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During the next century, atmospheric nitrogen (N) deposition is projected to more than double, potentially slowing litter decomposition by altering microbial community composition and function. If the flow of energy though detrital food webs is diminished by the slowing of decay under higher rates of atmospheric N deposition, this agent of global change could also negatively impact the abundance and composition of soil fauna. To test this hypothesis, we studied soil faunal communities in four sugar-maple-dominated forests that comprise a long-term N deposition experiment. To examine whether changes in soil faunal communities could then feed back to influence litter decay, litterbags with 13C-enriched aspen litter were placed in the forest floor in one study site. The microbial community within the litterbags was characterized using PLFA analysis. Overall, long-term experimental N deposition' reduced the abundance of microarthropods (ambient vs. experimental N deposition: 7844 vs. 4357 individuals/m², respectively; P = 0.004). We attribute this overall decline partly to the reduced energy flow entering the detrital food web, which has been documented in previous work in our system. Although there was no difference in microarthropod species richness between N deposition treatments, there was a shift in community composition within the most abundant group (Oribatida), indicating speciesspecific responses to N deposition. Experimental N deposition reduced the number of microarthropods colonizing litterbags by 41% (P = 0.014). This was associated with a reduction in 13C mobilization from leaf litter into microbial biomass. Overall, this study demonstrates that chronic N deposition has a detrimental effect on the soil detritus food web, and that the negative effect may feed back to influence litter decay and ecosystem functioning. [ABSTRACT FROM AUTHOR]

Industrialization has greatly affected the biogeochemistry of northern forests by increasing the atmospheric deposition of acid and nitrogen (N). In 1990, the US Congress amended the Clean Air Act to include tighter emissions regulations; this reduced acid deposition (by >50% in this study), but did not effectively lower N deposition. Here, we demonstrate that since this legislation was enacted, there have been marked decreases in sulfur (-16%), calcium (-17%), and aluminum (-42%) concentrations in sugar maple (Acer saccharum) foliage across the Upper Great Lakes region of the US, signaling a declining influence of acid deposition. In contrast, N deposition has persistently been over 75% greater than the amount of N needed to offset annual plant N sequestration, creating increases in N availability and soil N leaching. Recent emissions regulations will reduce N deposition somewhat, but further increases in soil N availability and leaching are likely. Policy decisions regarding N deposition will have to weigh increased carbon storage against negative impacts on water quality and species diversity. [ABSTRACT FROM AUTHOR]

Six terrestrial ecosystems in the USA were exposed to elevated atmospheric CO2 in single or multifactorial experiments for more than a decade to assess potential impacts. We retrospectively assessed soil bacterial community responses in all six-field experiments and found ecosystem-specific and common patterns of soil bacterial community response to elevated CO2. Soil bacterial composition differed greatly across the six ecosystems. No common effect of elevated atmospheric CO2 on bacterial biomass, richness and community composition across all of the ecosystems was identified, although significant responses were detected in individual ecosystems. The most striking common trend across the sites was a decrease of up to 3.5-fold in the relative abundance of Acidobacteria Group 1 bacteria in soils exposed to elevated CO2 or other climate factors. The Acidobacteria Group 1 response observed in exploratory 16S rRNA gene clone library surveys was validated in one ecosystem by 100-fold deeper sequencing and semi-quantitative PCR assays. Collectively, the 16S rRNA gene sequencing approach revealed influences of elevated CO2 on multiple ecosystems. Although few common trends across the ecosystems were detected in the small surveys, the trends may be harbingers of more substantive changes in less abundant, more sensitive taxa that can only be detected by deeper surveys. [ABSTRACT FROM AUTHOR]

Plant growth responses to rising atmospheric CO2 and O3 vary among genotypes and between species, which could plausibly influence the strength of competitive interactions for soil N. Ascribable to the size-symmetric nature of belowground competition, we reasoned that differential growth responses to CO2 and O3 should shift as juvenile individuals mature, thereby altering competitive hierarchies and forest composition. In a 12-year-long forest FACE experiment, we used tracer 15 N and whole-plant N content to assess belowground competitive interactions among five P opulus tremuloides genotypes, between a single P . tremuloides genotype and B etula papryrifera, as well as between the same single P . tremuloides genotype and A cer saccharum. Under elevated CO2, the amount of soil N and 15 N obtained by the P . tremuloides genotype common to each community was contingent on the nature of belowground competition. When this genotype competed with its congeners, it obtained equivalent amounts of soil N and tracer 15 N under ambient and elevated CO2; however, its acquisition of soil N under elevated CO2 increased by a significant margin when grown in competition with B . papyrifera (+30%) and A . saccharum (+60%). In contrast, elevated O3 had no effect on soil N and 15 N acquisition by the P . tremuloides genotype common in each community, regardless of competitive interactions. Under elevated CO2, the rank order of N acquisition among P . tremuloides genotypes shifted over time, indicating that growth responses to CO2 change during ontogeny; this was not the case under elevated O3. In the aspen-birch community, the competitive advantage elevated CO2 initially conveyed on birch diminished over time, whereas maple was a poor competitor for soil N in all regards. The extent to which elevated CO2 and O3 will shape the genetic structure and composition of future forests is, in part, contingent on the time-dependent effects of belowground competition on plant growth response. [ABSTRACT FROM AUTHOR]

1. During the next century, atmospheric nitrogen (N) deposition is projected to more than double, potentially leading to a decline in plant diversity as well as a change in plant community composition and structure. 2. In a decade-long field experiment, simulated atmospheric N deposition has slowed litter decay, resulting in an accumulation of forest floor (i.e. Oi & Oe horizons). We reasoned that a greater forest floor mass under simulated N deposition would impose a physical barrier to sugar maple Acer saccharum seedling establishment, thereby reducing seedling populations of an ecologically and economically important tree species. 3. To test this idea, we first quantified sugar maple seedling abundance in replicate northern hardwood forest stands receiving ambient atmospheric N (7-12 kg N ha−1 year−1) and experimental atmospheric N deposition, simulating future amounts in eastern North America (ambient plus 30 kg NO3− N ha−1 year−1). Then, we experimentally manipulated forest floor mass under ambient and simulated N deposition treatments. Finally, we transplanted first-year established seedlings into areas receiving ambient and simulated N deposition and quantified their mortality after 1 year. 4. First-year seedling abundance did not differ under ambient and simulated N deposition; however, there were greater abundances of second- and third-to-fifth-year seedlings under ambient N deposition ( P < 0·001). In all cases, experimental manipulation to increase forest floor mass, equivalent to that under simulated N deposition, resulted in significantly ( P = 0·001) fewer established individuals, regardless of whether the greater forest floor mass occurred under ambient or simulated N deposition. Finally, fewer 1-year-old transplanted seedlings survived when grown under simulated N, albeit that result was not statistically significant. 5. Synthesis and applications. The slowing of decay and the accumulation of forest floor under anthropogenic N deposition can negatively impact seedling survival and potentially alter stand development and structural diversity. As atmospheric N deposition increases globally, it becomes necessary to understand the mechanisms that lead to population changes for ecologically important tree species. The responses we document should be considered in simulations of future of forest' dynamics, as atmospheric N deposition continues to increase, specifically when sugar maple life-history traits are included to simulate regeneration, structural diversity and stand development. [ABSTRACT FROM AUTHOR]

Specific root respiration rates typically increase with increasing tissue N concentration. As a result, it is often assumed that external factors inducing greater root N concentration, such as chronic N deposition, will lead to increased respiration rates. However, enhanced N availability also alters root biomass, making the ecosystem-level consequences on whole-root-system respiration uncertain. The objective of this study was to determine the effects of chronic experimental N deposition on root N concentrations, specific respiration rates, and biomass for four northern hardwood forests in Michigan. Three of the six measurement plots at each location have received experimental N deposition (3 g [ABSTRACT FROM AUTHOR]

Ecology Letters (2011) 14: 1220-1226 Abstract The accumulation of anthropogenic CO2 in the Earth's atmosphere, and hence the rate of climate warming, is sensitive to stimulation of plant growth by higher concentrations of atmospheric CO2. Here, we synthesise data from a field experiment in which three developing northern forest communities have been exposed to factorial combinations of elevated CO2 and O3. Enhanced net primary productivity (NPP) ( c. 26% increase) under elevated CO2 was sustained by greater root exploration of soil for growth-limiting N, as well as more rapid rates of litter decomposition and microbial N release during decay. Despite initial declines in forest productivity under elevated O3, compensatory growth of O3-tolerant individuals resulted in equivalent NPP under ambient and elevated O3. After a decade, NPP has remained enhanced under elevated CO2 and has recovered under elevated O3 by mechanisms that remain un-calibrated or not considered in coupled climate-biogeochemical models simulating interactions between the global C cycle and climate warming. [ABSTRACT FROM AUTHOR]

Free-air CO2 enrichment (FACE) experiments have provided novel insights into the ecological mechanisms controlling the cycling and storage of carbon in terrestrial ecosystems and contribute to our ability to project how ecosystems respond to increasing CO2 in the Earth's atmosphere. Important lessons emerge by evaluating a set of hypotheses that initially guided the design and longevity of forested FACE experiments. Net primary productivity is increased by elevated CO2, but the response can diminish over time. Carbon accumulation is driven by the distribution of carbon among plant and soil components with differing turnover rates and by interactions between the carbon and nitrogen cycles. Plant community structure may change, but elevated CO2 has only minor effects on microbial community structure. FACE results provide a strong foundation for next-generation experiments in unexplored ecosystems and inform coupled climate-biogeochemical models of the ecological mechanisms controlling ecosystem response to the rising atmospheric CO2 concentration. [ABSTRACT FROM AUTHOR]

Ecology Letters (2011) 14: 1220-1226 Abstract The accumulation of anthropogenic CO2 in the Earth's atmosphere, and hence the rate of climate warming, is sensitive to stimulation of plant growth by higher concentrations of atmospheric CO2. Here, we synthesise data from a field experiment in which three developing northern forest communities have been exposed to factorial combinations of elevated CO2 and O3. Enhanced net primary productivity (NPP) ( c. 26% increase) under elevated CO2 was sustained by greater root exploration of soil for growth-limiting N, as well as more rapid rates of litter decomposition and microbial N release during decay. Despite initial declines in forest productivity under elevated O3, compensatory growth of O3-tolerant individuals resulted in equivalent NPP under ambient and elevated O3. After a decade, NPP has remained enhanced under elevated CO2 and has recovered under elevated O3 by mechanisms that remain un-calibrated or not considered in coupled climate-biogeochemical models simulating interactions between the global C cycle and climate warming. [ABSTRACT FROM AUTHOR]

Summary Elevated atmospheric CO2 generally increases plant productivity and subsequently increases the availability of cellulose in soil to microbial decomposers. As key cellulose degraders, soil fungi are likely to be one of the most impacted and responsive microbial groups to elevated atmospheric CO2. To investigate the impacts of ecosystem type and elevated atmospheric CO2 on cellulolytic fungal communities, we sequenced 10 677 cbhI gene fragments encoding the catalytic subunit of cellobiohydrolase I, across five distinct terrestrial ecosystem experiments after a decade of exposure to elevated CO2. The cbhI composition of each ecosystem was distinct, as supported by weighted Unifrac analyses (all P-values; < 0.001), with few operational taxonomic units (OTUs) being shared across ecosystems. Using a 114-member cbhI sequence database compiled from known fungi, less than 1% of the environmental sequences could be classified at the family level indicating that cellulolytic fungi in situ are likely dominated by novel fungi or known fungi that are not yet recognized as cellulose degraders. Shifts in fungal cbhI composition and richness that were correlated with elevated CO2 exposure varied across the ecosystems. In aspen plantation and desert creosote bush soils, cbhI gene richness was significantly higher after exposure to elevated CO2 (550 µmol mol−1) than under ambient CO2 (360 µmol mol−1 CO2). In contrast, while the richness was not altered, the relative abundance of dominant OTUs in desert soil crusts was significantly shifted. This suggests that responses are complex, vary across different ecosystems and, in at least one case, are OTU-specific. Collectively, our results document the complexity of cellulolytic fungal communities in multiple terrestrial ecosystems and the variability of their responses to long-term exposure to elevated atmospheric CO2. [ABSTRACT FROM AUTHOR]

Abstract: The impact of rising atmospheric carbon dioxide (CO2) may be mitigated, in part, by enhanced rates of net primary production and greater C storage in plant biomass and soil organic matter (SOM). However, C sequestration in forest soils may be offset by other environmental changes such as increasing tropospheric ozone (O3) or vary based on species-specific growth responses to elevated CO2. To understand how projected increases in atmospheric CO2 and O3 alter SOM formation, we used physical fractionation to characterize soil C and N at the Rhinelander Free Air CO2–O3 Enrichment (FACE) experiment. Tracer amounts of 15NH4 + were applied to the forest floor of Populus tremuloides, P. tremuloides–Betula papyrifera and P. tremuloides–Acer saccharum communities exposed to factorial CO2 and O3 treatments. The 15N tracer and strongly depleted 13C–CO2 were traced into SOM fractions over four years. Over time, C and N increased in coarse particulate organic matter (cPOM) and decreased in mineral-associated organic matter (MAOM) under elevated CO2 relative to ambient CO2. As main effects, neither CO2 nor O3 significantly altered 15N recovery in SOM. Elevated CO2 significantly increased new C in all SOM fractions, and significantly decreased old C in fine POM (fPOM) and MAOM over the duration of our study. Overall, our observations indicate that elevated CO2 has altered SOM cycling at this site to favor C and N accumulation in less stable pools, with more rapid turnover. Elevated O3 had the opposite effect, significantly reducing cPOM N by 15% and significantly increasing the C:N ratio by 7%. Our results demonstrate that CO2 can enhance SOM turnover, potentially limiting long-term C sequestration in terrestrial ecosystems; plant community composition is an important determinant of the magnitude of this response. [Copyright &y& Elsevier]

In order to better understand the nitrogen (N) cycle, a pulse of `5N03 was applied in 1998 to a sugar maple (Acer saccharum) dominated northern hardwood forest receiving long-term (1994-2008) simulated atmospheric N deposition. Sugar maple leaf litter and live fine-root `5N were quantified for four years prior to labeling and for 11 subsequent years. Continuous sampling of `5N following addition of the tracer enabled calculation of leaf litter and fine-root N pool turnover utilizing an exponential decay function. Fine-root `5N recovery peaked at 3.7% ± 1.7% the year the tracer was applied, while leaf litter `5N recovery peaked in the two years following tracer application at -∼8%. These results suggest shoots are primarily constructed from N taken up in previous years, while fine roots are constructed from new N. The residence time of N was 6.5 years in leaf litter and 3.1 years in fine roots. The longer residence time and higher recovery rate are evidence that leaves were a stronger sink for labeled N than fine roots, but the relatively short residence time of tracer N in both pools suggests that there is not tight intra-ecosystem cycling of N in this mature forest. [ABSTRACT FROM AUTHOR]

Bacteria and fungi drive the cycling of plant litter in forests, but little is known about their role in tropical rain forest nutrient cycling, despite the high rates of litter decay observed in these ecosystems. However, litter decay rates are not uniform across tropical rain forests. For example, decomposition can differ dramatically over small spatial scales between low-diversity, monodominant rain forests, and species-rich, mixed forests. Because the climatic patterns and soil parent material are identical in co-occurring mixed and monodominant forests, differences in forest floor accumulation, litter production, and decomposition between these forests may be biotically mediated. To test this hypothesis, we conducted field and laboratory studies in a monodominant rain forest in which the ectomycorrhizal tree Dicymbe corymbosa forms >80% of the canopy, and a diverse, mixed forest dominated by arbuscular mycorrhizal trees. After 2 years, decomposition was significantly slower in the monodominant forest ( P < 0.001), but litter production was significantly greater in the mixed forest ( P < 0.001). In the laboratory, we found microbial community biomass was greater in the mixed forest ( P = 0.02), and the composition of fungal communities was distinct between the two rain forest types ( P = 0.001). Sequencing of fungal rDNA revealed a significantly lower richness of saprotrophic fungi in the monodominant forest (19 species) relative to the species-rich forest (84 species); moreover, only 4% percent of fungal sequences occurred in both forests. These results show that nutrient cycling patterns in tropical forests can vary dramatically over small spatial scales, and that changes in microbial community structure likely drive the observed differences in decomposition. [ABSTRACT FROM AUTHOR]

Anthropogenic N deposition can slow the decay of plant detritus, leading to an accumulation of soil organic matter and the production of phenolic dissolved organic C (DOG), which can leach from soil to ground and surface waters. Actinobacteria are one of the few groups of saprotrophic microorganisms that oxidatively depolymerize lignin, producing substantial soluble polyphenolics in the process. In combination, these observations present the possibility that lignolytic Actinobacteria may become more important agents of lignin decay as atmospheric N deposition continues to increase during the next decade. To test this idea, we quantified actinobacterial abundance and community composition in a well-replicated field study in which atmospheric N deposition has been experimentally increased since 1994. Actinobacterial abundance was assessed using quantitative polymerase chain reaction of I 6S rRNA and community composition was evaluated using clone libraries and phylogenetic community analyses (i.e., LIBSHUFF and UniFrac). Contrary to our expectation, experimental atmospheric N deposition had no effect on actinobacterial abundance in the forest floor (∼1010 gene copies kg-1); however, it significantly decreased actinobacterial abundance by 47% and total DNA by 31% in surface soil. Our analyses revealed that experimental N deposition further elicited a significant membership change in forest floor and surface soil communities, as well as significant differences in the phylogenetic diversity of forest floor Actinobacteria. This shift in community composition occurred in concert with a slowing of plant litter decay, accumulation of soil organic matter, and a greater production of phenolic DOG. These observations are consistent with the idea that changes in actinobacterial community composition may underlie biogeochemical responses to experimental N deposition. [ABSTRACT FROM AUTHOR]

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]