James A. Teeri

Elevated atmospheric CO2 and feedback between carbon and nitrogen cycles

We tested a conceptual model describing the influence of elevated atmospheric CO2 on plant production, soil microorganisms, and the cycling of C and N in the plant-soil system. Our model is based on the observation that in nutrient-poor soils, plants (C3) grown in an elevated CO2 atmosphere often increase production and allocation to belowground structures. We predicted that greater belowground C inputs at elevated CO2 should elicit an increase in soil microbial biomass and increased rates of organic matter turnover and nitrogen availability. We measured photosynthesis, biomass production, and C allocation of Populus grandidentata Michx. grown in nutrient-poor soil for one field season at ambient and twice-ambient (i.e., elevated) atmospheric CO2 concentrations. Plants were grown in a sandy subsurface soil i) at ambient CO2 with no open top chamber, ii) at ambient CO2 in an open top chamber, and iii) at twice-ambient CO2 in an open top chamber. Plants were fertilized with 4.5 g N m−2 over a 47 d period midway through the growing season. Following 152 d of growth, we quantified microbial biomass and the availabilities of C and N in rhizosphere and bulk soil. We tested for a significant CO2 effect on plant growth and soil C and N dynamics by comparing the means of the chambered ambient and chambered elevated CO2 treatments.Rates of photosynthesis in plants grown at elevated CO2 were significantly greater than those measured under ambient conditions. The number of roots, root length, and root length increment were also substantially greater at elevated CO2. Total and belowground biomass were significantly greater at elevated CO2. Under N-limited conditions, plants allocated 50–70% of their biomass to roots. Labile C in the rhizosphere of elevated-grown plants was significantly greater than that measured in the ambient treatments; there were no significant differences between labile C pools in the bulk soil of ambient and elevated-grown plants. Microbial biomass C was significantly greater in the rhizosphere and bulk soil of plants grown at elevated CO2 compared to that in the ambient treatment. Moreover, a short-term laboratory assay of N mineralization indicated that N availability was significantly greater in the bulk soil of the elevated-grown plants. Our results suggest that elevated atmospheric CO2 concentrations can have a positive feedback effect on soil C and N dynamics producing greater N availability. Experiments conducted for longer periods of time will be necessary to test the potential for negative feedback due to altered leaf litter chemistry. ei]{gnH}{fnLambers} ei]{gnA C}{fnBorstlap}

GAS EXCHANGE, LEAF NITROGEN, AND GROWTH EFFICIENCY OF POPULUS TREMULOIDES IN A CO2-ENRICHED ATMOSPHERE

Predicting forest responses to rising atmospheric CO2 will require an understanding of key feedbacks in the cycling of carbon and nitrogen between plants and soil microorganisms. We conducted a study for 2.5 growing seasons with Populus tremuloides grown under experimental atmospheric CO2 and soil-N-availability treatments. Our objective was to integrate the combined influence of atmospheric CO2 and soil-N availability on the flow of C and N in the plant–soil system and to relate these processes to the performance of this widespread and economically important tree species. Here we consider treatment effects on photosynthesis and canopy development and the efficiency with which this productive capacity is translated into aboveground, harvestable yield. We grew six P. tremuloides genotypes at ambient (35 Pa) or elevated (70 Pa) CO2 and in soil of low or high N mineralization rate at the University of Michigan Biological Station, Pellston, Michigan, USA (45°35′ N, 84°42′ W). In the second year of growth, net...

Response of soil biota to elevated atmospheric CO2 in poplar model systems

Abstract We tested the hypotheses that increased belowground allocation of carbon by hybrid poplar saplings grown under elevated atmospheric CO2 would increase mass or turnover of soil biota in bulk but not in rhizosphere soil. Hybrid poplar saplings (Populus×euramericana cv. Eugenei) were grown for 5 months in open-bottom root boxes at the University of Michigan Biological Station in northern, lower Michigan. The experimental design was a randomized-block design with factorial combinations of high or low soil N and ambient (34 Pa) or elevated (69 Pa) CO2 in five blocks. Rhizosphere microbial biomass carbon was 1.7 times greater in high-than in low-N soil, and did not respond to elevated CO2. The density of protozoa did not respond to soil N but increased marginally (P < 0.06) under elevated CO2. Only in high-N soil did arbuscular mycorrhizal fungi and microarthropods respond to CO2. In high-N soil, arbuscular mycorrhizal root mass was twice as great, and extramatrical hyphae were 11% longer in elevated than in ambient CO2 treatments. Microarthropod density and activity were determined in situ using minirhizotrons. Microarthropod density did not change in response to elevated CO2, but in high-N soil, microarthropods were more strongly associated with fine roots under elevated than ambient treatments. Overall, in contrast to the hypotheses, the strongest response to elevated atmospheric CO2 was in the rhizosphere where (1) unchanged microbial biomass and greater numbers of protozoa (P < 0.06) suggested faster bacterial turnover, (2) arbuscular mycorrhizal root length increased, and (3) the number of microarthropods observed on fine roots rose.

Above- and belowground response of Populus grandidentata to elevated atmospheric CO2 and soil N availability

Soil N availability may play an important role in regulating the long-term responses of plants to rising atmospheric CO2 partial pressure. To further examine the linkage between above- and belowground C and N cycles at elevated CO2, we grew clonally propagated cuttings of Populus grandidentata in the field at ambient and twice ambient CO2 in open bottom root boxes filled with organic matter poor native soil. Nitrogen was added to all root boxes at a rate equivalent to net N mineralization in local dry oak forests. Nitrogen added during August was enriched with 15N to trace the flux of N within the plant-soil system. Above-and belowground growth, CO2 assimilation, and leaf N content were measured non-destructively over 142 d. After final destructive harvest, roots, stems, and leaves were analyzed for total N and 15N.There was no CO2 treatment effect on leaf area, root length, or net assimilation prior to the completion of N addition. Following the N addition, leaf N content increased in both CO2 treatments, but net assimilation showed a sustained increase only in elevated CO2 grown plants. Root relative extension rate was greater at elevated CO2, both before and after the N addition. Although final root biomass was greater at elevated CO2, there was no CO2 effect on plant N uptake or allocation. While low soil N availability severely inhibited CO2 responses, high CO2 grown plants were more responsive to N. This differential behavior must be considered in light of the temporal and spatial heterogeneity of soil resources, particularly N which often limits plant growth in temperate forests.

Acclimation of Photosynthetic Phenotype to Environmental Heterogeneity

Inducible C4-like photosynthetic metabolism in Hydrilla verticillata leaf tissue elicits variability in photosynthetic phenotype, expressed as CO2 compensation point (F). We conducted a field and laboratory study to investigate the ecological and adaptive significance of this physiological phenomenon. Spatial horizontal environmental hetero- geneity was observed within clonal populations of H. verticillata in Florida, USA. Measured at midday, the edge habitat at the expanding periphery of the clone exhibited a dissolved inorganic carbon (DIC) concentration of 0.7 mmol/L , pH 7.1, a dissolved oxygen (DO) level of 0. 13 mmol/L, and biomass of 0.2 kg/M2. The mat habitat, located 200 cm towards the interior of the surface mat, exhibited DIC 0.1 mmol/L, pH 10.2, DO 0.48 mmol/L, and biomass 0.8 kg/m2. DIC depletion and DO supersaturation characterized the mat habitat for most of the day and much of the growing season. Furthermore, net photosyn- thesis, daily carbon gain, and relative growth rate (RGR) of H. verticillata were reduced 80% by mat conditions compared to edge conditions. Fs of H. verticillata were positively correlated with CO2 and bicarbonate concentration, and negatively correlated with pH, DO, and biomass. Low and high F photosynthetic phenotypes were associated with the mat and edge habitats, respectively. Photosynthetic phenotype of H. verticillata appears to acclimate to environmental heterogeneity within a clone in the field. Net photosynthesis and daily carbon gain of low F phenotype H. verticillata was 128% and 40% greater than the high F phenotype when measured in the mat habitat, but was 21 % lower than the high F photosynthetic phenotype when measured in the edge habitat under low quantum flux. Laboratory experiments showed a negative curvilinear relation- ship between the F of H. verticillata and plant density. The data demonstrate that plasticity in photosynthetic phenotype of H. verticillata is a density-dependent, physiological response that optimizes carbon gain within a stressful heterogeneous environment. Evolution of facultative C4-like photosynthetic metabolism in H. verticillata may have been an adap- tation to the contraints imposed upon carbon gain by DIC and quantum flux limitation in the mat habitat.

Elevated-CO2-induced changes in the chemistry of quaking aspen (Populus tremuloides Michaux) leaf litter: subsequent mass loss and microbial response in a stream ecosystem

Changes in chemistry of quaking aspen (Populus tremuloides Michaux) leaf litter were examined under ambient (AMB = 360 ppm) and elevated (ELE = 720 ppm) levels of atmospheric CO2. Senesced ELE leaves were significantly higher in phenolic compounds, lignin, and C:N than AMB leaves. A 30-d in situ experiment in a northern Michigan stream analyzed changes in leaf mass, the concentration of phenolic compounds as a result of chemical leaching, and the growth responses of fungi and bacteria. ELE leaves lost less mass than AMB leaves after a 30-d incubation. Although ELE leaves were initially higher in total phenolic compounds and condensed tannins, differences between the treatments were no longer observed after 48 h of chemical leaching. Bacterial biomass and community respiration were higher on the AMB leaves for the first 12 d of incubation, whereas fungal biomass and community respiration were higher in the AMB treatment by the end of the 30-d experiment. Fungal biomass was negatively correlated with C:N and positively correlated with bacterial biomass on the ELE leaves, but not on the AMB leaves. These results indicate that a doubling in concentration of atmospheric CO2 could lead to leaf litter that is more recalcitrant toward microbial breakdown, which may decrease the availability of C and N for higher trophic levels.

Populus tremuloides photosynthesis and crown architecture in response to elevated CO2 and soil N availability

Abstract We tested the hypothesis that elevated CO2 would stimulate proportionally higher photosynthesis in the lower crown of Populus trees due to less N retranslocation, compared to tree crowns in ambient CO2. Such a response could increase belowground C allocation, particularly in trees with an indeterminate growth pattern such as Populus tremuloides. Rooted cuttings of P. tremuloides were grown in ambient and twice ambient (elevated) CO2 and in low and high soil N availability (89 ± 7 and 333 ± 16 ng N g−1 day−1 net mineralization, respectively) for 95 days using open-top chambers and open-bottom root boxes. Elevated CO2 resulted in significantly higher maximum leaf photosynthesis (Amax) at both soil N levels. Amax was higher at high N than at low N soil in elevated, but not ambient CO2. Photosynthetic N use efficiency was higher at elevated than ambient CO2 in both soil types. Elevated CO2 resulted in proportionally higher whole leaf A in the lower three-quarters to one-half of the crown for both soil types. At elevated CO2 and high N availability, lower crown leaves had significantly lower ratios of carboxylation capacity to electron transport capacity (Vcmax/Jmax) than at ambient CO2 and/or low N availability. From the top to the bottom of the tree crowns, Vcmax/Jmax increased in ambient CO2, but it decreased in elevated CO2 indicating a greater relative investment of N into light harvesting for the lower crown. Only the mid-crown leaves at both N levels exhibited photosynthetic down regulation to elevated CO2. Stem biomass segments (consisting of three nodes and internodes) were compared to the total Aleaf for each segment. This analysis indicated that increased Aleaf at elevated CO2 did not result in a proportional increase in local stem segment mass, suggesting that C allocation to sinks other than the local stem segment increased disproportionally. Since C allocated to roots in young Populus trees is primarily assimilated by leaves in the lower crown, the results of this study suggest a mechanism by which C allocation to roots in young trees may increase in elevated CO2.

Soil nematodes indicate food web responses to elevated atmospheric CO2

Summary To understand the impact of rising levels of atmospheric CO 2 on ecosystems, we need to understand plant responses to elevated CO 2 , as well as how those plant responses in turn affect their environment. An important component of the environment of a plant is the soil biota living near plant roots. Soil nematodes are representative of a large portion of this biota, since they are abundant and trophically diverse in most soils. In a three-year field experiment, we studied the responses of soil nematodes to increased root growth of trees growing in high and low nitrogen soils under ambient and twice-ambient atmospheric CO 2 , a two-by-two factorial experimental design. Our hypothesis was that in the high-N soil, increased root growth resulting from twice-ambient atmospheric CO 2 would positively affect nematode density, supporting a more abundant and trophically complex nematode community. Trembling aspen ( Populus tremuloides ) were grown in twenty open-top chambers under the four treatments, replicated five times. In low-N soil, twice-ambient CO 2 was associated with higher density of the most abundant plant-feeding taxon (Trichodoridae), lower density of one bacteriafeeding taxon (Rhabditidae), and lower evenness of the community, compared to ambient CO 2 . In high-N soil, twice-ambient CO 2 was associated with higher density of predator/omnivores, lower diversity, and a larger value of Bongers Maturity Index, compared to ambient CO 2 . In soils under young deciduous trees, such as the aspens in this experiment, increased root growth under elevated CO 2 may result in significant changes in soil food web community structure that may provide clues about the fate of carbon under elevated CO 2 .

POPULATION-LEVEL VARIATION IN PHOTOSYNTHETIC METABOLISM AND GROWTH IN SEDUM WRIGHTII'

Four populations of Sedum wrightii were studied with respect to their photosynthetic carbon metabolism. This leaf-succulent species exhibits crassulacean acid metabolism (CAM). Plants that possess CAM have the potential to vary their rates of transpirational water loss greatly in response to drought. Leaf thickness and biomass <513C values were measured in mid growing season for the populations in their native habitats. The populations exhibited significant differences in both leaf thickness and <513C value. The <513C values ranged from - 13.8%o to -22.9%o. In the field, populations differed in the proportion of day vs. night C02 uptake during growth. Three of the populations were compared in a controlled-environment study. It was found that there are both environmental and genetic determinants of the differences in photosynthetic carbon metabolism. We observed a significant correlation between plant growth and biomass 513C value. The largest plants exhibited the greatest proportion of day vs. night C02 uptake. These findings suggest that in this species there may be an inverse correlation between the ability to conserve water and the ability to gain carbon. The unusually wide range of photosynthetic phenotypes exhibited in this species may well explain its presence over a wide range of environments.

Elevated atmospheric CO2 alters leaf litter quality for stream ecosystems: an in situ leaf decomposition study

Trembling aspen (Populus tremuloides) seedlings were exposed to both elevated (720 ppm; ELEV) and ambient (370 ppm; AMB) concentrations of atmospheric CO2 for a 6-month growing season after which senesced leaves were collected and analyzed for differences in chemical composition. Elevated levels of atmospheric CO2 significantly increased total phenolic compounds, lignin levels, and C:N ratios, while decreasing the concentration of foliar nitrogen. ELEV and AMB leaf aggregates were placed into a headwater stream in the autumn of 1999 for 4 months to assess microbial activity, macroinvertebrate colonization, and leaf decomposition rates. Elevated CO2 significantly reduced 30 day microbial community respiration (−36.8%), and percent leaf mass remaining after 30 and 120 days of stream incubation (−9.4% and −13%, respectively). Low resolution of the experimental design for testing macroinvertebrate responses to altered leaves, including the free movement of macroinvertebrates among leaf aggregates, may explain the lack of treatment effect on invertebrate distribution between AMB and ELEV leaves. Elevated CO2-induced increases in leaf litter total phenolic compounds, lignins, and C:N appear to have negative effects on leaf decomposition, especially in the early stages of the decay process where microorganisms play a dominant role.