the first two growing seasons. CO2 treatments were ambient and elevated (ambient + 280 ppm). O3 treatments were elevated (hourly averages to 159 ppb, cumulative exposure >60 ppb O3 (SUM 06) of ~ 10.37 ppm ? hr), and a low control level (nearly all hourly averages <40 ppb, SUM 06 ~ 0.07 ppm ? hr). Significant (p<0.05) individual and interactive effects occurred with elevated CO2 and elevated O3. Elevated CO2 increased needle-level net photosynthetic rates over both seasons. Following the first season, the highest photosynthetic rates were for trees with elevated CO2, and which had previously received elevated O3. Elevated CO2 increased seedling growth as indicated by stem diameter, with he biggest increase at the low O3 level. Elevated CO2 decreased current year needle % N in the summer. For 1-year old needles in the fall there was a decrease in % N with elevated CO2 at low O3, but an increase in % N with elevated CO2 at elevated O3. Nitrogen fixation (measured by acetylene reduction) was low in Ponderosa pine litter and there were no significant CO2 or O3 effects. Neither elevated CO2 nor elevated O3 affected standing root biomass or root length density. Elevated O3 decreased the % N in coarse-fine (1-2 mm diameter) but not in fine (<1 mm diameter) roots. Both elevated CO2 and elevated O3 tended to increase the number of fungal colony forming units (CFUs) in the AC soil horizon, and elevated O3 tended to decrease bacterial CFUs in the C soil horizon; but only at p=0.054 to 0.075. When completed, this experiment will provide detailed information on interactive effects of CO2 and O3 on C and N biogeochemical cycling for a forest system by testing three hypotheses: 1) elevated O3 decreases C and N cycling, 2) elevated CO2 increases C and N cycling, and 3) elevated CO2 prevents negative effects of O3 on C and N cycling; and will address the role of litter/soil ecology in any treatment effects on C and N cycling.