Interactive effects of CO2 and O3 on a ponderosa pine plant/litter/soil mesocosm

To study individual and combined impacts of two important atmospheric trace gases, CO 2 and O 3 , on C and N cycling in forest ecosystems; a multi-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 could be studied in detail. This paper describes the approach and methodology used, and presents preliminary data for the first two growing seasons. CO 2 treatments were ambient and elevated (ambient + 280 ppm). O 3 treatments were elevated (hourly averages to 159 ppb, cumulative exposure > 60 ppb O 3 . SUM 06 ∼10.37 ppm h), and a low control level (nearly all hourly averages <40 ppb, SUM 06 ∼0.07 ppm h). Significant (P<0.05) individual and interactive effects occurred with elevated CO 2 and elevated O 3 . Elevated CO 2 increased needle-level net photosynthetic rates over both seasons. Following the first season, the highest photosynthetic rates were for trees which had previously received elevated O 3 in addition to elevated CO 2 . Elevated CO 2 increased seedling stem diameters, with the greatest increase at low O 3 . Elevated CO 2 decreased current year needle % N in the summer. For 1-year-old needles measured in the fall there was a decrease in % N with elevated CO 2 at low O 3 , but an increase in % N with elevated CO 2 at elevated O 3 . Nitrogen fixation (measured by acetylene reduction) was low in ponderosa pine litter and there were no significant CO 2 or O 3 effects. Neither elevated CO 2 nor elevated O 3 affected standing root biomass or root length density. Elevated O 3 decreased the % N in coarse-fine (1 2 mm diameter) but not in fine ( < 1 mm diameter) roots. Both elevated CO 2 and elevated O 3 tended to increase the number of fungal colony forming units (CFUs) in the AC soil horizon, and elevated O 3 tended to decrease bacterial CFUs in the C soil horizon. Thus, after two growing seasons we showed interactive effects of O 3 and CO 2 in combination, in addition to responses to CO 2 or O 3 alone for a ponderosa pine plant/litter/soil system.