Effects of Tropospheric O3 on Trembling Aspen and Interaction with Co2: Results from an O3-Gradient and a Face Experiment

Over the years, a series of trembling aspen (Populus tremuloides Michx.) clones differing in O₃ sensitivity have been identified from OTC studies. Three clones (216 and 271[(O₃ tolerant] and 259 [O₃ sensitive]) have been characterized for O₃ sensitivity by growth and biomass responses, foliar symptoms, gas exchange, chlorophyll content, epicuticular wax characteristics, and antioxidant production. In this study we compared the responses of these same clones exposed to O₃ under field conditions along a natural O₃ gradient and in a Free-Air CO₂ and O₃ Enrichment (FACE) facility. In addition, we examined how elevated CO₂ affected O₃ symptom development. Visible O₃ symptoms were consistently seen (5 out of 6 years) at two of the three sites along the O₃ gradient and where daily one-hour maximum concentrations were in the range of 96 to 125 ppb. Clonal differences in O₃ sensitivity were consistent with our OTC rankings. Elevated CO₂ (200 ppm over ambient and applied during daylight hours during the growing season) reduced visible foliar symptoms for all three clones from 31 to 96% as determined by symptom development in elevated O₃ versus elevated O₃ + CO₂ treatments. Degradation of the epicuticular wax surface of all three clones was found at the two elevated O₃ gradient sites. This degradation was quantified by a coefficient of occlusion which was a measure of stomatal occlusion by epicuticular waxes. Statistically significant increases in stomatal occlusion compared to controls were found for all three clones and for all treatments including elevated CO₂, elevated O₃, and elevated CO₂ + O₃. Our results provide additional evidence that current ambient O₃ levels in the Great Lakes region are causing adverse effects on trembling aspen. Whether or not elevated CO₂ in the future will alleviate some of these adverse effects, as occurred with visible symptoms but not with epicuticular wax degradation, is unknown.     

Contributions of nitrification and denitrification to N2O emissions from soils at different water-filled pore space

A combination of stable isotope and acetylene (0.01% v/v) inhibition techniques were used for the first time to determine N₂O production during denitrification, autotrophic nitrification and heterotrophic nitrification in a fertilised (200 kg N ha^–1) silt loam soil at contrasting (20–70%) water-filled pore space (WFPS). ¹⁵N-N₂O emissions from ¹⁴NH₄¹⁵NO₃ replicates were attributed to denitrification and ¹⁵N-N₂O from ¹⁵NH₄¹⁵NO₃ minus that from ¹⁴NH₄¹⁵NO₃ replicates was attributed to nitrification and heterotrophic nitrification in the presence of acetylene, as there was no dissimilatory nitrate reduction to ammonium or immobilisation and remineralisation of ¹⁵N-NO₃^–. All of the N₂O emitted at 70% WFPS (31.6 mg N₂O-N m^–2 over 24 days; 1.12 g N₂O-N g dry soil^–1; 0.16% of N applied) was produced during denitrification, but at 35–60% WFPS nitrification was the main process producing N₂O, accounting for 81% of ¹⁵N-N₂O emitted at 60% WFPS, and 7.9 g ¹⁵N-N₂O m^–2 (0.28 ng ¹⁵N-N₂O g dry soil^–1) was estimated to be emitted over 7 days during heterotrophic nitrification in the 50% WFPS treatment and accounted for 20% of ¹⁵N-N₂O from this treatment. Denitrification was the predominant N₂O-producing process at 20% WFPS (2.6 g ¹⁵N-N₂O m^–2 over 7 days; 0.09 ng ¹⁵N-N₂O g dry soil^–1; 85% of ¹⁵N-N₂O from this treatment) and may have been due to the occurrence of aerobic denitrification at this WFPS. Our results demonstrate the usefulness of a combined stable isotope and acetylene approach to quantify N₂O emissions from different processes and to show that several processes may contribute to N₂O emission from agricultural soils depending on soil WFPS.     

Seasonal contribution of C3 and C4 species to ecosystem respiration and photosynthesis estimated from isotopic measurements of atmospheric CO2 at a grassland in Japan

Measurements of δ¹³C in atmospheric CO₂ and plant samples were made in 2003, along with CO₂ flux measurements, at a grassland site in Tsukuba in central Japan. The objective of the study was to obtain estimates of relative seasonal contributions of C₃ and C₄ plants to the net CO₂ flux over a C₃/C₄ grassland area influenced by the Asian monsoon. C₄ contribution to the ecosystem respiration (f_4R) increased from June to September, and then became constant at 63–67% through October and November. The seasonal variation in f_4R reflected the biomass composition of C₃ and C₄ plants at the measurement site. The seasonal C₄ contribution to photosynthesis (f_4P) was significantly different from its contribution to f_4R in May (8%) while it showed similar values to f_4R after June. The seasonal variations in f_4R and f_4P reflect the biomass composition ratio. Such seasonal transition from C₃ to C₄ of relative contribution to the carbon flux is similar to that observed in humid prairie ecosystems, while it is different from that in dry prairie ecosystems where the contribution of C₃ plants is lower and the seasonal maximum of the C₄-plant contribution occurs earlier compared to the humid prairie ecosystems.