Three-year growth responses of Pinus taeda L. to simulated rain chemistry,soil magnesium status,and ozone

Height, diameter, and biomass were measured for loblolly pine (Pinus taeda L) seedlings grown in soil containing 15 or 35 Μg Mg g^?1 and exposed from May to October in 1987, 1988, and 1989 to three O₃ concentrations (sub-ambient, ambient, or twice-ambient) and to rain pH levels of 3.8 or 5.2. Reduction in biomass accumulation in seedlings exposed to twice-ambient O₃ vs sub-ambient O₃ was 14% (P = 0.03) in 1987, 11.4% (P = 0.002) by 1988, and 8% (P = 0.15) by 1989. The greatest height growth occurred in seedlings exposed to twice-ambient O₃, and the greatest stem diameter growth occurred in seedlings exposed to sub-ambient O₃. A comparison of stem volume (d²h) with stem biomass suggested that tissue density was reduced by elevated O₃. Biomass accumulation response to rainfall chemistry was small (5.5% reduction in the low pH treatment in 1989) and not statistically significant for most plant tissues. Growth response to soil Mg status was not significant. Hoewever, in 1989 treatment interactions between rainfall chemistry and soil Mg status were observed. Height was 5% greater (P = 0.02) and biomass was 6% greater (P = 0.10) in seedlings grown in higher-Mg soil and receiving higher-pH rainfall than seedlings grown in any of the other pH-Mg treatment combinations. The data suggest direct adverse effects of near ambient O₃ and indirect, slower acting and interacting adverse effects of rainfall chemistry and soil nutrient status on growth of loblolly pine.     

Influence of O3, rainfall acidity,and soil Mg status on growth and ectomycorrhizal colonization of loblolly pine roots

Root biomass, length, and branching frequency, and number and type of mycorrhizal short roots were determined for loblolly pine seedlings grown at two levels of soil Mg and exposed to chronic levels of O₃ and simulated acidic rainfall. Seedlings were planted in a sandy loam soil having approximately 15 or 35 mg kg^?1 Mg and were exposed to subambient; ambient, or twice ambient concentrations of O₃ in open top chambers from May through October. Seedlings also received ambient amounts of simulated rainfall at pH 3.8 or 5.2. Root biomass, length, and branching frequency were not significantly affected by O₃, rainfall acidity, or soil Mg treatments. Seedlings grown in the subambient O₃ treatment had a greater number of short roots infected with mycorrhizae than seedlings grown in ambient or twice ambient O₃ treatments, but trends were not statistically significant. Increasing rainfall acidity and soil Mg concentration resulted in a significantly (P = 0.07) greater number of mycorrhizal short roots, due primarily to an increased occurrence of one corraloid mycorrhizal type. Results suggest that mycorrhizal fungi differ in their response to O₃, rainfall acidity, and soil Mg status, and suggest that mycorrhizal infection is more sensitive than seedling root growth to O₃, acidic rainfall, and soil Mg status.     

Ozone,acidic precipitation,and soil Mg impacts on soil and loblolly pine seedling nutrient status after three growing seasons

Recent studies have suggested that the growth of loblolly pine (Pinus taeda L.) has declined in the southeastern United States, possibly due to acidic deposition and air pollutants, especially under conditions of low nutrient availability. Consequently, the potential for individual and synergistic impacts of O₃, acidic precipitation, and soil Mg status on the nutrient status of loblolly pine seedlings and soil was investigated over a 3 yr study period. Thirty-six open top chambers equipped with a rainfall exclusion/addition system were utilized to administer three levels of O₃ (subambient, ambient, or twice ambient) and two acidic precipitation treatments (pH 3.8 or 5.2) to seedlings growing in 24-L plastic pots containing soil having either 35 or 15 mg kg^?1 of exchangeable Mg. Each chamber contained 36 pots, and each treatment combination was replicated six times for a total of 1296 individual pots. After three seasons, throughfall and foliar nutrition data indicated that foliar leaching was not accelerated by increasing the acidity of precipitation from pH 5.2 to 3.8 and that increasing O₃ did not act to exacerbate foliar leaching. Further, foliar nutrient concentrations were not significantly affected by precipitation pH or O₃ treatments. Soil and soil solution data also indicate no accelerated soil leaching associated with chronic acidic precipitation. Differences in soil Mg treatments were reflected in soil solution and seedling Mg contents, but the 15 mg kg^?1 soil Mg treatment was not sufficiently low enough to induce Mg deficiency in the seedlings.     

The influence of acid precipitation and ozone on nitrogen nutrition of young loblolly pine

Field grown loblolly pine (Pinus taeda L.) seedlings from two half-sibling families were exposed to three levels of acid precipitation and four levels of O₃ in open top chambers at Auburn, AL. At the end of one growing season, dry weight and total N accumulation of seedlings was directly related. to rainfall acidity, possibly indicating a fertilizer response in the N deficient soil. Increasing O₃ levels elicited an increase in N content of permanent tissues that was not related to a dry weight response. The origin of this N was believed to be increased internal translocation associated with enhanced premature needle senescence observed at higher O₃ levels.     

Growth of Pinus Taeda L. Seedlings varies with Family and ozone exposure level

Loblolly pine seedlings of five half-sib families were grown under ambient, subambient (approximately 0.6 × ambient), and elevated [ambient + 60 ppb O₃ (120 μg m^?3)] O₃ levels for one growing season in open-topped chambers. Diameter and height of the seedlings were measured periodically over the growing season, and above ground and below ground biomass were determined at harvest. Significant genetic differences were found in above ground volume (D ²H) 1 mo after 03 fumigation began and continued until harvest. Biomass of secondary needles and coarse and fine roots also differed significantly among families. Elevated O3 resulted in significantly decreased D ²H and secondary needle biomass relative to seedlings grown in ambient air. Seedlings receiving subambient O₃ levels were intermediate in size, but were not significantly different from seedlings fumigated at ambient O₃ levels. Root and stem biomass did not differ significantly among treatments. A significant interaction of O₃ and genotype was detected, suggesting that the response of loblolly pine to O₃ is influenced by genotype.     

Deciduous Conifers: High N Deposition and O3 Exposure Effects on Growth and Biomass Allocation in Ponderosa Pine

Ponderosa pines (Pinus ponderosa Dougl. ex. Laws) 21 to 60 yr old were used to assess the relative importance of environmental stressors (O₃, drought) versus an enhancer (N deposition) on foliar retention, components of aboveground growth, and whole tree biomass allocation. Sites were chosen across a well-described gradient in ozone exposure (40 to 80 ppb per h, 24 h basis, 6 month growing season) and nitrogen deposition (5 to 40 kg ha^-1 yr^-1) in the San Bernardino Mountains east of Los Angeles, California. A high level of chlorotic mottle indicated high O₃ injury at sites closest to the pollution source, despite potential for the mitigating effects of N deposition. At the least polluted site, foliar biomass was evenly distributed across three of the five needle-age classes retained. At the most polluted site, 95% of the foliar biomass was found in the current year's growth. High N deposition and O₃ exposure combined to shift biomass allocation in pine to that of a deciduous tree with one overwintering needle age class. Based on whole tree harvests, root biomass was lowest at sites with the highest pollution exposure, confirming previous chamber exposure and field studies. Aboveground growth responses in the high-pollution sites were opposite to those expected for O₃ injury. Needle and lateral branch elongation growth, and measures of wood production increased with increasing proximity to the pollution source. An enhancement of these growth attributes suggested that N deposition dominated the ponderosa pine response despite high O₃ exposure.     

Effects of gaseous air pollutants on secondary chemistry of Scots pine and norway spruce seedlings

Plant secondary compounds have an important role in defense responses against herbivores and pathogens. This study summarises published and some unpublished data from a series of fumigation experiments where Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies (L.) Karst.) seedlings were exposed to different concentrations of gaseous air pollutants, ozone (O₃), sulphur dioxide (SO₂) or nitrogen dioxide (NO₂), in growth chambers. Concentrations of monoterpenes, resin acids and total phenolics were studied. Overall, needle monoterpenes were not affected by pollutants. Only very high level of O₃ (600 ppb) decreased concentration of some individual monoterpenes in pine needles. O₃ did not have effect on concentrations of resin acids in pine needles. In contrast, the concentration of some individual resin acids increased in O₃-exposed pine shoots and in O₃-exposed needles of one spruce clone. The highest dose of SO₂ decreased concentrations of resin acids in pine needles, but low exposure levels did not have effects. However, SO₂ had no effects on the resin acids concentrations of spruce needles, except some minor individual compounds were affected in clonal spruces. Increased concentrations of resin acids was found in pine shoots exposed to NO₂. Total phenolics of needles were not affected by pollutants. These observations suggest that among secondary compounds there is variation in sensitivity to air pollutants and genetically different trees have different responses to air pollutants.     

Carbon Isotope Composition, Macronutrient Concentrations, and Carboxylating Enzymes in Relation to the Growth of Pinus halepensis Mill. When Subject to Ozone Stress

We present here the effects of ambient ozone (O₃)-induced decline in carbon availability, accelerated foliar senescence, and a decrease in aboveground biomass accumulation in the Aleppo pine (Pinus halepensis Mill.). Aleppo pine seedlings were continuously exposed in open-top chambers for 39 months to three different types of O₃ treatments, which are as follows: charcoal-filtered air, nonfiltered air (NFA), and nonfiltered air supplemented with 40 ppb O₃ (NFA+). Stable carbon isotope discrimination (??) and derived time-integrated c _i/c _a ratios were reduced after an accumulated ozone exposure over a threshold of 40 ppb (AOT40) value from April to September of around 20,000 ppb·h. An AOT40 of above 67,000 ppb·h induced reductions in ribulose-1,5-biphosphate carboxylase/oxygenase activity, aboveground C and needle N and K concentrations, the C/N ratio, Ca concentrations in twigs under 3 mm, and the aerial biomass, as well as increases in needle P concentrations and phosphoenolpyruvate carboxylase (PEPC) activity and the N and K concentrations in twigs under 3 mm. Macronutrients losses, the limitations placed on carbon uptake, and increases in catabolic processes may be the causes of carbon gain diminution in leaves which was reflected as a reduction in aboveground biomass at tree level. Stimulation of PEPC activity, the consequent decreased ??, and compensation processes in nutrient distribution may increase O₃ tolerance and might be interpreted as part of Aleppo pine acclimation response to O₃.     

Phosphorus sorption curves for evaluating phosphorus requirements of loblolly pine (pinus taeda)

Abstract

Bray P2, Mehlich 1, and Mehlich 2 methods identified soils on which loblolly pine would respond to P fertilizer in a greenhouse study using six soils from the West Gulf Coastal Plain. The P extracted by any of the three methods did not correlate with growth on unfertilized soils or to the maximum amount of response when P was applied.

Phosphorus sorption curves were used to estimate the amount of P in soil solution. The concentration of P in roots and tops was proportional to the log of the amount of P in solution. For 90% of maximum dry weight of loblolly seedlings, about 0.22 mg/liter of P in soil solution was necessary. The phosphorus sorption method has the potential of predicting the P requirements of loblolly pine, but field testing is necessary.     

Fall exposure of beech saplings (Fagus sylvatica L.) to ozone and simulated acidic mist: Effects on gas exchange and leachability

Two yr old beech saplings (Fagus sylvatica L.) were exposed under an early autumn climate to four levels of O₃ (10, 40, 75, and 110 nL L O₃) with or without preceding simulated acidic mist (pH 3.5). As a first response in gas exchange to O₃, stomatal conductance was transiently increased followed by a decline in net photosynthesis. Thus, regarding exposure effects on stomatal behavior, both O₃ and acidic mist increased the pollutant flux into the leaf interior. The subsequent impairment of cell permeability consisted of dose-dependent increases in efflux rates of dichlorofluorescein, indicating greater membrane permeabilities. While the efflux rates of Mg and Ca correlated with those of the fluorescent dye, the leachability of K was increased only at the highest O₃ levels. Changes in compartmentation of K, Ca, Mg, and S was derived from selective cell wall extraction; thereafter, the exposure to O₃ and acidic mist resulted in increased extracellular fractions of these elements.     

Physiological Effects of Exposure to Arsenic,Mercury, Antimony and Selenium in the Aquatic Moss Fontinalis antipyretica Hedw.

This study assessed the effect of ambient air pollution on leaf characteristics of white willow, northern red oak, and Scots pine. Willow, oak, and pine saplings were planted at sixteen locations in Belgium, where nitrogen dioxide (NO₂), ozone (O₃), sulfur dioxide (SO₂), and particulate matter (PM₁₀) concentrations were continuously measured. The trees were exposed to ambient air during 6 months (April–September 2010), and, thereafter, specific leaf area (SLA), stomatal resistance (R _s), leaf fluctuating asymmetry (FA), drop contact angle (CA), relative chlorophyll content, and chlorophyll fluorescence (F _v/F _m) were measured. Leaf characteristics of willow, oak, and pine were differently related to the ambient air pollution, indicating a species-dependent response. Willow and pine had a higher SLA at measuring stations with higher NO₂ and lower O₃ concentrations. Willow had a higher R _s and pine had a higher F _v/F _m at measuring stations with a higher NO₂ and lower O₃ concentrations, while oak had a higher F _v/F _m and a lower FA at measuring stations with a higher NO₂ and lower O₃ concentrations. FA and R _s of willow, oak, and pine, SLA of oak, and CA of willow were rather an indicator for local adaptation to the micro-environment than an indicator for the ambient air pollution.     

Ozone effects on Aleppo pine seedlings (Pinus halepensis Mill.) grown in open-top chambers

A three-year term experiment was performed in eastern Spain to assess the effects of ozone (O₃) on the physiological behaviour of Aleppo pine (Pinus halepensis). Seedlings of this species were enclosed in open-top chambers during the summer and exposed to either Charcoal Filtered Air (CFA) to exclude O₃, Non Filtered Air (NFA) or Non Filtered Air plus 40 ppb O₃, 9h/day, 5 days/week (NFA+40). Gas exchange, foliar chlorophyll and nutrient levels were measured throughout the year. Clear seasonal fluctuations were observed since chlorophyll levels, stomatal conductance and net photosynthesis rates were lower during the summer, and higher in winter. O₃-induced effects were apparent on previous year needles from the second year of exposure. A reduction in net photosynthesis, stomatal conductance, chlorophyll, N and P was found in the NFA+40 plots. The results showed the sensitivity of this species to ozone, since a three-month exposure to realistic O₃ concentrations over three consecutive years induced important alterations in Aleppo pine performance. The application of the results to the definition of a critical O₃ level for forest trees is discussed.     

Effects of acidic precipitation,O3, and soil Mg status on throughfall,soil, and seedling loblolly pine nutrient concentrations

The individual and combined impacts of acidic precipitation, O₃, and soil Mg status on nutrient concentrations of throughfall, soil solution, soil, and seedling components were evaluated after one growing season. Loblolly pine seedlings were planted in a sandy loam soil having approximately 15 or 35 mg kg^?1 of exchangeable Mg and were exposed to subambient, ambient, or twice ambient concentrations of O₃ in open top chambers from May through October. Seedlings also received ambient amounts of simulated precipitation at pH 4.0 or 5.3 during this period. Concentrations of nutrients analyzed increased slightly in throughfall, but did not respond significantly to precipitation pH treatments. No increased nutrient leaching and no soil acidification were evident in soil exposed to low pH precipitation after one growing season. Ozone treatments had no significant effects on tissue nutrient concentrations. Interaction responses were very limited and do not suggest any significant synergisms.     

Role of air pollution in forest decline in Eastern North America

Evidence for and against three general mechanisms by which air pollution stress may cause or contribute to forest decline in eastern North America is examined. These mechanisms are (1) soil acidification, cation nutrient depletion, and subsequent Al toxicity, (2) direct and indirect effects of gaseous pollutants on the physiology and growth of forest trees; and (3) excess nitrogen deposition and subsequent soil acidification or physiological injury. Recent studies have revealed reductions in base saturation in some sites (both polluted and pristine), but the consequences of these changes to forest health and nutrition are not resolved. Aluminum toxicity may contribute to forest decline in red spruce (Picearubens L.) in high-elevation sites, but the data are contradictory. In forested landscapes in which regional decline (reduced tree growth and/or dieback) is reported, O₃ is the only gaseous pollutant documented as a contributing factor in the eastern North America. Whereas this secondary pollutant exhibits a regional distribution and occurs at potentially phytotoxic levels, a linkage between O₃ and forest decline is substantive only for loblolly pine (Pinustaeda Sarg.) in the southeast. Studies in high elevations do not support the hypothesis that O₃ or hydrogen peroxide directly affects the growth of red spruce or fraser fir (Abiesfraseri Poir.).However, preliminary data link chronic-level O₃ exposure with indirect effects, principally changes in drought tolerance in low-elevations (e.g., P. taeda) nd winter hardiness in high elevations (e.g., P. rubens).The emerging data on the role of air pollution as an environmental stress indicate that indirect effects (i.e., responses in which pollution alters the plant's ability to compete for limited resources or withstand other environmental stresses) are more important than direct effects (e.g., foliar necrotic lesions). Nitrogen saturation has been offered as an hypothesis for P. rubens decline, but no experimental data exist supporting this hypothesis.     

Baseline element concentrations in soils and plants,Bull Island,Cape Romain National Wildlife Refuge,South Carolina,U.S.A.

Baseline element concentrations are given for Spanish moss (Tillandsia usneoides), loblolly pine (Pinus taeda), and associated soils. Baseline and variability data for ash, Al, Ba, C, Ca, Cd, Ce, Co, Cr, Cu, Fe, K, La, Li, Mg, Mn, Na, Nb, Nd, Ni, P, Pb, S, Sc, Sr, Th, Ti, V, Y, and Zn are reported; however, not all variables are reported for all media because, in some media, certain elements were below the analytical detection limit. Spatial variation in element concentration among and within 0.5 km grid cells are given for each of the media. In general, only a few elements in Spanish moss showed statistically significant landscape patterns, whereas several elements in loblolly pine and in soils exhibited differences among sampling grids. Significant differences in the concentration of three elements in Spanish moss and eight elements (including total S) in loblolly pine were observed between two sampling dates (November and June); however, the absolute amount of these differences was small. Except for perhaps Ni and Pb concentrations in Spanish moss, element levels in all sample media exhibited ranges that indicate natural rather than anthropogenic additions of trace elements.     

Effects of gaseous air pollutants on aphid performance on Scots pine and Norway spruce seedlings

Aphids are frequently found on conifers, but mass outbreaks are seldom reported. On trees stressed by air pollutants the natural resistance can be broken and insect attack combined with pollution stress may promote plant damages. To evaluate effects of air pollution on conifer aphids Scots pine and Norway spruce seedlings have been exposed to gaseous pollutants (O₃, SO₂ and NO₂) in growth chambers. The studied aphid species were Cinara pilicornis Hartig on Norway spruce, C. pinea (Mordv.) and Schizolachnus pineti Fabr. on Scots pine in SO₂ fumigations and S. pineti in O₃ and NO₂ fumigations. C. pilicornis nymphs had peaked dose response to SO₂ concentration. Both the first and third instar larvae of C. pilicornis showed highest mean relative growth rate (MRGR) at 100 ppb SO₂ concentration. MRGR of C. pinea peaked at 50 and 150 ppb SO₂ The response of S. pineti was more inconsistent During fumigation the peak MRGR of S. pineti was at 100 ppb and after exposure at 50 ppb SO₂. MRGR of S. pineti nymphs was not significantly affected during fumigation or after the end of fumigation experiment by 100 ppb O₃ or 100 ppb NO₂ or the mixtures. The results suggest that SO₂ affects more distinctively on aphid performance on conifers than O₃ or NO₂. Especially stem-feeding aphids on spruce can exploit physiological disturbance of host plant under pollution stress.     

The individual and combined effects of ozone and simulated acid rain on chlorophyll contents,carbon allocation and biomass accumulation of armand pine seedlings

The seedlings of armand pine (Pinus armand Franch.) were applied to exposure, alone or in combination, to charcoal filtered air (CF) or ozone (O₃) at 300±15 nl/l(ppb) for 8 h a day, 6 days a week, and simulated rain of pH 6.8, 3.0 or 2.3, six times a week, alone and in combination, for 14 weeks from June 15 to September 20, 1993. No significant interactive effects of O₃ and simulated acid rain were observed on chlorophyll contents, carbon allocation and biomass accumulation of the seedlings in the present study. The O₃ caused reductions in biomass accumulation of whole-plant and below-ground parts, but not that of above-ground parts without an acute visible foliar injury. At the same time, the O₃ reduced R/S ratio, but raised F/C ratio. Therefore, O₃ also altered carbon allocation pattern. On the other hand, chlorophyll contents were increased by simulated acid rain, but other determined parameters were not altered.     

Growth responses of maize,soybean, and tomato to sulphur dioxide and ozone exposure patterns utilizing an incomplete block factorial experimental design

Maize (Zea mays L.), soybean (Glycine max L.), and tomato (Lycopersicon esculentum Mill.) plants were grown in a controlled environment and exposed for 6 hr daily for 7 days to O₃ at 0.15 μL L^?1 and/or SO₂ at 0.30 μL L^?1 (daily exposures). Some plants exposed daily to O₃ were also exposed to SO₂ for 6 hr on the first, third, fifth, or seventh day of O₃ exposure (variable exposures) and some plants exposed daily to SO₂ were treated similarly with O₃ to determine the growth effects of O₃ or S0₂ pre- and/or post-treatments on S0₂ and O₃ mixture response. Growth sensitivity to 6 hr S0₂ or 6 hr O₃ treatments was generally affected by the previous history of O₃ or SO₂ exposure, respectively. Species differed in the number of days of O₃ or SO₂ treatments required to elicit maximum sensitivity to a single 6 hr O₃ and SO₂ treatment. Linear contrasts compared variable with daily exposures for the S0₂ and O₃ regimes. Plants exposed to the gas mixture for a single day (variable exposures) tended to be smaller than those exposed to the gas mixture daily, with the exception of soybean exposed to SO₂ during daily O₃. The six treatments were carried out in eight exposure chambers, as a partially balanced incomplete block design in blocks of four due to separate environmental control of the exposure facilities. The partially balanced incomplete block design proved to be about 2.6 times as efficient as a complete block design. The inclusion of covariates further increased precision.     

Loblolly pine plant community structure and soil solution aluminum,organic acids,calcium, magnesium,and pH

Abstract

Vegetative cover may influence soil chemical properties by producing organic acids that can modify the toxicity are responsible for high levels of nontoxic soil solution aluminum (Al). This study was conducted to determine the effect of loblolly pine (Pinus taeda L.) plant community structure, i.e., differing vegetative covers, on exchangeable cation distribution, and soil solution concentration of aluminum (Al) and organic acids. Soil was sampled were taken from 8‐year‐old forest plots with differing plant community structure (herbaceous‐hardwood‐pine, herbaceous‐pine, hardwood‐pine, and pine only), and measured for exchangeable cations. Soil solution extracts were analyzed for Al, organic acids, and the major cations and anions. There were significant differences in exchangeable Al, and calcium (Ca) among communities. Soil pH was highest in the herbaceous‐pine community, while other communities did not differ in soil pH. Oxalic, citric, malic, and succinic acids were detected in soil solution for all communities, but did not differ significantly among communities. Calculated Al activity was less than those determined by the 8‐hydroxy quinoline (15 sec) method. For these conditions, calculated Al activities did not change by accounting for the Ca‐ and magnesium (Mg)‐organic acid complex. The Al activities found were similar to previous reported levels that were considered nontoxic.     

Effects of elevated CO2 and O3 on soil respiration under ponderosa pine

Soil respiration represents the integrated response of plant roots and soil organisms to environmental conditions and the availability of C in the soil. A multi-year study was conducted in outdoor sun-lit controlled-environment chambers containing a reconstructed ponderosa pine/soil-litter system. The study used a 2×2 factorial design with two levels of CO₂ and two levels of O₃ and three replicates of each treatment. The objectives of our study were to assess the effects of long-term exposure to elevated CO₂ and O₃, singly and in combination, on soil respiration, fine root growth and soil organisms. Fine root growth and soil organisms were included in the study as indicators of the autotrophic and heterotrophic components of soil respiration. The study evaluated three hypotheses: (1) elevated CO₂ will increase C assimilation and allocation belowground increasing soil respiration; (2) elevated O₃ will decrease C assimilation and allocation belowground decreasing soil respiration and (3) as elevated CO₂ and O₃ have opposing effects on C assimilation and allocation, elevated CO₂ will eliminate or reduce the negative effects of elevated O₃ on soil respiration. A mixed-model covariance analysis was used to remove the influences of soil temperature, soil moisture and days from planting when testing for the effects of CO₂ and O₃ on soil respiration. The covariance analysis showed that elevated CO₂ significantly reduced the soil respiration while elevated O₃ had no significant effect. Despite the lack of a direct CO₂ stimulation of soil respiration, there were significant interactions between CO₂ and soil temperature, soil moisture and days from planting indicating that elevated CO₂ altered soil respiration indirectly. In elevated CO₂, soil respiration was more sensitive to soil temperature changes and less sensitive to soil moisture changes than in ambient CO₂. Soil respiration increased more with days from planting in elevated than in ambient CO₂. Elevated CO₂ had no effect on fine root biomass but increased abundance of culturable bacteria and fungi suggesting that these increases were associated with increased C allocation belowground. Elevated CO₂ had no significant effect on microarthropod and nematode abundance. Elevated O₃ had no significant effects on any parameter except it reduced the sensitivity of soil respiration to changes in temperature.