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Anthropogenic release of biologically available N has increased atmospheric N deposition in forest ecosystems, which may slow decomposition by reducing the lignolytic activity of white-rot fungi. We investigated the potential for atmospheric N deposition to reduce the abundance and alter the composition of lignolytic basidiomycetes in a regional network of four northern hardwood forest stands receiving experimental NO3 − deposition (30 kg NO3 −−N ha−1 year−1) for a decade. To estimate the abundance of basidiomycetes with lignolytic potential, we used PCR primers targeting laccase (polyphenol oxidase) and quantitative fluorescence PCR to estimate gene copy number. Natural variation in laccase gene size permitted use of length heterogeneity PCR to profile basidiomycete community composition across two sampling dates in forest floor and mineral soil. Although past work has identified significant and consistent negative effects of NO3 − deposition on lignolytic enzyme activity, microbial biomass, soil respiration, and decomposition rate, we found no consistent effect of NO3 − deposition on basidiomycete laccase gene abundance or community profile. Rather, laccase abundance under NO3 − deposition was lower (−52%), higher (+223%), or unchanged, depending on stand. Only a single stand exhibited a significant change in basidiomycete laccase gene profile. Basidiomycete laccase genes occurring in mineral soil were a subset of the genes observed in the forest floor. Moreover, significant effects on laccase abundance were confined to the forest floor, suggesting that species composition plays some role in determining how lignolytic basidiomycetes are affected by N deposition. Community profiles differed between July and October sampling dates, and basidiomycete communities sampled in October had lower laccase gene abundance in the forest floor, but higher laccase abundance in mineral soil. Although experimental N deposition significantly suppresses lignolytic activity in these forests, this change is not related to the abundance or community composition of basidiomycete fungi with laccase genes. Understanding the expression of laccases and other lignolytic enzymes by basidiomycete fungi and other lignin-decaying organisms appears to hold promise for explaining the consistent decline in lignolytic activity elicited by experimental N deposition. [ABSTRACT FROM AUTHOR]

Anthropogenic nitrogen (N) deposition affects a wide range of soil processes including phenol oxidase (PO) activity and soil organic matter dynamics. Depression of phenol oxidase activity in response to N saturation is believed to be mediated by the activity of white-rot basidiomycetes, whose production of extracellular oxidative enzymes can be limited by high N availability. We examined the effect of short-term N deposition on basidiomycete laccase gene diversity and relative abundance in temperate oak forest soil in which significant decreases in phenol oxidase and increased SOM have been recorded in response to experimental N deposition. UniFrac was used to compare the composition of laccase genes between three control- and three nitrogen-fertilized (80 kg−1 ha−1 per year) oak forest soils. The relative abundance of laccase genes was determined from qPCR analysis of laccase and basidiomycete ITS gene abundances. Our results indicate that there was no significant shift in the composition of laccase genes between control- and N-fertilized soils, nor was there a significant change in the relative abundance of laccase genes. These data suggest that N deposition effects on mineral soil PO activity do not result from changes in laccase gene diversity of white-rot basidiomycetes but are likely the result of altered microbial abundance or expression in this ecosystem type. Furthermore, laccase gene composition may be tied to factors that structure microbial communities in general, as soil laccase gene communities are more similar to other forest soils than with the corresponding litter. [ABSTRACT FROM AUTHOR]

A diverse range of fungi associate with ectomycorrhizal (EcM) root tips, however, their identity and the biotic and abiotic filters structuring these communities remain unknown. We employed a metabarcoding approach to characterize fungal communities associating with the EcM root tips of Quercus rubra along a natural soil nitrogen gradient. EcM communities and ectomycorrhizal associated fungi (EcAF) were tightly linked across the breadth of the soil gradient. Notably, EcAF communities were primarily shaped by the morphological attributes of EcM communities, particularly the relative abundance of EcM taxa forming rhizomorphic hyphae. Edaphic properties (soil C:N and net N mineralization) exerted minimal influence, suggesting a strong role of biotic interactions in EcAF community assembly. The presence of plants forming ericoid mycorrhizal associations also shapes the prevalence of ericoid mycorrhizal fungi associating with EcM root tips. Overall, EcAF communities were dominated by helotialean fungi, ericoid mycorrhizal fungi, dark septate endophytes, and the white-rot fungi Mycena. [ABSTRACT FROM AUTHOR]

The article presents an analysis of the study which looks into the relevance of the simulated atmospheric NO3- deposition on the soil organic matter. With reference to this, research on the field accordingly devised an organic matter and nitrogen budgets in the replication of the northern hardwood stands which have been given off an ambient and experimental NO3- deposition. In doing this, the team claimed that NO3- manifests an accumulation of nitrogen in soil organic matter.

• The Rhinelander free-air CO2 enrichment (FACE) experiment is designed to understand ecosystem response to elevated atmospheric carbon dioxide (+CO2) and elevated tropospheric ozone (+O3). The objectives of this study were: to understand how soil respiration responded to the experimental treatments; to determine whether fine-root biomass was correlated to rates of soil respiration; and to measure rates of fine-root turnover in aspen ( Populus tremuloides) forests and determine whether root turnover might be driving patterns in soil respiration. • Soil respiration was measured, root biomass was determined, and estimates of root production, mortality and biomass turnover were made. • Soil respiration was greatest in the +CO2 and +CO2 +O3 treatments across all three plant communities. Soil respiration was correlated with increases in fine-root biomass. In the aspen community, annual fine-root production and mortality (g m−2) were positively affected by +O3. • After 10 yr of exposure, +CO2 +O3-induced increases in belowground carbon allocation suggest that the positive effects of elevated CO2 on belowground net primary productivity (NPP) may not be offset by negative effects of O3. For the aspen community, fine-root biomass is actually stimulated by +O3, and especially +CO2 +O3. [ABSTRACT FROM AUTHOR]

Most explanations for the positive effect of plant species diversity on productivity have focused on the efficiency of resource use, implicitly assuming that resource supply is constant. To test this assumption, we grew seedlings of Echinacea purpurea in soil collected beneath 10-year-old, experimental plant communities containing one, two, four, eight, or 16 native grassland species. The results of this greenhouse bioassay challenge the assumption of constant resource supply; we found that bioassay seedlings grown in soil collected from experimental communities containing 16 plant species produced 70% more biomass than seedlings grown in soil collected beneath monocultures. This increase was likely attributable to greater soil N availability, which had increased in higher diversity communities over the 10-year-duration of the experiment. In a distinction akin to the selection/complementarity partition commonly made in studies of diversity and productivity, we further determined whether the additive effects of functional groups or the interactive effects of functional groups explained the increase in fertility with diversity. The increase in bioassay seedling biomass with diversity was largely explained by a concomitant increase in N-fixer, C4 grass, forb, and C3 grass biomass with diversity, suggesting that the additive effects of these four functional groups at higher diversity contributed to enhance N availability and retention. Nevertheless, diversity still explained a significant amount of the residual variation in bioassay seedling biomass after functional group biomass was included in a multiple regression, suggesting that interactions also increased fertility in diverse communities. Our results suggest a mechanism, the fertility effect, by which increased plant species diversity may increase community productivity over time by increasing the supply of nutrients via both greater inputs and greater retention. [ABSTRACT FROM AUTHOR]

The ability of an ecosystem to retain anthropogenic nitrogen (N) deposition is dependent upon plant and soil sinks for N, the strengths of which may be altered by chronic atmospheric N deposition. Sugar maple ( Acer saccharum Marsh.), the dominant overstory tree in northern hardwood forests of the Lake States region, has a limited capacity to take up and assimilate NO. However, it is uncertain whether long-term exposure to NO deposition might induce NO uptake by this ecologically important overstory tree. Here, we investigate whether 10 years of experimental NOdeposition (30 kg N ha−1 y−1) could induce NO uptake and assimilation in overstory sugar maple (approximately 90 years old), which would enable this species to function as a direct sink for atmospheric NO deposition. Kinetic parameters for NH and NO uptake in fine roots, as well as leaf and root NO reductase activity, were measured under conditions of ambient and experimental NO deposition in four sugar maple-dominated stands spanning the geographic distribution of northern hardwood forests in the Upper Lake States. Chronic NO deposition did not alter the V max or K m for NO and NH uptake nor did it influence NO reductase activity in leaves and fine roots. Moreover, the mean V max for NH uptake (5.15 μmol 15N g−1 h−1) was eight times greater than the V max for NO uptake (0.63 μmol 15N g−1 h−1), indicating a much greater physiological capacity for NH uptake in this species. Additionally, NO reductase activity was lower than most values for woody plants previously reported in the literature, further indicating a low physiological potential for NO assimilation in sugar maple. Our results demonstrate that chronic NO deposition has not induced the physiological capacity for NO uptake and assimilation by sugar maple, making this dominant species an unlikely direct sink for anthropogenic NO deposition. [ABSTRACT FROM AUTHOR]

High levels of atmospheric nitrogen (N) deposition in Europe and North America were maintained throughout the 1990s, and global N deposition is expected to increase by a factor of 2.5 over the next century. Available soil N limits primary production in many terrestrial ecosystems, and some computer simulation models have predicted that increasing atmospheric N deposition may result in greater terrestrial carbon (C) storage in woody biomass. However, empirical evidence demonstrating widespread increases in woody biomass C storage due to atmospheric N deposition is uncommon. Increased C storage in soil organic matter due to chronic N inputs has rarely been reported and is often not considered in computer simulation models of N deposition effects. Since 1994, we have experimentally simulated chronic N deposition by adding 3 g N m−2 yr−1 to four different northern hardwood forests, which span a 500 km geographic gradient in Michigan. Each year we measured tree growth. In 2004, we also examined soil C content to a depth of 70 cm. When we compared the control treatment with the NO3− deposition treatment after a decade of experimentation, ecosystem C storage had significantly increased in both woody biomass (500 g C m−2) and surface soil (0–10 cm) organic matter (690 g C m−2). The increase in surface soil C storage was apparently driven by altered rates of organic matter decomposition, rather than an increase in detrital inputs to soil. Our results, for study locations stretching across hundreds of kilometers, support the hypothesis that chronic N deposition may increase C storage in northern forests, potentially contributing to a sink for anthropogenic CO2 in the northern Hemisphere. [ABSTRACT FROM AUTHOR]

As human activity continues to increase CO2 and O3, broad expanses of north temperate forests will be simultaneously exposed to elevated concentrations of these trace gases. Although both CO2 and O3 are potent modifiers of plant growth, we do not understand the extent to which they alter competition for limiting soil nutrients, like nitrogen (N). We quantified the acquisition of soil N in two 8-year-old communities composed of trembling aspen genotypes ( n= 5) and trembling aspen–paper birch which were exposed to factorial combinations of CO2 (ambient and 560 μL L−1) and O3 (ambient = 30–40 vs. 50–60 nL L−1). Tracer amount of 15NH4+ were applied to soil to determine how these trace gases altered the competitive ability of genotypes and species to acquire soil N. One year after isotope addition, we assessed N acquisition by measuring the amount of 15N tracer contained in the plant canopy (i.e. recent N acquisition), as well as the total amount of canopy N (i.e. cumulative N acquisition). Exposure to elevated CO2 differentially altered recent and cumulative N acquisition among aspen genotypes, changing the rank order in which they obtained soil N. Elevated O3 also altered the rank order in which aspen genotypes obtained soil N by eliciting increases, decreases and no response among genotypes. If aspen genotypes respond similarly under field conditions, then rising concentrations of CO2 and O3 could alter the structure of aspen populations. In the aspen–birch community, elevated CO2 increased recent N (i.e. 15N) acquisition in birch (68%) to a greater extent than aspen (19%), suggesting that, over the course of this experiment, birch had gained a competitive advantage over aspen. The response of genotypes and species to rising CO2 and O3 concentrations, and how these responses are modified by competitive interactions, has the potential to change the future composition and productivity of northern temperate forests. [ABSTRACT FROM AUTHOR]

Anthropogenic O3 and CO2-induced declines in soil N availability could counteract greater plant growth in a CO2-enriched atmosphere, thereby reducing net primary productivity (NPP) and the potential of terrestrial ecosystems to sequester anthropogenic CO2. Presently, it is uncertain how increasing atmospheric CO2 and O3 will alter plant N demand and the acquisition of soil N by plants as well as the microbial supply of N from soil organic matter. To address this uncertainty, we initiated an ecosystem-level 15N tracer experiment at the Rhinelander (Wisconsin, USA) free air CO2-03 enrichment (FACE) facility to understand how projected increases in atmospheric CO2 and O3 alter the distribution and flow of N in developing northern temperate forests. Tracer amounts of 15NH4+ were applied to the forest floor of developing Populus tremuloides and P. tremuloides-Betula papyrift'ra communities that have been exposed to factorial CO2 and O3 treatments for seven years. One year after isotope addition, both forest communities exposed to elevated CO2 obtained greater amounts of 15N (29%) and N (40%) from soil, despite no change in soil N availability or plant N-use efficiency. As such, elevated CO2 increased the ability of plants to exploit soil for N, through the development of a larger root system. Conversely, elevated O3 decreased the amount of 15N (-15%) and N (-29%) in both communities, a response resulting from lower rates of photosynthesis, decreases in growth, and smaller root systems that acquired less soil N. Neither CO2 nor O3 altered the amount of N or 15N recovery in the forest floor, microbial biomass, or soil organic matter. Moreover, we observed no interaction between CO2 and O3 on the amount of N or 15N in any ecosystem pool, suggesting that O3 could exert a negative effect regardless of CO2 concentration. In a CO2-enriched atmosphere, greater belowground growth and a more thorough exploitation of soil for growth-limiting N is an important mechanism sustaining the enhancement of NPP in developing forests (0-8 years following establishment). However, as CO2 accumulates in the Earth's atmosphere, future O3 concentrations threaten to diminish the enhancement of plant growth, decrease plant N acquisition, and lessen the storage pf anthropogenic C in temperate forests. [ABSTRACT FROM AUTHOR]

As human activity continues to increase CO2 and O3, broad expanses of north temperate forests will be simultaneously exposed to elevated concentrations of these trace gases. Although both CO2 and O3 are potent modifiers of plant growth, we do not understand the extent to which they alter competition for limiting soil nutrients, like nitrogen (N). We quantified the acquisition of soil N in two 8-year-old communities composed of trembling aspen genotypes ( n= 5) and trembling aspen–paper birch which were exposed to factorial combinations of CO2 (ambient and 560 μL L−1) and O3 (ambient = 30–40 vs. 50–60 nL L−1). Tracer amount of 15NH4+ were applied to soil to determine how these trace gases altered the competitive ability of genotypes and species to acquire soil N. One year after isotope addition, we assessed N acquisition by measuring the amount of 15N tracer contained in the plant canopy (i.e. recent N acquisition), as well as the total amount of canopy N (i.e. cumulative N acquisition). Exposure to elevated CO2 differentially altered recent and cumulative N acquisition among aspen genotypes, changing the rank order in which they obtained soil N. Elevated O3 also altered the rank order in which aspen genotypes obtained soil N by eliciting increases, decreases and no response among genotypes. If aspen genotypes respond similarly under field conditions, then rising concentrations of CO2 and O3 could alter the structure of aspen populations. In the aspen–birch community, elevated CO2 increased recent N (i.e. 15N) acquisition in birch (68%) to a greater extent than aspen (19%), suggesting that, over the course of this experiment, birch had gained a competitive advantage over aspen. The response of genotypes and species to rising CO2 and O3 concentrations, and how these responses are modified by competitive interactions, has the potential to change the future composition and productivity of northern temperate forests. [ABSTRACT FROM AUTHOR]

Forest ecosystems are important sinks for rising concentrations of atmospheric CO2. In previous research, we showed that net primary production (NPP) increased by 23 ± 2% when four experimental forests were grown under atmospheric concentrations of CO2 predicted for the latter half of this century. Because nitrogen (N) availability commonly limits forest productivity, some combination of increased N uptake from the soil and more efficient use of the N already assimilated by trees is necessary to sustain the high rates of forest NPP under free-air CO2 enrichment (FACE). In this study, experimental evidence demonstrates that the uptake of N increased under elevated CO2 at the Rhinelander, Duke, and Oak Ridge National Laboratory FACE sites, yet fertilization studies at the Duke and Oak Ridge National Laboratory FACE sites showed that tree growth and forest NPP were strongly limited by N availability. By contrast, nitrogen-use efficiency increased under elevated CO2 at the POP-EUROFACE site, where fertilization studies showed that N was not limiting to tree growth. Some combination of increasing fine root production, increased rates of soil organic matter decomposition, and increased allocation of carbon (C) to mycorrhizal fungi is likely to account for greater N uptake under elevated CO2. Regardless of the specific mechanism, this analysis shows that the larger quantities of C entering the below-ground system under elevated CO2 result in greater N uptake, even in N-limited ecosystems. Biogeochemical models must be reformulated to allow C transfers below ground that result in additional N uptake under elevated CO2. [ABSTRACT FROM AUTHOR]

We determined soil microbial community composition and function in a field experiment in which plant communities of increasing species richness were exposed to factorial elevated CO2 and nitrogen (N) deposition treatments. Because elevated CO2 and N deposition increased plant productivity to a greater extent in more diverse plant assemblages, it is plausible that heterotrophic microbial communities would experience greater substrate availability, potentially increasing microbial activity, and accelerating soil carbon (C) and N cycling. We, therefore, hypothesized that the response of microbial communities to elevated CO2 and N deposition is contingent on the species richness of plant communities. Microbial community composition was determined by phospholipid fatty acid analysis, and function was measured using the activity of key extracellular enzymes involved in litter decomposition. Higher plant species richness, as a main effect, fostered greater microbial biomass, cellulolytic and chitinolytic capacity, as well as the abundance of saprophytic and arbuscular mycorrhizal (AM) fungi. Moreover, the effect of plant species richness on microbial communities was significantly modified by elevated CO2 and N deposition. For instance, microbial biomass and fungal abundance increased with greater species richness, but only under combinations of elevated CO2 and ambient N, or ambient CO2 and N deposition. Cellobiohydrolase activity increased with higher plant species richness, and this trend was amplified by elevated CO2. In most cases, the effect of plant species richness remained significant even after accounting for the influence of plant biomass. Taken together, our results demonstrate that plant species richness can directly regulate microbial activity and community composition, and that plant species richness is a significant determinant of microbial response to elevated CO2 and N deposition. The strong positive effect of plant species richness on cellulolytic capacity and microbial biomass indicate that the rates of soil C cycling may decline with decreasing plant species richness. [ABSTRACT FROM AUTHOR]

Environmental perturbations can alter the composition of plant communities, either directly, by altering growth of some species more than others, or indirectly, by altering the strength of interspecific interactions among species. The relative importance of direct and indirect effects is not at all well known. We used an experimental approach to quantifying direct and indirect effects of fungicide on the composition of a plant community. To separate the direct and indirect impacts of fungicide we grew plant species in monoculture and mixed communities, and with and without the systemic fungicide benomyl. We predicted that direct effects of fungicide would be important at low but not high nutrient availability, while indirect effects would be more important at high nutrient availability. After 3 years there was little impact of fungicide on arbuscular mycorrhizal fungal colonization, and on soil microbial community composition assessed as the relative abundance of different phospholipid fatty acids. Like fertilizer, fungicide increased plant biomass. However, in contrast to fertilizer, this did not result in a decline in species evenness. Although not significant, the direct effects of fungicide tended to oppose the indirect effects of both fungicide and interspecific interactions on plant community composition. Experiments relying on fungicide treatments must be interpreted extremely cautiously, because the impact of fungicide is potentially the integrated response of plants to multiple factors, including arbuscular mycorrhizal fungi, pathogenic and saprophytic fungi, and nutrient inputs. [ABSTRACT FROM AUTHOR]

We tested the main and interactive effects of elevated carbon dioxide concentration ([CO2]), nitrogen (N), and light availability on leaf photosynthesis, and plant growth and survival in understory seedlings grown in an N-limited northern hardwood forest. For two growing seasons, we exposed six species of tree seedlings ( Betula papyrifera, Populus tremuloides, Acer saccharum, Fagus grandifolia, Pinus strobus, and Prunus serotina) to a factorial combination of atmospheric CO2 (ambient, and elevated CO2 at 658 μmol CO2 mol−1) and N deposition (ambient and ambient +30 kg N ha−1 yr−1) in open-top chambers placed in an understory light gradient. Elevated CO2 exposure significantly increased apparent quantum efficiency of electron transport by 41% ( P<0. 0001), light-limited photosynthesis by 47% ( P<0. 0001), and light-saturated photosynthesis by 60% ( P<0. 003) compared with seedlings grown in ambient [CO2]. Experimental N deposition significantly increased light-limited photosynthesis as light availability increased ( P<0. 037). Species differed in the magnitude of light-saturated photosynthetic response to elevated N and light treatments ( P<0. 016). Elevated CO2 exposure and high N availability did not affect seedling growth; however, growth increased slightly with light availability ( R2=0. 26, P<0. 0001). Experimental N deposition significantly increased average survival of all species by 48% ( P<0. 012). However, seedling survival was greatest (85%) under conditions of both high [CO2] and N deposition ( P<0. 009). Path analysis determined that the greatest predictor for seedling survival in the understory was total biomass ( R2=0. 39, P<0. 001), and that carboxylation capacity ( Vcmax) was a better predictor for seedling growth and survival than maximum photosynthetic rate ( Amax). Our results suggest that increasing [CO2] and N deposition from fossil fuel combustion could alter understory tree species recruitment dynamics through changes in seedling survival, and this has the potential to alter future forest species composition. [ABSTRACT FROM AUTHOR]

Seedling responses to elevated atmospheric CO2 concentration ([CO2]) and solar irradiance were measured over two growing seasons in shade-tolerant Acer saccharum Marsh. and Fagus grandifolia J.F. Ehrh. and shade-intolerant Prunus serotina, a J.F. Ehrh. and Betula papyrifera Marsh. Seedlings were exposed to a factorial combination of [CO2] (ambient and elevated (658 μmol mol−1)) and understory shade (deep and moderate) in open-top chambers placed in a forest understory. The elevated [CO2] treatment increased mean light-saturated net photosynthetic rate by 63% in the shade-tolerant species and 67% in the shade-intolerant species. However, when measured at the elevated [CO2], long-term enhancement of photosynthesis was 10% lower than the instantaneous enhancement seen in ambient-[CO2]-grown plants (P < 0.021). Overall, growth light environment affected long-term photosynthetic enhancement by elevated [CO2]: as the growth irradiance increased, proportional enhancement due to elevated [CO2] decreased from 97% for plants grown in deep shade to 47% for plants grown in moderate shade. Results suggest that in N-limited northern temperate forests, trees grown in deep shade may display greater photosynthetic gains from a CO2-enriched atmosphere than trees growing in more moderate shade, because of greater downregulation in the latter environment. If photosynthetic gains by deep-shade-grown plants in response to elevated [CO2] translate into improved growth and survival of shade-intolerant species, it could alter the future composition and dynamics of successional forest communities. [ABSTRACT FROM PUBLISHER]

Abstract: In forests of the Great Lakes region, experimental deposition has suppressed soil respiration and enhanced DOC export. Reasons for these responses are unknown, but they could arise via two alternatives: (i) direct suppression of microbial activity by or (ii) indirect suppression of the microbial community via changes in litter biochemistry in response to greater N availability. To test the second alternative, we conducted a controlled laboratory experiment to examine how chronic experimental deposition affects the contributions of fresh leaf litter to microbial respiration and DOC export. The study reported here used manipulations of mineral soil and fresh leaf litter from a previously studied northern hardwood forest stand in northern Lower Michigan that has received 9 years of ambient and experimental (three times ambient) atmospheric deposition. We found that cumulative microbial respiration over the 6-week incubation was substantially greater in fresh litter plus mineral soil (20.2–13.4mg C) versus mineral soil alone (4.4–4.1mg C); however, experimental deposition had no effect on microbial respiration across the litter–mineral soil manipulations. DOC production (∼75%) was primarily associated with leaching from fresh litter. In contrast, mineral soil was a significant sink for litter-derived DOC. Significantly, the mineral soil sink was less pronounced in soil receiving experimental deposition in which ∼30% more DOC was leached compared to the ambient deposition treatment. Furthermore, mineral soil was also both a source and sink for soluble phenolics; however, deposition suppressed a mineral–soil sink for phenolics derived from fresh leaf litter. These results suggest that increases in DOC export and declines in soil respiration in response to deposition in the field are not related to obvious changes in litter biochemistry or to the microbial metabolism of this material. Alternatively, these patterns may be linked to decreased abiotic sinks for litter-derived DOC in mineral soil, an unexpected ecosystem consequence of increased anthropogenic deposition. [Copyright &y& Elsevier]

Elevated atmospheric CO2 and O3 have the potential to affect the primary productivity of the forest overstory, but little attention has been given to potential responses of understory vegetation. Our objective was to document the effects of elevated atmospheric CO2 and O3 on understory species composition and biomass and to quantify nitrogen (N) acquisition by the understory vegetation. The research took place at the aspen free-air CO2 and O3 enrichment (FACE) experiment, which has four treatments (control, elevated CO2, elevated O3, and elevated CO2+O3) and three tree communities: aspen, aspen/birch, and aspen/maple. In June 2003, each FACE ring was uniformly labeled with 15N applied as NH4Cl. Understory biomass was harvested in June of 2004 for productivity, N, and 15N measurements, and photosynthetically active radiation (PAR) was measured below the canopy. The understory was divided into five species groups, which dominate in this young aggrading forest: Taraxacum officinale (dandelion), Solidago sp. (goldenrod), Trifolium repens and T. pretense (clover), various species from the Poaceae family (grass), and composited minor components (CMC). Understory species composition, total and individual species biomass, N content, and 15N recovery showed overstory community effects, but the direct effects of treatments was masked by the high variability of these data. Total understory biomass increased with increasing light, and thus was greatest under the open canopy of the aspen/maple community, as well as the more open canopy of the elevated O3 treatments. Species were different from one another in terms of 15N recovery, with virtually no 15N recovered in clover and the greatest amount recovered in dandelion. Thus, understory species composition and biomass appear to be driven by the structure of the overstory community, which is determined by the tree species present and their response to the treatments. However, N acquisition by the understory does not appear to be affected by either the overstory community or the treatments at this point. [ABSTRACT FROM AUTHOR]

Atmospheric CO2 and O3 concentrations are increasing due to human activity and both trace gases have the potential to alter C cycling in forest ecosystems. Because soil microorganisms depend on plant litter as a source of energy for metabolism, changes in the amount or the biochemistry of plant litter produced under elevated CO2 and O3 could alter microbial community function and composition. Previously, we have observed that elevated CO2 increased the microbial metabolism of cellulose and chitin, whereas elevated O3 dampened this response. We hypothesized that this change in metabolism under CO2 and O3 enrichment would be accompanied by a concomitant change in fungal community composition. We tested our hypothesis at the free-air CO2 and O3 enrichment (FACE) experiment at Rhinelander, Wisconsin, in which Populus tremuloides, Betula papyrifera, and Acer saccharum were grown under factorial CO2 and O3 treatments. We employed extracellular enzyme analysis to assay microbial metabolism, phospholipid fatty acid (PLFA) analysis to determine changes in microbial community composition, and polymerase chain reaction–denaturing gradient gel electrophoresis (PCR–DGGE) to analyze the fungal community composition. The activities of 1,4-β-glucosidase (+37%) and 1,4,-β- N-acetylglucosaminidase (+84%) were significantly increased under elevated CO2, whereas 1,4-β-glucosidase activity (−25%) was significantly suppressed by elevated O3. There was no significant main effect of elevated CO2 or O3 on fungal relative abundance, as measured by PLFA. We identified 39 fungal taxonomic units from soil using DGGE, and found that O3 enrichment significantly altered fungal community composition. We conclude that fungal metabolism is altered under elevated CO2 and O3, and that there was a concomitant change in fungal community composition under elevated O3. Thus, changes in plant inputs to soil under elevated CO2 and O3 can propagate through the microbial food web to alter the cycling of C in soil. [ABSTRACT FROM AUTHOR]

Atmospheric CO2 and O3 concentrations are increasing due to human activity and both trace gases have the potential to alter C cycling in forest ecosystems. Because soil microorganisms depend on plant litter as a source of energy for metabolism, changes in the amount or the biochemistry of plant litter produced under elevated CO2 and O3 could alter microbial community function and composition. Previously, we have observed that elevated CO2 increased the microbial metabolism of cellulose and chitin, whereas elevated O3 dampened this response. We hypothesized that this change in metabolism under CO2 and O3 enrichment would be accompanied by a concomitant change in fungal community composition. We tested our hypothesis at the free-air CO2 and O3 enrichment (FACE) experiment at Rhinelander, Wisconsin, in which Populus tremuloides, Betula papyrifera, and Acer saccharum were grown under factorial CO2 and O3 treatments. We employed extracellular enzyme analysis to assay microbial metabolism, phospholipid fatty acid (PLFA) analysis to determine changes in microbial community composition, and polymerase chain reaction–denaturing gradient gel electrophoresis (PCR–DGGE) to analyze the fungal community composition. The activities of 1,4-β-glucosidase (+37%) and 1,4,-β- N-acetylglucosaminidase (+84%) were significantly increased under elevated CO2, whereas 1,4-β-glucosidase activity (−25%) was significantly suppressed by elevated O3. There was no significant main effect of elevated CO2 or O3 on fungal relative abundance, as measured by PLFA. We identified 39 fungal taxonomic units from soil using DGGE, and found that O3 enrichment significantly altered fungal community composition. We conclude that fungal metabolism is altered under elevated CO2 and O3, and that there was a concomitant change in fungal community composition under elevated O3. Thus, changes in plant inputs to soil under elevated CO2 and O3 can propagate through the microbial food web to alter the cycling of C in soil. [ABSTRACT FROM AUTHOR]

Rising atmospheric carbon dioxide (CO2) concentration ([CO2]) could alter terrestrial carbon (C) cycling by affecting plant growth, litter chemistry and decomposition. How the concurrent increase in tropospheric ozone (O3) concentration ([O3]) will interact with rising atmospheric [CO2] to affect C cycling is unknown. A major component of carbon cycling in forests is fine root production, mortality and decomposition. To better understand the effects of elevated [CO2] and [O3] on the dynamics of fine root C, we conducted a combined field and laboratory incubation experiment to monitor decomposition dynamics and changes in fine root litter chemistry. Free-air CO2 enrichment (FACE) technology at the FACTS-II Aspen FACE project in Rhinelander, Wisconsin, elevated [CO2] (535 μl 1−1) and [O3] (53 nl 1−1) in intact stands of pure trembling aspen (Populus tremuloides Michx.) and in mixed stands of trembling aspen plus paper birch (Betula papyrifera Marsh.) and trembling aspen plus sugar maple (Acer saccharum Marsh.). We hypothesized that the trees would react to increased C availability (elevated [CO2]) by increasing allocation to C-based secondary compounds (CBSCs), thereby decreasing rates of decomposition. Because of its lower growth potential, we reasoned this effect would be greatest in the aspen–maple community relative to the aspen and aspen–birch communities. As a result of decreased C availability, we expected elevated [O3] to counteract shifts in C allocation induced by elevated [CO2]. Concentrations of CBSCs were rarely significantly affected by the CO2 and O3 treatments in decomposing fine roots. Rates of microbial respiration and mass loss from fine roots were unaffected by the treatments, although the production of dissolved organic C differed among communities. We conclude that elevated [CO2] and [O3] induce only small changes in fine root chemistry that are insufficient to significantly influence fine root decomposition. If changes in soil C cycling occur in the future, they will most likely be brought about by changes in litter production. [ABSTRACT FROM PUBLISHER]

Plant communities on nutrient-poor soils are thought to use nutrients more efficiently to produce biomass than plant communities on nutrient-rich soils. Yet, increased efficiency with declining soil nutrients has not been demonstrated empirically in lowland tropical rain forests, where plant growth is thought to be strongly limited by soil nutrients, especially phosphorus (P). We tested for higher P uptake and use efficiency across a 16-fold soil P gradient in lowland Borneo by measuring the P content of aboveground net primary productivity (fine litter production plus new tree growth; ANPP) for 24 months. Extractable soil P was positively related to litter production, tree growth, and ANPP. Efficiency of P response (ANPP/available soil P), uptake (P uptake/available soil P), and use (ANPP/P uptake) increased monotonically with declining soil P and, was significantly higher on P-rich soil than P-poor soil. Increased P uptake and use efficiency with declining soil P enabled higher than expected plant productivity on low P soils and thus strongly influenced spatial patterns of aboveground productivity throughout this lowland landscape. A complementary P use efficiency index, the integrated canopy P (Pc) use efficiency of production (ANPP/Pc × residence time of Pc), was similar across the P gradient, but underlying dynamics varied significantly with soil P: on rich soils, ANPP/Pc was high and Pc residence time was low, while the converse held on poor soils. These contrasting strategies enabled rapid tree growth on nutrient-rich soils, where P limitation is relatively weak, and higher P conservation on nutrient-poor soils, where P limitation is relatively strong. The occurrence of contrasting P use strategies on high and low P soils has important implications for understanding spatial patterns of aboveground productivity, P cycling, and canopy tree species composition across the P gradient. [ABSTRACT FROM AUTHOR]

A highly controversial issue in global biogeochemistry is the regulation of terrestrial carbon (C) sequestration by soil nitrogen (N) availability. This controversy translates into great uncertainty in predicting future global terrestrial C sequestration. We propose a new framework that centers on the concept of progressive N limitation (PNL) for studying the interactions between C and N in terrestrial ecosystems. In PNL, available soil N becomes increasingly limiting as C and N are sequestered in long-lived plant biomass and soil organic matter. Our analysis focuses on the role of PNL in regulating ecosystem responses to rising atmospheric carbon dioxide concentration, but the concept applies to any perturbation that initially causes C and N to accumulate in organic forms. This article examines conditions under which PNL may or may not constrain net primary production and C sequestration in terrestrial ecosystems. While the PNL-centered framework has the potential to explain diverse experimental results and to help researchers integrate models and data, direct tests of the PNL hypothesis remain a great challenge to the research community. [ABSTRACT FROM AUTHOR]

Chronic N additions to forest ecosystems can enhance soil N availability, potentially leading to reduced C allocation to root systems. This in turn could decrease soil CO2 efflux. We measured soil respiration during the first, fifth, sixth and eighth years of simulated atmospheric NO3− deposition (3 g N m−2 yr−1) to four sugar maple-dominated northern hardwood forests in Michigan to assess these possibilities. During the first year, soil respiration rates were slightly, but not significantly, higher in the NO3−-amended plots. In all subsequent measurement years, soil respiration rates from NO3−-amended soils were significantly depressed. Soil temperature and soil matric potential were measured concurrently with soil respiration and used to develop regression relationships for predicting soil respiration rates. Estimates of growing season and annual soil CO2 efflux made using these relationships indicate that these C fluxes were depressed by 15% in the eighth year of chronic NO3− additions. The decrease in soil respiration was not due to reduced C allocation to roots, as root respiration rates, root biomass, and root turnover were not significantly affected by N additions. Aboveground litter also was unchanged by the 8 years of treatment. Of the remaining potential causes for the decline in soil CO2 efflux, reduced microbial respiration appears to be the most likely possibility. Documented reductions in microbial biomass and the activities of extracellular enzymes used for litter degradation on the NO3−-amended plots are consistent with this explanation. [ABSTRACT FROM AUTHOR]

Human activity has substantially increased atmospheric $NO3-$ deposition in many regions of the Earth, which could lead to the N saturation of terrestrial ecosystems. Sugar maple (Acer saccharum Marsh.) dominated northern hardwood forests in the Upper Great Lakes region may be particularly sensitive to chronic $NO3-$ deposition, because relatively moderate experimental increases (three times ambient) have resulted in substantial N leaching over a relatively short duration (5–7 years). Although microbial immobilization is an initial sink (i.e., within 1–2 days) for anthropogenic $NO3-$ in this ecosystem, we have an incomplete understanding of the processes controlling the longer-term (i.e., after 1 year) retention and flow of anthropogenic N. Our objectives were to determine: (i) whether chronic $NO3-$ additions have altered the N content of major ecosystem pools, and (ii) the longer-term fate of $15NO3-$ in plots receiving chronic $NO3-$ addition. We addressed these objectives using a field experiment in which three northern hardwood plots receive ambient atmospheric N deposition (ca. 0.9 g N m-2 year-1) and three plots which receive ambient plus experimental N deposition (3.0 g NO3--N m-2 year-1). Chronic $NO3-$ deposition significantly increased the N concentration and content (g N/m2) of canopy leaves, which contained 72% more N than the control treatment. However, chronic $NO3-$ deposition did not significantly alter the biomass, N concentration or N content of any other ecosystem pool. The largest portion of 15N recovered after 1 year occurred in overstory leaves and branches (10%). In contrast, we recovered virtually none of the isotope in soil organic matter (SOM), indicating that SOM was not a sink for anthropogenic $NO3-$ over a 1 year duration. Our results indicate that anthropogenic $NO3-$ initially assimilated by the microbial community is released into soil solution where it is subsequently taken up by overstory trees and allocated to the canopy. Anthropogenic N appears to be incorporated into SOM only after it is returned to the forest floor and soil via leaf litter fall. Short- and long-term isotope tracing studies provided very different results and illustrate the need to understand the physiological processes controlling the flow of anthropogenic N in terrestrial ecosystems and the specific time steps over which they operate. [ABSTRACT FROM AUTHOR]

Increases in atmospheric CO2 and tropospheric O3 may affect forest N cycling by altering plant litter production and the availability of substrates for microbial metabolism. Three years following the establishment of our free-air CO2–O3 enrichment experiment, plant growth has been stimulated by elevated CO2 resulting in greater substrate input to soil; elevated O3 has counteracted this effect. We hypothesized that rates of soil N cycling would be enhanced by greater plant productivity under elevated CO2, and that CO2 effects would be dampened by O3. We found that elevated CO2 did not alter gross N transformation rates. Elevated O3 significantly reduced gross N mineralization and microbial biomass N, and effects were consistent among species. We also observed significant interactions between CO2 and O3: (i) gross N mineralization was greater under elevated CO2 (1.0 mg N kg−1 day−1) than in the presence of both CO2 and O3 (0.5 mg N kg−1 day−1) and (ii) gross NH4+ immobilization was also greater under elevated CO2 (0.8 mg N kg−1 day−1) than under CO2 plus O3 (0.4 mg N kg−1 day−1). We used a laboratory 15N tracer method to quantify transfer of inorganic N to organic pools. Elevated CO2 led to greater recovery of NH4+-15N in microbial biomass and corresponding lower recovery in the extractable NO3− pool. Elevated CO2 resulted in a substantial increase in NO3−-15N recovery in soil organic matter. We observed no O3 main effect and no CO2 by O3 interaction effect on 15N recovery in any soil pool. All of the above responses were most pronounced beneath Betula papyrifera and Populus tremuloides, which have grown more rapidly than Acer saccharum. Although elevated CO2 has increased plant productivity, the resulting increase in plant litter production has yet to overcome the influence of the pre-existing pool of soil organic matter on soil microbial activity and rates of N cycling. Ozone reduces plant litter inputs and also appears to affect the composition of plant litter in a way that reduces microbial biomass and activity. [ABSTRACT FROM AUTHOR]

The article presents a study which examines the extent to which net primary productivity (NPP) of plants will be sustained in soil nitrogen (N) cycling under elevated carbon dioxide (CO2). The use of common field and laboratory methods to measure microbial N, gross N mineralization, and immobilization in free-air CO2 enrichment experiments is noted. Results indicating that atmospheric CO2 concentration has no effect on any microbial N cycling pool is explained.

Increases in atmospheric CO2 and tropospheric O3 may affect forest N cycling by altering plant litter production and the availability of substrates for microbial metabolism. Three years following the establishment of our free-air CO2–O3 enrichment experiment, plant growth has been stimulated by elevated CO2 resulting in greater substrate input to soil; elevated O3 has counteracted this effect. We hypothesized that rates of soil N cycling would be enhanced by greater plant productivity under elevated CO2, and that CO2 effects would be dampened by O3. We found that elevated CO2 did not alter gross N transformation rates. Elevated O3 significantly reduced gross N mineralization and microbial biomass N, and effects were consistent among species. We also observed significant interactions between CO2 and O3: (i) gross N mineralization was greater under elevated CO2 (1.0 mg N kg−1 day−1) than in the presence of both CO2 and O3 (0.5 mg N kg−1 day−1) and (ii) gross NH4+ immobilization was also greater under elevated CO2 (0.8 mg N kg−1 day−1) than under CO2 plus O3 (0.4 mg N kg−1 day−1). We used a laboratory 15N tracer method to quantify transfer of inorganic N to organic pools. Elevated CO2 led to greater recovery of NH4+-15N in microbial biomass and corresponding lower recovery in the extractable NO3− pool. Elevated CO2 resulted in a substantial increase in NO3−-15N recovery in soil organic matter. We observed no O3 main effect and no CO2 by O3 interaction effect on 15N recovery in any soil pool. All of the above responses were most pronounced beneath Betula papyrifera and Populus tremuloides, which have grown more rapidly than Acer saccharum. Although elevated CO2 has increased plant productivity, the resulting increase in plant litter production has yet to overcome the influence of the pre-existing pool of soil organic matter on soil microbial activity and rates of N cycling. Ozone reduces plant litter inputs and also appears to affect the composition of plant litter in a way that reduces microbial biomass and activity. [ABSTRACT FROM AUTHOR]

Human activity causes increasing background concentrations of the greenhouse gases CO2 and O3. Increased levels of CO2 can be found in all terrestrial ecosystems. Damaging O3 concentrations currently occur over 29% of the world's temperate and subpolar forests but are predicted to affect fully 60% by 2100 (ref. 3). Although individual effects of CO2 and O3 on vegetation have been widely investigated, very little is known about their interaction, and long-term studies on mature trees and higher trophic levels are extremely rare. Here we present evidence from the most widely distributed North American tree species, Populus tremuloides, showing that CO2 and O3, singly and in combination, affected productivity, physical and chemical leaf defences and, because of changes in plant quality, insect and disease populations. Our data show that feedbacks to plant growth from changes induced by CO2 and O3 in plant quality and pest performance are likely. Assessments of global change effects on forest ecosystems must therefore consider the interacting effects of CO2 and O3 on plant performance, as well as the implications of increased pest activity. [ABSTRACT FROM AUTHOR]

We exposed Populus tremuloides Michx. and Acer saccharum Marsh. to a factorial combination of ambient and elevated atmospheric CO2 concentrations ([CO2]) and high-nitrogen (N) and low-N soil treatments in open-top chambers for 3 years. Our objective was to compare photosynthetic acclimation to elevated [CO2] between species of contrasting shade tolerance, and to determine if soil N or shading modify the acclimation response. Sun and shade leaf responses to elevated [CO2] and soil N were compared between upper and lower canopy leaves of P. tremuloides and between A. saccharum seedlings grown with and without shading by P. tremuloides. Both species had higher leaf N concentrations and photosynthetic rates in high-N soil than in low-N soil, and these characteristics were higher for P. tremuloides than for A. saccharum. Electron transport capacity (Jmax) and carboxylation capacity (Vcmax) generally decreased with atmospheric CO2 enrichment in all 3 years of the experiment, but there was no evidence that elevated [CO2] altered the relationship between them. On a leaf area basis, both Jmax and Vcmax acclimated to elevated [CO2] more strongly in shade leaves than in sun leaves of P. tremuloides. However, the apparent [CO2] × shade interaction was largely driven by differences in specific leaf area (m2 g−1) between sun and shade leaves. In A. saccharum, photosynthesis acclimated more strongly to elevated [CO2] in sun leaves than in shade leaves on both leaf area and mass bases. We conclude that trees rooted freely in the ground can exhibit photosynthetic acclimation to elevated [CO2], and the response may be modified by light environment. The hypothesis that photosynthesis acclimates more completely to elevated [CO2] in shade-tolerant species than in shade-intolerant species was not supported. [ABSTRACT FROM PUBLISHER]

Summary It has been hypothesized that greater production of total nonstructural carbohydrates (TNC) in foliage grown under elevated atmospheric carbon dioxide (CO2) will result in higher concentrations of defensive compounds in tree leaf litter, possibly leading to reduced rates of decomposition and nutrient cycling in forest ecosystems of the future. To evaluate the effects of elevated atmospheric CO2 on litter chemistry and decomposition, we performed a 111 day laboratory incubation with leaf litter of trembling aspen (Populus tremuloides Michaux) produced at 36 Pa and 56 Pa CO2 and two levels of soil nitrogen (N) availability. Decomposition was quantified as microbially respired CO2 and dissolved organic carbon (DOC) in soil solution, and concentrations of nonstructural carbohydrates, N, carbon (C), and condensed tannins were monitored throughout the incubation. Growth under elevated atmospheric CO2 did not significantly affect initial litter concentrations of TNC, N, or condensed tannins. Rates of decomposition, measured as both microbially respired CO2 and DOC did not differ between litter produced under ambient and elevated CO2. Total C lost from the samples was 38 mg g-1 litter as respired CO2 and 138 mg g-1 litter as DOC, suggesting short-term pulses of dissolved C in soil solution are important components of the terrestrial C cycle. We conclude that litter chemistry and decomposition in trembling aspen are minimally affected by growth under higher concentrations of CO2. [ABSTRACT FROM AUTHOR]

Physiological and biomass responses of six genotypes of Populus tremuloides Michx., grown in ambient (357 μmol mol−1) or twice ambient (707 μmol mol−1) CO2 concentration ([CO2]) and in low-N or high-N soils, were studied in 1995 and 1996 in northern Lower Michigan, USA. There was a significant CO2 × genotype interaction in photosynthetic responses. Net CO2 assimilation (A) was significantly enhanced by elevated [CO2] for five genotypes in high-N soil and for four genotypes in low-N soil. Enhancement of A by elevated [CO2] ranged from 14 to 68%. Genotypes also differed in their biomass responses to elevated [CO2], but biomass responses were poorly correlated with A responses. There was a correlation between magnitude of A enhancement by elevated [CO2] and stomatal sensitivity to CO2. Genotypes with low stomatal sensitivity to CO2 had a significantly higher A at elevated [CO2] than at ambient [CO2], but elevated [CO2] did not affect the ratio of intercellular [CO2] to leaf surface [CO2]. Stomatal conductance and A of different genotypes responded differentially to recovery from drought stress. Photosynthetic quantum yield and light compensation point were unaffected by elevated [CO2]. We conclude that P. tremuloides genotypes will respond differentially to rising atmospheric [CO2], with the degree of response dependent on other abiotic factors, such as soil N and water availability. The observed genotypic variation in growth could result in altered genotypic representation within natural populations and could affect the composition and structure of plant communities in a higher [CO2] environment in the future. [ABSTRACT FROM PUBLISHER]

It is uncertain whether elevated atmospheric CO2 will increase C storage in terrestrial ecosystems without concomitant increases in plant access to N. Elevated CO2 may alter microbial activities that regulate soil N availability by changing the amount or composition of organic substrates produced by roots. Our objective was to determine the potential for elevated CO2 to change N availability in an experimental plant-soil system by affecting the acquisition of root-derived C by soil microbes. We grew Populus tremuloides (trembling aspen) cuttings for 2 years under two levels of atmospheric CO2 (36.7 and 71.5 Pa) and at two levels of soil N (210 and 970 µg N g–1). Ambient and twice-ambient CO2 concentrations were applied using open-top chambers, and soil N availability was manipulated by mixing soils differing in organic N content. From June to October of the second growing season, we measured midday rates of soil respiration. In August, we pulse-labeled plants with 14CO2 and measured soil 14CO2 respiration and the 14C contents of plants, soils, and microorganisms after a 6-day chase period. In conjunction with the August radio-labeling and again in October, we used 15N pool dilution techniques to measure in situ rates of gross N mineralization, N immobilization by microbes, and plant N uptake. At both levels of soil N availability, elevated CO2 significantly increased whole-plant and root biomass, and marginally increased whole-plant N capital. Significant increases in soil respiration were closely linked to increases in root biomass under elevated CO2. CO2 enrichment had no significant effect on the allometric distribution of biomass or 14C among plant components, total 14C allocation belowground, or cumulative (6-day) 14CO2 soil respiration. Elevated CO2 significantly increased microbial 14C contents, indicating greater availability of microbial substrates derived from roots. The near doubling of microbial 14C contents at elevated CO2 was a relatively small quantitative change in the belowground C cycle of our experimental system, but represents an ecologically significant effect on the dynamics of microbial growth. Rates of plant N uptake during both 6-day periods in August and October were significantly greater at elevated CO2, and were closely related to fine-root biomass. Gross N mineralization was not affected by elevated CO2. Despite significantly greater rates of N immobilization under elevated CO2, standing pools of microbial N were not affected by elevated CO2, suggesting that N was cycling through microbes more rapidly. Our results contained elements of both positive and negative feedback hypotheses, and may be most relevant to young, aggrading ecosystems, where soil resources are not yet fully exploited by plant roots. If the turnover of microbial N increases, higher rates of N immobilization may not decrease N availability to plants under elevated CO2. [ABSTRACT FROM AUTHOR]

There is considerable uncertainty about how rates of soil carbon (C) and nitrogen (N) cycling will change as CO2 accumulates in the Earth's atmosphere. We summarized data from 47 published reports on soil C and N cycling under elevated CO2 in an attempt to generalize whether rates will increase, decrease, or not change. Our synthesis centres on changes in soil respiration, microbial respiration, microbial biomass, gross N mineralization, microbial immobilization and net N mineralization, because these pools and processes represent important control points for the below-ground flow of C and N. To determine whether differences in C allocation between plant life forms influence soil C and N cycling in a predictable manner, we summarized responses beneath graminoid, herbaceous and woody plants grown under ambient and elevated atmospheric CO2. The below-ground pools and processes that we summarized are characterized by a high degree of variability (coefficient of variation 80-800%), making generalizations within and between plant life forms difficult. With few exceptions, rates of soil and microbial respiration were more rapid under elevated CO2, indicating that (1) greater plant growth under elevated CO2 enhanced the amount of C entering the soil, and (2) additional substrate was being metabolized by soil microorganisms. However, microbial biomass, gross N mineralization, microbial immobilization and net N mineralization are characterized by large increases and declines under elevated CO2, contributing to a high degree of variability within and between plant life forms. From this analysis we conclude that there are insufficient data to predict how microbial activity and rates of soil C and N cycling will change as the atmospheric CO2 concentration continues to rise. We argue that current gaps in our understanding of fine-root biology limit our ability to predict the response of soil microorganisms to rising atmospheric CO2, and that understanding differences in fine-root longevity and biochemistry between plant species are necessary for developing a predictive model of soil C and N cycling under elevated CO2. [ABSTRACT FROM AUTHOR]

The article focuses on a study related to impact of soil-nitrogen availability and atmospheric carbon dioxide on root growth and morphology of Populus tremuloides. It mentions that the soil-nitrogen availability limit the growth response of forests to interact with atmospheric carbon dioxide and ecosystem carbon and nitrogen cycling. It also mentions that the relationship between total soil respiration and root biomass.

The article focuses on a study regarding gas exchange, growth efficiency and leaf nitrogen in Populus tremuloides in atmospheric CO2. Topics discussed include need of cycling of nitrogen and carbon between plants and soil microorganisms, need of soil-N (nitrogen in soil) availability in cycling, and impact of photosynthesis and canopy development in plant growth.

The article focuses on a study related to use of atmospheric CO2, soil-N (nitrogen in soil) availability and biomass in altering plant growth regulators in genotypes of Populus tremuloides. Topics discussed include use of terrestrial ecosystems for changing carbon and nitrogen cycles, increased total plant biomass high-N soil, and decline in plant tissues due to lower nitrogen concentration by CO2.

An introduction is presented which discusses various topics including growth efficiency of Populus tremuloides in carbon dioxide (CO2)-enriched atmosphere, response of soil microorganisms at interface between carbon (C) and nitrogen (N) cycle and allocation of nitrogen by Populus tremuloides.

In most natural ecosystems a significant portion of carbon fixed through photosynthesis is allocated to the production and maintenance of fine roots, the ephemeral portion of the root system that absorbs growth-limiting moisture and nutrients. In turn, senescence of fine roots can be the greatest source of C input to forest soils. Consequently, important questions in ecology entail the extent to which increasing atmospheric CO2 may alter the allocation of carbon to, and demography of, fine roots. Using microvideo and image analysis technology, we demonstrate that elevated atmospheric CO2 increases the rates of both fine root production and mortality. Rates of root mortality also increased substantially as soil nitrogen availability increased, regardless of CO2concentration. Nitrogen greatly influenced the proportional allocation of carbon to leaves vs. fine roots. The amount of available nitrogen in the soil appears to be the most important factor regulating fine root demography in Populus trees. [ABSTRACT FROM AUTHOR]

We grew cuttings of two early (mid Oct.) and two late (early Nov.) leaf-fall Populus tremuloides Michx. genotypes (referred to as genotype pairs) for c. 150 din open-top chambers to understand how twice-ambient (elevated) CO2, and soil N availability would affect growth and C allocation. For the study, we selected genotypes differing in leaf area duration to find out if late-season photosynthesis influenced C allocation to roots. Both elevated CO2 and high soil N availability significantly increased estimated whole-tree photosynthesis, but they did so in different ways. Elevated CO2 stimulated leaf-level photosynthesis rates, whereas high soil N availability resulted in greater total plant leaf area. The early leaf-fall genotype pair had significantly higher photosynthesis rates per unit leaf area than the late leaf-fall genotype pair and elevated CO2 enhanced this difference. The early leaf-fall genotype pair had less leaf area than the late leaf-fall genotype pair, and their rate of leaf area development decreased earlier in the season. Across both genotype pairs, high soil N availability significantly increased fine root length production and mortality by increasing both the amount of root length present, and by decreasing the life span of individual roots. Elevated CO2 resulted in significantly increased fine root production and mortality in high N but not low N soil and did not affect fine root life span. The early leaf-fall genotype pair had significantly greater fine root length production than the late leaf-fall genotype pair across all CO2 and N treatments. These differences in belowground C allocations are consistent with the hypothesis that belowground C and N cycling is strongly influenced by soil N availability and will increase under elevated atmospheric CO2. In addition, this study reinforces the need for better understanding of the variation in tree responses to elevated CO2, within and among species. [ABSTRACT FROM AUTHOR]

Two important processes which may limit productivity gains in forest ecosystems with atmospheric CO2 are reduction in photosynthetic capacity following prolonged exposure to high CO2 and diminution of positive growth responses when soil nutrients, particularly N, are limiting. To examine the interacting effects of soil fertility and CO2 enrichment on photosynthesis and growth in trees we grew hybrid poplar (Populus x euramericana) for 158 d in the field at ambient and twice ambient CO2 and in soil with low or high N availability. We measured the timing and rate of canopy development, and final above- and belowground dry weight. Single leaf net CO2 assimilation (A) increased at elevated CO2 over the majority of the growing season in both fertility treatments. At high fertility, the maximum size of individual leaves, total leaf number, and seasonal leaf area duration (LAD) also increased at elevated CO2, leading to a 49% increase in total dry weight. In contrast, at low fertility leaf area growth was unaffected by CO2 treatment. Total dry weight nonetheless increased 25% due to CO2 effects on A. Photosynthetic capacity (A at constant internal p(CO2), (C1)) was reduced in high CO2 plants after 100 d growth at low fertility and 135 d growth at high fertility. Analysis of A responses to changing C1 indicated that this negative adjustment of photosynthesis was due to a reduction in the maximum rate of CO2 fixation by Rubisco. Maximum rate of electron transport and phosphate regeneration capacity were either unaffected or declined at elevated CO2, Carbon dioxide effects on leaf respiration were most pronounced at high fertility, with increased respiration mid-season and no change (area basis) or reduced (mass basis) respiration late-season in elevated compared to ambient CO2 plants. This temporal variation correlated with changes in leaf N concentration and leaf mass per area. Our results demonstrate the importance of considering both structural and physiological pathways of net C gain in predicting tree responses to rising CO2 under conditions of suboptimal soil fertility. [ABSTRACT FROM AUTHOR]

Root respiration may account for as much as 60% of total soil respiration. Therefore, factors that regulate the metabolic activity of roots and associated microbes are an important component of terrestrial carbon budgets. Root systems are often sampled by diameter and depth classes to enable researchers to process samples in a systematic and timely fashion. We recently discovered that small, lateral roots at the distal end of the root system have much greater tissue N concentrations than larger roots, and this led to the hypothesis that the smallest roots have significantly higher rates of respiration than larger roots. This study was designed to determine if root respiration is related to root diameter or the location of roots in the soil profile. We examined relationships among root respiration rates and N concentration in four diameter classes from three soil depths in two sugar maple (Acer saccharum Marsh.) forests in Michigan. Root respiration declined as root diameter increased and was lower at deeper soil depths than at the soil surface. Surface roots (0–10 cm depth) respired at rates up to 40% greater than deeper roots, and respiration rates for roots < 0.5 mm in diameter were 2.4 to 3.4 times higher than those for roots in larger diameter classes. Root N concentration explained 70% of the observed variation in respiration across sites and size and depth classes. Differences in respiration among root diameter classes and soil depths appeared to be consistent with hypothesized effects of variation in root function on metabolic activity. Among roots, very fine roots in zones of high nutrient availability had the highest respiration rates. Large roots and roots from depths of low nutrient availability had low respiration rates consistent with structural and transport functions rather than with active nutrient uptake and assimilation. These results suggest that broadly defined root classes, e.g., fine roots are equivalent to all roots < 2.0 mm in diameter, do not accurately reflect the functional categories typically associated with fine roots. Tissue N concentration or N content (mass × concentration N) may be a better indicator of root function than root diameter. [ABSTRACT FROM PUBLISHER]