Comparing global models of terrestrial net primary productivity (NPP): analysis of the seasonal atmospheric CO2 signal

Eight terrestrial biospheric models (TBMs) calculating the monthly distributions of both net primary productivity (NPP) and soil heterotrophic respiration (R_H) in the Potsdam NPP Model Intercomparison workshop are used to simulate seasonal patterns of atmospheric CO₂ concentration. For each model, we used net ecosystem productivity (NEP = NPP – R_H) as the source function in the TM2 atmospheric transport model from the Max-Planck Institute for Meteorology. Comparing the simulated concentration fields with detrended measurements from 25 monitoring stations spread over the world, we found that the decreasing seasonal amplitude from north to south is rather well reproduced by all the models, though the amplitudes are slightly too low in the north. The agreement between the simulated and observed seasonality is good in the northern hemisphere, but poor in the southern hemisphere, even when the ocean is accounted for. Based on a Fourier analysis of the calculated zonal atmospheric signals, tropical NEP plays a key role in the seasonal cycle of the atmospheric CO₂ in the whole southern hemisphere. The relatively poor match between measured and predicted atmospheric CO₂ in this hemisphere suggests problems with all the models. The simulation of water relations, a dominant regulator of NEP in the tropics, is a leading candidate for the source of these problems.     

Acclimation response of spring wheat in a free-air CO2 enrichment (FACE) atmosphere with variable soil nitrogen regimes. 1. Leaf position and phenology determine acclimation response

We have examined the photosynthetic acclimation of wheat leaves grown at an elevated CO₂ concentration, and ample and limiting N supplies, within a field experiment using free-air CO₂ enrichment (FACE). To understand how leaf age and developmental stage affected any acclimation response, measurements were made on a vertical profile of leaves every week from tillering until maturity. The response of assimilation (A) to internal CO₂ concentration (C_i) was used to estimate the in vivo carboxylation capacity (Vc_max) and maximum rate of ribulose-1,5-bisphosphate limited photosynthesis (A _sat). The total activity of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), and leaf content of Rubisco and the Light Harvesting Chlorophyll a/b protein associated with Photosystem II (LHC II), were determined. Elevated CO₂ did not alter Vc_max in the flag leaf at either low or high N. In the older shaded leaves lower in the canopy, acclimatory decline in Vc_max and A _sat was observed, and was found to correlate with reduced Rubisco activity and content. The dependency of acclimation on N supply was different at each developmental stage. With adequate N supply, acclimation to elevated CO₂ was also accompanied by an increased LHC II/Rubisco ratio. At low N supply, contents of Rubisco and LHC II were reduced in all leaves, although an increased LHC II/Rubisco ratio under elevated CO₂ was still observed. These results underscore the importance of leaf position, leaf age and crop developmental stage in understanding the acclimation of photosynthesis to elevated CO₂ and nutrient stress. This revised version was published online in June 2006 with corrections to the Cover Date.     

迁地保存中土壤因子对渐危植物珊瑚菜生长发育的影响

将原生长在海滨沙滩上的珊瑚菜(GlehnialittoralisFr.SchmidtexMiq.)迁地移栽至下列6种不同配比的土壤基质上,即全沙(Ⅰ)、1 2沙+1 2腐殖土(Ⅱ)、1 2沙+1 2黄棕壤土(Ⅲ)、黄棕壤土(Ⅳ)、腐殖土(V)和1 3沙+1 3腐殖土+1 3黄棕壤土(Ⅵ)。在定期定株连续观察基础上,比较分析了生长在上述6种不同基质中的珊瑚菜的成活率、生长发育状况和花果数量,发现在Ⅵ类基质上的珊瑚菜移栽成活率最高;在Ⅱ类基质上的营养生长最好;在Ⅲ类基质上的结实率及种子的饱满程度最高;而Ⅳ类基质对珊瑚菜的生长发育最不利。根据珊瑚菜生长发育状况的10个生物学指标,对上述各类基质进行了综合评估,认为对珊瑚菜迁地移栽最为有利的土壤基质依次为Ⅲ>Ⅱ>Ⅰ>Ⅵ>Ⅴ>Ⅳ。     

Changes in net photosynthesis and growth of Pinus eldarica seedlings in response to atmospheric CO2 enrichment

Pinus eldarica L. trees, rooted in the natural soil of an agricultural field at Phoenix, Arizona, were grown from the seedling stage in clear-plastic-wall open-top enclosures maintained at four different atmospheric CO₂ concentrations for 15 months. Light response functions were determined for one tree from each treatment by means of whole-tree net CO₂ exchange measurements at the end of this period, after which rates of carbon assimilation of an ambient-treatment tree were measured across a range of atmospheric CO₂ concentrations. The first of these data sets incorporates the consequences of both the CO₂-induced enhancement of net photosynthesis per unit needle area and the CO₂-induced enhancement of needle area itself (due primarily to the production of more needles), whereas the second data set reflects only the first of these effects. Hence the division of the normalized results of the first data set by the normalized results of the second set yields a representation of the increase in whole-tree net photosynthesis due to enhanced needle production caused by atmospheric CO₂ enrichment. In the solitary trees we studied, the relative contribution of this effect increased rapidly with the CO₂ concentration of the air to increase whole-tree net photosynthesis by nearly 50% at a CO₂ concentration approximately 300 μmol mol⁻¹ above ambient.     

The effects of elevated atmospheric CO2 and soil P placement on cotton root deployment

Root proliferation into nutrient rich zones is an important mechanism in the exploitation of soil nutrients by plants. No studies have examined atmospheric CO₂ effects on cotton (Gossypium hirsutum L.) root distribution as affected by localized phosphorus (P). Cotton plants were grown in a Troup sand (loamy, thermic Grossarenic Kandiudults) using 17.2-l containers placed in open top field chambers (OTC) under ambient (360 mol mol^–1) or enriched (720 mol mol^–1) atmospheric CO₂ concentrations for 40 days. Equivalent amounts of P were added (150 mg P per kg of soil) to 100, 50, 25, 12.5, and 6.25% of the total soil volume; control containers with no added P were also included. Under extremely low P (controls), cotton was unresponsive to CO₂ enrichment. In treatments with both fertilized and unfertilized soil volumes, root proliferation was greater in the unfertilized soil under elevated CO₂ conditions. Stimulation of root growth occurred in the P-fertilized soil fraction; the pattern of stimulation was similar under both CO₂ levels. Under ambient CO₂, cotton plant response was positive (shoot mass, and total root mass and length) when soil P was confined to relatively small proportions of the total soil volume (6.25 and 12.5%). However, elevated CO₂ grown plants tended to respond to P regardless of its distribution.     

Influence of atmospheric CO2 enrichment, soil N, and water stress on needle surface wax formation in Pinus palustris (Pinaceae)

Interactive effects of increasing atmospheric CO2 with resource limitations on production of surface wax in plants have not been studied. Pinus palustris seedlings were grown for 1 yr at two levels of soil N (40 or 400 kg N_ha-1_yr-1) and water stress (-0.5 or -1.5 MPa xylem pressure potential) in open-top field chambers under two levels of CO2 (365 or 720 mumol/mol). Needle surface wax content was determined at 8 mo (fall) and 12 mo (spring) and epicuticular wax morphology was examined using scanning electron microscopy (SEM) at 12 mo. Wax content expressed on both a leaf area and dry mass basis was increased due to main effects of low N and water stress. No main effects of CO2 were observed; however, a CO2 x N interaction at 12 mo indicated that under low soil N the elevated CO2 treatment had less wax (surface area or dry mass basis) compared to its ambient counterpart. Morphologically, low N needle surfaces appeared rougher compared to those of high N needles due to more extensive wax ridges. Although the main effect of water treatment on wax density was not reflected by changes in wax morphology, the CO2 x N interaction was paralleled by alterations in wax appearance. Decreases in density and less prominent epicuticular wax ridges resulting from growth under elevated CO2 and limiting N suggest that dynamics of plant/atmosphere and plant/pathogen interactions may be altered.     

Projected ecosystem impact of the Prairie Heating and CO2 Enrichment experiment

The Prairie Heating and CO2 Enrichment (PHACE) experiment has been initiated at a site in southern Wyoming (USA) to simulate the impact of warming and elevated atmospheric CO2 on ecosystem dynamics for semiarid grassland ecosystems. The DAYCENT ecosystem model was parametrized to simulate the impact of elevated CO2 at the open-top chamber (OTC) experiment in north-eastern Colorado (1996-2001), and was also used to simulate the projected ecosystem impact of the PHACE experiments during the next 10 yr. Model results suggest that soil water content, plant production, soil respiration, and nutrient mineralization will increase for the high-CO2 treatment. Soil water content will decrease for all years, while nitrogen mineralization, soil respiration, and plant production will both decrease and increase under warming depending on yearly differences in water stress. Net primary production (NPP) will be greatest under combined warming and elevated CO2 during wet years. Model results are consistent with empirical field data suggesting that water and nitrogen will be critical drivers of the semiarid grassland response to global change.     

Tropospheric O(3) compromises net primary production in young stands of trembling aspen, paper birch and sugar maple in response to elevated atmospheric CO(2)

Concentrations of atmospheric CO(2) and tropospheric ozone (O(3)) are rising concurrently in the atmosphere, with potentially antagonistic effects on forest net primary production (NPP) and implications for terrestrial carbon sequestration. Using free-air CO(2) enrichment (FACE) technology, we exposed north-temperate forest communities to concentrations of CO(2) and O(3) predicted for the year 2050 for the first 7 yr of stand development. Site-specific allometric equations were applied to annual nondestructive growth measurements to estimate above- and below-ground biomass and NPP for each year of the experiment. Relative to the control, elevated CO(2) increased total biomass 25, 45 and 60% in the aspen, aspen-birch and aspen-maple communities, respectively. Tropospheric O(3) caused 23, 13 and 14% reductions in total biomass relative to the control in the respective communities. Combined fumigation resulted in total biomass response of -7.8, +8.4 and +24.3% relative to the control in the aspen, aspen-birch and aspen-sugar maple communities, respectively. These results indicate that exposure to even moderate levels of O(3) significantly reduce the capacity of NPP to respond to elevated CO(2) in some forests.     

Lifetime and temporal occurrence of ectomycorrhizae on ponderosa pine (Pinus ponderosa Laws.) seedlings grown under varied atmospheric CO2 and nitrogen levels

Climate change (elevated atmospheric CO2, and altered air temperatures, precipitation amounts and seasonal patterns) may affect ecosystem processes by altering carbon allocation in plants, and carbon flux from plants to soil. Mycorrhizal fungi, as carbon sinks, are among the first soil biota to receive carbon from plants, and thereby influence carbon release from plants to soil. One step in this carbon release is via fine root and mycorrhizal turnover. It is necessary to know the lifetime and temporal occurrence of roots and mycorrhizae to determine the capacity of the soil ecosystem to sequester carbon assimilated aboveground. In this study, ponderosa pine (Pinus ponderosa Laws) seedlings were grown under three levels of atmospheric CO2 (ambient, 525 and 700 mol CO2 mol-1) and three levels of annual nitrogen additions (0,100 and 200 kg N ha-1) in open-top chambers. At a two-month frequency during 18 months, we observed ectomycorrhizal root tips observed using minirhizotron tubes and camera. The numbers of new mycorrhizal root tips, the numbers of tips that disappeared between two consecutive recording events, and the standing crop of tips at each event were determined. There were more mycorrhizal tips of all three types seen during the summer compared with other times of the year. When only the standing crop of mycorrhizal tips was considered, effects of the CO2 and N addition treatments on carbon allocation to mycorrhizal tips was weakly evident. However, when the three types of tips were considered collectively, tips numbers flux of carbon through mycorrhizae was greatest in the: (1) high CO2 treatment compared with the other CO2 treatments, and (2) intermediate N addition treatment compared with the other N addition treatments. A survival analysis on the entire 18 month cohort of tips was done to calculate the median lifetime of the mycorrhizal root tips. Average median lifetime of the mycorrhizal tips was 139 days and was not affected by nitrogen and CO2 treatments.     

Nonlinearity of photosynthetic responses to growth in rising atmospheric CO2: an experimental and modelling study

Nonlinear responses of photosynthesis to the CO₂ concentration at which plants were grown (C_g) have been often reported in the literature. This study was designed to develop mechanistic understanding of the nonlinear responses with both experimental and modelling approaches. Soybean (Glycine max) was grown in five levels of C_g (280, 350, 525, 700, 1000 ppm) with either a high or low rate of nitrogen fertilization. When the rate of nitrogen fertilization was high, the photosynthetic rate measured at C_g was highest in plants from the 700 ppm CO₂ treatment. When the rate of nitrogen fertilization was low, little variation was observed in the photosynthetic rates of plants from the different treatments measured at their respective C_g. Measurements of CO₂-induced changes in mass-based leaf nitrogen concentration (n_m, an index of changes in biochemical processes) and leaf mass per unit area (h, an index of morphological properties) were used in a model and indicate that the nonlinearity of photosynthetic responses to C_g is largely determined by relative changes in photosynthetic sensitivity, biochemical downregulation, and morphological upregulation. In order to further understand the nonlinear responses, we compiled data from the literature on CO₂-induced changes in n_m and h. These compiled data indicate that h generally increases and n_m usually decreases with increasing C_g, but that the trajectories and magnitudes of the changes in h and n_m vary with species and growth environments. Integration of these variables (n_m and h) into a biochemically based model of photosynthesis enabled us to predict diverse responses of photosynthesis to C_g. Thus a general mechanism is suggested for the highly variable, nonlinear responses of photosynthesis to C_g reported in the literature.     

Effects of six years atmospheric CO2 enrichment on plant,soil, and soil microbial C of a calcareous grassland

Stimulated plant production and often even larger stimulation of photosynthesis at elevated CO₂ raise the possibility of increased C storage in plants and soils. We analysed ecosystem C partitioning and soil C fluxes in calcareous grassland exposed to elevated CO₂ for 6 years. At elevated CO₂, C pools increased in plants (+23%) and surface litter (+24%), but were not altered in microbes and soil organic matter. Soils were fractionated into particle size and density separates. The amount of low-density macroorganic C, an indicator of particulate soil C inputs from root litter, was not affected by elevated CO₂. Incorporation of C fixed during the experiment (C_new) was tracked by C isotopic analysis of soil fractions which were labelled due to ¹³C depletion of the commercial CO₂ used for atmospheric enrichment. This data constrains estimates of C sequestration (absolute upper bound) and indicates where in soils potentially sequestered C is stored. C_new entered soils at an initial rate of 210±42 g C m^–2 year^–1, but only 554±39 g C_new m^–2 were recovered after 6 years due to the low mean residence time of 1.8 years. Previous process-oriented measurements did not indicate increased plant–soil C fluxes at elevated CO₂ in the same system (¹³C kinetics in soil microbes and fine roots after pulse labelling, and minirhizotron observations). Overall experimental evidence suggests that C storage under elevated CO₂ occurred only in rapidly turned-over fractions such as plants and detritus, and that potential extra soil C inputs were rapidly re-mineralised. We argue that this inference does not conflict with the observed increases in photosynthetic fixation at elevated CO₂, because these are not good predictors of plant growth and soil C fluxes for allometric reasons. C sequestration in this natural system may also be lower than suggested by plant biomass responses to elevated CO₂ because C storage may be limited by stabilisation of C_new in slowly turned-over soil fractions (a prerequisite for long-term storage) rather than by the magnitude of C inputs per se.     

Effects of increased atmospheric CO2 and N supply on photosynthesis, growth and cell composition of the cyanobacterium Spirulina platensis (Arthrospira)

The consequences of the addition of CO2 (1%) in cultures of S. platensis are examined in terms of biomass yield, cell composition and external medium composition. CO2 enrichment was tested under nitrogen saturating and nitrogen limiting conditions. Increasing CO2 levels did not cause any change in maximum growth rate while it decreased maximum biomass yield. Protein and pigments were decreased and carbohydrate increased by high CO2, but the capability to store carbohydrates was saturated. C:N ratio remained unchanged while organic carbon released to the external medium was enhanced, suggesting that organic carbon release in S. platensis is an efficient mechanism for the maintenance of the metabolic integrity, balancing the cell C:N ratio in response to environmental CO2 changes. CO2 affected the pigment content: Phycocyanin, chlorophyll and carotenoids were reduced in around 50%, but the photosynthetic parameters were slightly changed. We propose that in S. platensis CO2 could act promoting degradation of pigments synthetised in excess in normal CO2 conditions, that are not necessary for light harvesting. Nitrogen assimilation was significantly not affected by CO2, and it is proposed that the inability to stimulate N assimilation by CO2 enrichment determined the lack of response in maximum growth rate. This revised version was published online in June 2006 with corrections to the Cover Date.     

CO2 enrichment in vitro. Effect on autotrophic and heterotrophic cultures of Nicotiana tabacum (var. Samsun)

Summary Plantlets of Nicotiana tabacum (var. Samsun) were grown under CO₂ enriched air supplied by a Warburg buffer. Growth of all plant parts was enhanced. The maximum growth increase was found for roots (120%).Addition of 30 g.l^-1 sucrose in the medium resulted in a three time faster growth. However, the effect of CO₂ enrichment was still positive in these conditions, although less pronounced than in autotrophic cultures.     

Combined effects of CO2 and light on large and small isolates of the unicellular N2-fixing cyanobacterium Crocosphaera watsonii from the western tropical Atlantic Ocean

We examined the combined effects of light and pCO₂ on growth, CO₂-fixation and N₂-fixation rates by strains of the unicellular marine N₂-fixing cyanobacterium Crocosphaera watsonii with small (WH0401) and large (WH0402) cells that were isolated from the western tropical Atlantic Ocean. In low-pCO₂-acclimated cultures (190 ppm) of WH0401, growth, CO₂-fixation and N₂-fixation rates were significantly lower than those in cultures acclimated to higher (present-day ～385 ppm, or future ～750 ppm) pCO₂ treatments. Growth rates were not significantly different, however, in low-pCO₂-acclimated cultures of WH0402 in comparison with higher pCO₂ treatments. Unlike previous reports for C. watsonii (strain WH8501), N₂-fixation rates did not increase further in cultures of WH0401 or WH0402 when acclimated to 750 ppm relative to those maintained at present-day pCO₂. Both light and pCO₂ had a significant negative effect on gross : net N₂-fixation rates in WH0402 and trends were similar in WH0401, implying that retention of fixed N was enhanced under elevated light and pCO₂. These data, along with previously reported results, suggest that C. watsonii may have wide-ranging, strain-specific responses to changing light and pCO₂, emphasizing the need for examining the effects of global change on a range of isolates within this biogeochemically important genus. In general, however, our data suggest that cellular N retention and CO₂-fixation rates of C. watsonii may be positively affected by elevated light and pCO₂ within the next 100 years, potentially increasing trophic transfer efficiency of C and N and thereby facilitating uptake of atmospheric carbon by the marine biota.     

Effects of CO2 and osmolality on hybridoma cells: growth, metabolism and monoclonal antibody production

CO2 partial pressure (pCO2) in industrial cell culture reactors may reach 150–200 mm Hg, which can significantly inhibit cell growth and recombinant protein production. Due to equilibrium with bicarbonate, increased pCO2 at constant pH results in a proportional increase in osmolality. Hybridoma AB2-143.2 cell growth rate decreased with increasing pCO2 in well-plate culture, with a 45% decrease at 195 mm Hg with partial osmolality compensation (to 361 mOsm kg- 1). Inhibition was more extensive without osmolality compensation, with a 63% decrease in growth rate at 195 mm Hg and 415 mOsm kg-1. Also, the hybridoma death rate increased with increasing pCO2, with 31- and 64-fold increases at 250 mm Hg pCO2 for 401 and 469 mOsm kg- 1, respectively. The specific glucose consumption and lactate production rates were 40–50% lower at 140 mm Hg pCO2. However, there was little further inhibition of glycolysis at higher pCO2. The specific antibody production rate was not significantly affected by pCO2 or osmolality within the range tested. Hybridomas were also exposed to elevated pCO2 in continuous culture. The viable cell density decreased by 25–40% at 140 mm Hg. In contrast to the well-plate cultures, the death rate was lower at the new steady state at 140 mm Hg. This was probably due to higher residual nutrient and lower byproduct levels at the lower cell density (at the same dilution rate), and was associated with increased cell-specific glucose and oxygen uptake. Thus, the apparent effects of pCO2 may vary with the culture system. VMdZ and RK contributed equally to the results in this article. This revised version was published online in August 2006 with corrections to the Cover Date.     

The likely impact of elevated [CO2], nitrogen deposition, increased temperature and management on carbon sequestration in temperate and boreal forest ecosystems: a literature review

Temperate and boreal forest ecosystems contain a large part of the carbon stored on land, in the form of both biomass and soil organic matter. Increasing atmospheric [CO2], increasing temperature, elevated nitrogen deposition and intensified management will change this C store. Well documented single-factor responses of net primary production are: higher photosynthetic rate (the main [CO2] response); increasing length of growing season (the main temperature response); and higher leaf-area index (the main N deposition and partly [CO2] response). Soil organic matter will increase with increasing litter input, although priming may decrease the soil C stock initially, but litter quality effects should be minimal (response to [CO2], N deposition, and temperature); will decrease because of increasing temperature; and will increase because of retardation of decomposition with N deposition, although the rate of decomposition of high-quality litter can be increased and that of low-quality litter decreased. Single-factor responses can be misleading because of interactions between factors, in particular those between N and other factors, and indirect effects such as increased N availability from temperature-induced decomposition. In the long term the strength of feedbacks, for example the increasing demand for N from increased growth, will dominate over short-term responses to single factors. However, management has considerable potential for controlling the C store.     

大气CO2增加对陆地生态系统微量气体地-气交换的影响

简要综述了近年来国内外在大气CO2浓度增加对微量气体交换影响方面的研究进展,首先介绍了有关大气CO2浓度增加的研究技术和方法,比较了目前两种常用技术开顶箱（OTC）和开放式空气CO2增加（FACE）方法的优缺点,然后着重阐述了用OTC和FACE研究陆地生态系统CH4、N2O、CO2等微量气体的地气交换对大气CO2浓度增加的响应,综合现有的资料表明,大气CO2浓度增加,会促进绿色植物生物量增加,同时改变生物质的C/N,降低有机质的分解速率,增强了陆地生态系统对大气CO2的固特作用;大气CO2浓度增加会提高产甲烷菌的活性和影响CH4的排放过程,有可能导致湿地生态系统CH4的排放增加;大气CO2浓度增加对N2O排放影响的研究较少,且尚无一致的结论;另外,对于其他微量气体,尚没有盯关研究报道,鉴于此,今后应加强大气CO2浓度增加的微量气体地气交换响应研究。     

大气CO2浓度增高对麦田土壤硝化和反硝化细菌的影响

硝化和反硝化细菌是土壤中与氮转化有关的微生物菌群 ,大气CO2 浓度升高可能对它们的数量产生影响。位于中国无锡的稻 麦轮作农田生态系统FACE平台 2 0 0 1年 6月开始运行。本试验在 2 0 0 3年小麦生长季研究了土壤 (0～ 5cm和 5～ 10cm土层 )中硝化和反硝化细菌在大气CO2 浓度升高条件下的变化。试验采用最大可能法 (MPN)计这两种微生物菌群的数量。结果表明 ,0～ 5cm土层硝化菌数拔节期和成熟期FACE低于对照 ,而孕穗期FACE高于对照 ,5～ 10cm土层硝化菌数越冬期与成熟期FACE低于对照 ,大气CO2 浓度升高使得麦田土壤硝化细菌数目减少。 0～ 5cm土层各个生长期反硝化菌数FACE与对照均没有明显差异 ,5～ 10cm土层反硝化菌数拔节期FACE低于对照 ,大气CO2 浓度升高对麦田土壤反硝化菌的影响不大。     

Effects of carbon dioxide enrichment on response of mycorrhizal pitch pine (Pinus rigida) to aluminum: growth and mineral nutrition

 Carbon dioxide enrichment may increase the Al tolerance of trees by increasing root growth, root exudation and/or mycorrhizal colonization. The effect of elevated CO₂ on the response of mycorrhizal pitch pine (Pinus rigida Mill.) seedlings to Al was determined in two experiments with different levels of nutrients, 0.1- or 0.2-strength Clark solution. During each experiment, seedlings inoculated with the ectomycorrhizal fungus Pisolithus tinctorius (Pers.) Coker & Couch were grown 13 weeks in sand irrigated with nutrient solution (pH 3.8) containing 0, 6.25, 12.5, or 25 mg/l Al (0, 232, 463, or 927 μM Al) in growth chambers fumigated with 350 (ambient) or 700 (elevated) μl/l CO₂. At ambient CO₂, in the absence of Al, mean total dry weights (DW) of seedlings at the high nutrient level were 164% higher than those at the low level. Total DW at elevated CO₂, in the absence of Al, was significantly greater than that in ambient CO₂ at the low (+34%) and high (+16%) nutrient levels. Root and shoot DW at both nutrient levels decreased with increasing Al concentrations with Al reducing root growth more than shoot growth. Although visible symptoms of Al toxicity in roots and needles were reduced by CO₂ enrichment, there were no significant CO₂ × Al interactions for shoot or root DW. The percentage of seedling roots that became mycorrhizal was negatively related to nutrient level and was greater at elevated than at ambient CO₂ levels. Generally, elevated CO₂ had little effect on concentration of mineral nutrients in roots and needles. Aluminum reduced concentrations of most nutrients by inhibiting uptake. Received: 18 June 1997 / Accepted: 8 December 1997