Analysis of the Effect of an Open Hole on the Buckling of a Compressed Composite Plate

This paper investigates the effect of an open hole on the stability of a compressed laminated composite plate. The study was carried out in two ways: using experimental tests and numerical analysis. As a result of the experiment, the buckling form and path of the plate were recorded. The form of buckling was determined using the ARAMIS non-contact measurement system. The critical load value was determined from the working path using the approximation method. The experimental results were verified by numerical analysis based on the finite element method. FEM investigations were carried out in terms of a linear eigenproblem analysis. This allowed the bifurcation load and the corresponding buckling form of the numerical model of the plate to be determined. Investigating the effect of the hole in the compressed plate at a critical state showed high agreement between the proposed test methods. No clear effect of the hole size on the buckling of the plate was observed. In contrast, a clear effect of the hole on the critical load value was determined. The maximum decrease in the critical load value was 14%. The same decrease was observed for the stiffness of the post-critical characteristics. It was shown that the [45|-45|90|0]s composite plate had more than three times lower strength compared to [0|-45|45|90]s and [0|90|0|90]s. The novelty of this article is the development of a research methodology based on new interdisciplinary research methods for describing the influence of the central hole on the stability of compressed composite plates. The ABAQUS system was used for the numerical analysis.

[Assessment of Critical Loads of Nitrogen Deposition in Natural Ecosystems of China]

Excessive nitrogen (N) deposition causes a series of environmental problems, including biodiversity loss. Therefore, assessing current N deposition thresholds of natural ecosystems is critical for regional N management and pollution control. In this study, the critical loads of N deposition in mainland China were estimated using the steady-state mass balance method, and the spatial distribution of ecosystems that exceeded the critical load was evaluated. The results showed that areas with critical loads of N deposition higher than 56, in the range of 14-56, and lower than 14 kg·(hm²·a)^-1 accounted for 6%, 67%, and 27% of that in China, respectively. The areas with higher critical loads of N deposition were mainly distributed in the eastern Tibetan Plateau, northeastern Inner Mongolia, and parts of south China. Lower critical loads of N deposition were mainly distributed in the western Tibetan Plateau, northwest China, and parts of southeast China. Moreover, the areas where N deposition exceeded the critical loads accounted for 21% of that in mainland China, being mainly distributed in southeast and northeast China. The exceedances of critical loads of N deposition in northeast China, northwest China, and the Qinghai-Tibet Plateau were generally lower than 14 kg·(hm²·a)^-1. Therefore, the management and control of N in these areas that exceeded the critical load of deposition is more worthy of future attention.

Tuning the Surface Characteristics and Mechanical Properties of Y2O3 Coatings on a Graphene Matrix via Laser Micro Melting

The effects of laser parameters on the microstructure and properties of plasma-sprayed yttrium oxide coating on the graphite matrix were investigated. Tensile strength, porosity, roughness, and scratch meter tests were carried out to evaluate the critical load and mechanical properties of the coating after spraying and laser micro-melting. When the porosity and surface roughness of the coating are minimum, the critical load of the coating is 7.85 N higher than that of the spraying surface. After laser micromelting, the crystal phase of Y₂O₃ coating surface does not change, the crystallinity is improved, and fine grain strengthening occurs. When the laser power density is 75 W/mm², the scanning speed is 30 mm/s, and the defocusing distance is 40 mm, the film base bonding performance and wear resistance of the material reach the maximum value. The failure of Y₂O₃ coating is mainly due to the degradation of mechanical properties such as film base bonding strength, surface porosity, and surface roughness, which leads to the local collapse of the material. The coating after laser micro-melting only presents particle disintegration at the end of the scratch area.

Modeled Vegetation Community Trajectories: Effects from Climate Change, Atmospheric Nitrogen Deposition, and Soil Acidification Recovery

Forest understory plant communities in the United States harbor most of the vegetation diversity of forests and are often sensitive to changes in climate and atmospheric deposition of nitrogen (N). As temperature increases from human-caused climate change and soils recover from long term atmospheric deposition of N and sulfur (S), it is unclear how these important ecosystem components will respond. We used the newly developed US-PROPS model - based on species response functions for over 1,500 species - to evaluate the potential impacts of atmospheric N deposition and climate change on species occurrence probability for a case study in the forested ecosystems of the Great Smoky Mountains National Park (GRSM), an iconic park in the southeastern United States. We evaluated six future scenarios from various combinations of two potential recoveries of soil pH (no change, +0.5 pH units) and three climate futures (no change, +1.5, +3.0 deg C). Species critical loads (CLs) of N deposition and projected responses for each scenario were determined. Critical loads were estimated to be low (< 2 kg N/ha/yr) to protect all species under current and expected future conditions across broad regions of GRSM and these CLs were exceeded at large spatial extents among scenarios. Northern hardwood, yellow pine, and chestnut oak forests were among the most N-sensitive vegetation map classes found within GRSM. Potential future air temperature conditions generally led to decreases in the maximum occurrence probability for species. Therefore, CLs were considered "unattainable" in these situations because the specified level of protection used for CL determination (i.e., maximum occurrence probability under ambient conditions) was not attainable. Although some species showed decreases in maximum occurrence probability with simulated increases in soil pH, most species were favored by increased pH. The importance of our study is rooted in the methodology described here for establishing regional CLs and for evaluating future conditions, which is transferable to other national parks in the U.S. and in Europe where the original PROPS model was developed.

Modeling the Relationship between Load and Repetitions to Failure in Resistance Training: A Bayesian Analysis

PURPOSE

To identify the relationship between load and the number of repetitions performed to momentary failure in the pin press exercise, the present study compared different statistical model types and structures using a Bayesian approach.

METHODS

Thirty resistance-trained men and women were tested on two separate occasions. During the first visit, participants underwent assessment of their one-repetition maximum (1-RM) in the pin press exercise. On the second visit, they performed sets to momentary failure at 90%, 80% and 70% of their 1-RM in a fixed order during a single session. The relationship between relative load and repetitions performed to failure was fitted using linear regression, exponential regression and the critical load model. Each model was fitted according to the Bayesian framework in two ways: using an across-subjects pooled data structure and using a multilevel structure. Models were compared based on the variance explained (R²) and leave-one-out cross-validation information criterion (LOOIC).

RESULTS

Multilevel models, which incorporate higher-level commonalities into individual relationships, demonstrated a substantially better fit (R²: 0.97-0.98) and better predictive accuracy compared to generalized pooled-data models (R²: 0.89-0.93). The multilevel 2-parameter exponential regression emerged as the best representation of data in terms of model fit, predictive accuracy and model simplicity.

CONCLUSION

The relationship between load and repetitions performed to failure follows an individually expressed exponential trend in the pin press exercise. To accurately predict the load that is associated with a certain repetition maximum, the relationship should therefore be modeled on a subject-specific level.

Optimal Design of Double-Walled Corrugated Board Packaging

Designing corrugated board packaging is a real challenge, especially when the packaging material comes from multiple recycling. Recycling itself is a pro-ecological and absolutely necessary process, but the mechanical properties of materials that are processed many times deteriorate with the number of cycles. Manufacturers are trying to use unprecedented design methods to preserve the load-bearing capacity of packaging, even when the material itself is of deteriorating quality. An additional obstacle in the process of designing the structure of paper packaging is the progressive systematic reduction of the grammage (the so-called lightweight process) of corrugated cardboard. Therefore, this research presents a critical look at the process of optimal selection of corrugated cardboard for packaging structures, depending on the paper used. The study utilizes analytical, simplified formulas to estimate the strength of cardboard itself as well as the strength of packaging, which are then analyzed to determine their sensitivity to changes in cardboard components, such as the types of paper of individual layers. In the performed sensitivity analysis, numerical homogenization was used, and the influence of initial imperfections on the packaging mechanics was determined. The paper presents a simple algorithm for the optimal selection of the composition of corrugated cardboard depending on the material used and the geometry of the packaging, which allows for a more conscious production of corrugated cardboard from materials derived, e.g., from multiple recycling.

Non-Local Sensitivity Analysis and Numerical Homogenization in Optimal Design of Single-Wall Corrugated Board Packaging

The optimal selection of the composition of corrugated cardboard dedicated to specific packaging structures is not an easy task. The use of lighter boards saves material, but at the same time increases the risk of not meeting the guaranteed load capacity. Therefore, the answer to the question “in which layer the basis weight of the paper should be increased?” is not simple or obvious. The method proposed here makes it easy to understand which components and to what extent they affect the load-bearing capacity of packages of various dimensions. The use of numerical homogenization allows for a quick transformation of a cardboard sample, i.e., a representative volume element (RVE) into a flat plate structure with effective parameters describing the membrane and bending stiffness. On the other hand, the use of non-local sensitivity analysis makes it possible to find the relationship between the parameters of the paper and the load capacity of the packaging. The analytical procedures presented in our previous studies were used here to determine (1) the edge crush resistance, (2) critical load, and (3) the load capacity of corrugated cardboard packaging. The method proposed here allows for obtaining a comprehensive and hierarchical list of the parameters that play the most important role in the process of optimal packaging design.

Application of Variational Method to Stability Analysis of Cantilever Vertical Plates with Bimodular Effect

In the design of cantilevered balconies of buildings, many stability problems exist concerning vertical plates, in which reaching a critical load plays an important role during the stability analysis of the plate. At the same time, the concrete forming vertical plate, as a typical brittle material, has larger compressive strength but lower tensile strength, which means the tensile and compression properties of concrete are different. However, due to the complexities of such analyses, this difference has not been considered. In this study, the variational method is used to analyze stability problems of cantilever vertical plates with bimodular effect, in which different loading conditions and plate shapes are also taken into account. For the effective implementation of a variational method, the bending strain energy based on bimodular theory is established first, and critical loads of four stability problems are obtained. The results indicate that the bimodular effect, as well as different loading types and plate shapes, have influences on the final critical loads, resulting in varying degrees of buckling. In particular, if the average value of the tensile modulus and compressive modulus remain unchanged, the introduction of the bimodular effect will weaken, to some extent, the bending stiffness of the plate. Among the four stability problems, a rectangular plate with its top and bottom loaded is most likely to buckle; next is a rectangular plate with its top loaded, followed by a triangular plate with its bottom loaded. A rectangular plate with its bottom loaded is least likely to buckle. This work may serve as a theoretical reference for the refined analysis of vertical plates. Plates are made of concrete or similar material whose bimodular effect is relatively obvious and cannot be ignored arbitrarily; otherwise the greater inaccuracies will be encountered in building designs.

Laboratory Study on the Stability of Large-Size Graded Crushed Stone under Cyclic Rotating Axial Compression

In this paper, the stability of large-size graded crushed stone used for road base or cushioning under repeated load is investigated. Using an in-house developed device, large-size crushed stone mix was compacted and molded by the vibration and rotary compaction method. Cyclic rotating axial compression was applied, and the shakedown theory was used to study the cumulative deformation of the large-size crushed stone specimens. The effects of gradation parameters on the cumulative strain and stability behavior were analyzed, and the critical stability and failure loads were determined according to the shakedown theory. The test results indicate that there are three obvious instability behavior stages of large-size graded crushed stone under cyclic rotating axial compression: elastic stability, plastic creep, and incremental plastic failure. Large-size graded crushed stone has a higher critical stability load stiffness than conventional-size graded crushed stone. The critical shakedown load of the specimen is mainly affected by the skeleton structure performance, and the critical failure load by the properties of the crushed stone material. Increasing the content and compactness of large-size crushed stone in the specimen can improve the stiffness and stability performance, and to achieve improvements, the content of large-size crushed stone should be controlled between 22% and 26%. The critical shakedown load increases with the increase in the California bearing ratio (CBR) value, while, on the other hand, the CBR value has little relationship with the critical failure load.

Experimental Optical Testing and Numerical Verification by CuFSM of Compression Columns with Modified Channel Sections

Thin-walled channel columns with non-standard cross-section shapes loaded with gradually increasing compressive force applied at the geometric centre of gravity of the cross-section were the subject of the investigations presented in this paper. The aim of the research was to determine which of the columns has the most favourable geometrical characteristics in terms of the applied load. The main investigation was an experimental study carried out using two methods: strain gauging and the optical method. Based on strain gauging, the critical forces were determined using the strain averaging method and the linear regression tangent to compression plot method. In addition, modern optical tests were performed using the ARAMIS system. The buckling forces at which the first signs of buckling appear and the buckling modes of columns were determined. The results obtained from the experimental tests were used to validate the results of numerical tests carried out using the Finite Strip Method (CuFSM). Based on this method, the values of critical forces and the percentage contribution of individual buckling forms to the loss of stability of the compressed columns were determined.

Applications of the Critical Power Model to Dynamic Constant External Resistance Exercise: A Brief Review of the Critical Load Test

The study and application of the critical power (CP) concept has spanned many decades. The CP test provides estimates of two distinct parameters, CP and W', that describe aerobic and anaerobic metabolic capacities, respectively. Various mathematical models have been used to estimate the CP and W' parameters across exercise modalities. Recently, the CP model has been applied to dynamic constant external resistance (DCER) exercises. The same hyperbolic relationship that has been established across various continuous, whole-body, dynamic movements has also been demonstrated for upper-, lower-, and whole-body DCER exercises. The asymptote of the load versus repetition relationship is defined as the critical load (CL) and the curvature constant is L'. The CL and L' can be estimated from the same linear and non-linear mathematical models used to derive the CP. The aims of this review are to (1) provide an overview of the CP concept across continuous, dynamic exercise modalities; (2) describe the recent applications of the model to DCER exercise; (3) demonstrate how the mathematical modeling of DCER exercise can be applied to further our understanding of fatigue and individual performance capabilities; and (4) make initial recommendations regarding the methodology for estimating the parameters of the CL test.

Calculation of Honeycomb Paperboard Resistance to Edge Crush Test

The article presents the method of calculating the edge crush test (ECT) of honeycomb paperboard. Calculations were made on the basis of mechanical properties of paper raw materials used for the production of cellular paperboard and geometrical parameters describing cellular paperboard. The presented method allows ECT calculation of honeycomb paperboard in the main directions in the paperboard plane; i.e., machine direction (MD) and cross direction (CD). The proposed method was verified by comparing the results of calculations with the results of ECT measurements of paperboard with different geometrical parameters made of different fibrous materials.

Experimentally derived nitrogen critical loads for northern Great Plains vegetation

The critical load concept facilitates communication between scientists and policy makers and land managers by translating the complex effects of air pollution on ecosystems into unambiguous numbers that can be used to inform air quality targets. Anthropogenic atmospheric nitrogen (N) deposition adversely affects a variety of ecosystems, but the information used to derive critical loads for North American ecosystems is sparse and often based on experiments investigating N loads substantially higher than current or expected atmospheric deposition. In a 4-yr field experiment in the northern Great Plains (NGP) of North America, where current N deposition levels range from ~3 to 9 kg N·ha^-1 ·yr^-1 , we added 12 levels of N, from 2.5 to 100 kg N·ha^-1 ·yr^-1 , to three sites spanning a range of soil fertility and productivity. Our results suggest a conservative critical load of 4-6 kg N·ha^-1 ·yr^-1 for the most sensitive vegetation type we investigated, badlands sparse vegetation, a community that supports plant species adapted to low fertility conditions, where N addition at this rate increased productivity and litter load. In contrast, for the two more productive vegetation types characteristic of most NGP grasslands, a critical load of 6-10 kg N·ha^-1 ·yr^-1 was identified. Here, N addition at this level altered plant tissue chemistry and increased nonnative species. These critical loads are below the currently suggested range of 10-25 kg N·ha^-1 ·yr^-1 for NGP vegetation and within the range of current or near-future deposition, suggesting that N deposition may already be inducing fundamental changes in NGP ecosystems.

Assessing Ecological Risks from Atmospheric Deposition of Nitrogen and Sulfur to US Forests Using Epiphytic Macrolichens

Critical loads of atmospheric deposition help decision-makers identify levels of air pollution harmful to ecosystem components. But when critical loads are exceeded, how can the accompanying ecological risk be quantified? We use a 90% quantile regression to model relationships between nitrogen and sulfur deposition and epiphytic macrolichens, focusing on responses of concern to managers of US forests: Species richness and abundance and diversity of functional groups with integral ecological roles. Analyses utilized national-scale lichen survey data, sensitivity ratings, and modeled deposition and climate data. We propose 20, 50, and 80% declines in these responses as cut-offs for low, moderate, and high ecological risk from deposition. Critical loads (low risk cut-off) for total species richness, sensitive species richness, forage lichen abundance and cyanolichen abundance, respectively, were 3.5, 3.1, 1.9, and 1.3 kg N and 6.0, 2.5, 2.6, and 2.3 kg S ha⁻¹ yr⁻¹. High environmental risk (80% decline), excluding total species richness, occurred at 14.8, 10.4, and 6.6 kg N and 14.1, 13, and 11 kg S ha⁻¹ yr⁻¹. These risks were further characterized in relation to geography, species of conservation concern, number of species affected, recovery timeframes, climate, and effects on interdependent biota, nutrient cycling, and ecosystem services.

A Nanomechanical Analysis of Deformation Characteristics of 6H-SiC Using an Indenter and Abrasives in Different Fixed Methods

The super-precise theory for machining single crystal SiC substrates with abrasives needs to be improved for its chemical stability, extremely hard and brittle. A Berkovich indenter was used to carry out a systematic static stiffness indentation experiments on single crystal 6H-SiC substrates, and then these substrates were machined by utilizing fixed, free, and semi-fixed abrasives, and the nanomechanical characteristics and material removal mechanisms using abrasives in different fixed methods were analyzed theoretically. The results indicated that the hardness of C faces and Si faces of single crystal 6H-SiC under 500 mN load were 38.596 Gpa and 36.246 Gpa respectively, and their elastic moduli were 563.019 Gpa and 524.839 Gpa, respectively. Moreover, the theoretical critical loads for the plastic transition and brittle fracture of C face of single crystal 6H-SiC were 1.941 mN and 366.8 mN, while those of Si face were 1.77 mN and 488.67 mN, respectively. The 6H-SiC materials were removed by pure brittle rolling under three-body friction with free abrasives, and the process parameters determined the material removal modes of 6H-SiC substrates by grinding with fixed abrasives, nevertheless, the materials were removed under full elastic-plastic deformation in cluster magnetorheological finishing with semi-fixed abrasives.

Bryophytes and Organic layers Control Uptake of Airborne Nitrogen in Low-N Environments

The effects of atmospheric nitrogen (N) deposition on ecosystem functioning largely depend on the retention of N in different ecosystem compartments, but accumulation and partitioning processes have rarely been quantified in long-term field experiments. In the present study we analysed for the first time decadal-scale flows and allocation patterns of N in a heathland ecosystem that has been subject to airborne N inputs over decades. Using a long-term 15N tracer experiment, we quantified N retention and flows to and between ecosystem compartments (above-ground/below-ground vascular biomass, moss layer, soil horizons, leachate). After 9 years, about 60% of the added 15N-tracer remained in the N cycle of the ecosystem. The moss layer proved to be a crucial link between incoming N and its allocation to different ecosystem compartments (in terms of a short-term capture, but long-term release function). However, about 50% of the 15N captured and released by the moss layer was not compensated for by a corresponding increase in recovery rates in any other compartment, probably due to denitrification losses from the moss layer in the case of water saturation after rain events. The O-horizon proved to be the most important long-term sink for added 15N, as reflected by an increase in recovery rates from 18 to 40% within 8 years. Less than 2.1% of 15N were recovered in the podzol-B-horizon, suggesting that only negligible amounts of N were withdrawn from the N cycle of the ecosystem. Moreover, 15N recovery was low in the dwarf shrub above-ground biomass (<3.9% after 9 years) and in the leachate (about 0.03% within 1 year), indicating still conservative N cycles of the ecosystem, even after decades of N inputs beyond critical load thresholds. The continuous accumulation of reactive forms of airborne N suggests that critical load-estimates need to account for cumulative effects of N additions into ecosystems.

Terrestrial acidification and ecosystem services: effects of acid rain on bunnies, baseball, and Christmas trees

Often termed “acid rain,” combined nitrogen and sulfur deposition can directly and indirectly impact the condition and health of forest ecosystems. Researchers use critical loads (CLs) to describe response thresholds, and recent studies on acid-sensitive biological indicators show that forests continue to be at risk from terrestrial acidification. However, rarely are impacts translated into changes in “ecosystem services” that impact human well-being. Further, the relevance of this research to the general public is seldom communicated in terms that can motivate action to protect valuable resources. To understand how changes in biological indicators affect human well-being, we used the STEPS (Stressor–Ecological Production function–final ecosystem Services) Framework to quantitatively and qualitatively link CL exceedances to ecosystem service impacts. We specified the cause-and-effect ecological processes linking changes in biological indicators to final ecosystem services. The Final Ecosystem Goods and Services Classification System (FEGS-CS) was used within the STEPS Framework to classify the ecosystem component and the beneficiary class that uses or values the component. We analyzed two acid-sensitive tree species, balsam fir (Abies balsamea) and white ash (Fraxinus americana), that are common in northeastern USA. These well-known species provide habitat for animals and popular forest products that are relatable to a broad audience. We identified 160 chains with 10 classes of human beneficiaries for balsam fir and white ash combined, concluding that there are resources at risk that the public may value. Two stories resulting from these explorations into the cascading effects of acid rain on terrestrial resources are ideal for effective science communication: the relationship between (1) balsam fir as a popular Christmas tree and habitat for the snowshoe hare, a favorite of wildlife viewers, and (2) white ash because it is used for half of all baseball bats, fine wood products, and musical instruments. Thus, rather than focusing on biological indicators that may only be understood or appreciated by specific stakeholders or experts, this approach extends the analysis to include impacts on FEGS and humans. It also lays the foundation for developing stakeholder-specific narratives, quantitative measures of endpoints, and for conducting demand-based valuations of affected ecosystem services.

Effects of grass species and grass growth on atmospheric nitrogen deposition to a bog ecosystem surrounded by intensive agricultural land use

We applied a (15)N dilution technique called "Integrated Total Nitrogen Input" (ITNI) to quantify annual atmospheric N input into a peatland surrounded by intensive agricultural practices over a 2-year period. Grass species and grass growth effects on atmospheric N deposition were investigated using Lolium multiflorum and Eriophorum vaginatum and different levels of added N resulting in increased biomass production. Plant biomass production was positively correlated with atmospheric N uptake (up to 102.7 mg N pot(-1)) when using Lolium multiflorum. In contrast, atmospheric N deposition to Eriophorum vaginatum did not show a clear dependency to produced biomass and ranged from 81.9 to 138.2 mg N pot(-1). Both species revealed a relationship between atmospheric N input and total biomass N contents. Airborne N deposition varied from about 24 to 55 kg N ha(-1) yr(-1). Partitioning of airborne N within the monitor system differed such that most of the deposited N was found in roots of Eriophorum vaginatum while the highest share was allocated in aboveground biomass of Lolium multiflorum. Compared to other approaches determining atmospheric N deposition, ITNI showed highest airborne N input and an up to fivefold exceedance of the ecosystem-specific critical load of 5-10 kg N ha(-1) yr(-1).

Slow recovery of High Arctic heath communities from nitrogen enrichment

Arctic ecosystems are strongly nutrient limited and exhibit dramatic responses to nitrogen (N) enrichment, the reversibility of which is unknown. This study uniquely assesses the potential for tundra heath to recover from N deposition and the influence of phosphorus (P) availability on recovery. We revisited an experiment in Svalbard, established in 1991, in which N was applied at rates representing atmospheric N deposition in Europe (10 and 50 kg N ha(-1)  yr(-1) ; 'low' and 'high', respectively) for 3-8 yr. We investigated whether significant effects on vegetation composition and ecosystem nutrient status persisted up to 18 yr post-treatment. Although the tundra heath is no longer N saturated, N treatment effects persist and are strongly P-dependent. Vegetation was more resilient to N where no P was added, although shrub cover is still reduced in low-N plots. Where P was also added (5 kg P ha(-1)  yr(-1) ), there are still effects of low N on community composition and nutrient dynamics. High N, with and without P, has many lasting impacts. Importantly, N + P has caused dramatically increased moss abundance, which influences nutrient dynamics. Our key finding is that Arctic ecosystems are slow to recover from even small N inputs, particularly where P is not limiting.

Forest floor vegetation response to nitrogen deposition in Europe

Chronic nitrogen (N) deposition is a threat to biodiversity that results from the eutrophication of ecosystems. We studied long-term monitoring data from 28 forest sites with a total of 1,335 permanent forest floor vegetation plots from northern Fennoscandia to southern Italy to analyse temporal trends in vascular plant species cover and diversity. We found that the cover of plant species which prefer nutrient-poor soils (oligotrophic species) decreased the more the measured N deposition exceeded the empirical critical load (CL) for eutrophication effects (P = 0.002). Although species preferring nutrient-rich sites (eutrophic species) did not experience a significantly increase in cover (P = 0.440), in comparison to oligotrophic species they had a marginally higher proportion among new occurring species (P = 0.091). The observed gradual replacement of oligotrophic species by eutrophic species as a response to N deposition seems to be a general pattern, as it was consistent on the European scale. Contrary to species cover changes, neither the decrease in species richness nor of homogeneity correlated with nitrogen CL exceedance (ExCLemp N). We assume that the lack of diversity changes resulted from the restricted time period of our observations. Although existing habitat-specific empirical CL still hold some uncertainty, we exemplify that they are useful indicators for the sensitivity of forest floor vegetation to N deposition.

Evidence for acidification-driven ecosystem collapse of Danish Erica tetralix wet heathland

We report observations of disappearance of Erica tetralix in wet heathland, which is unlikely to be caused by competition, as E. tetralix is dying before its place is taken up by other species. To investigate the causes, we used both old and new data. Results showed that presence of Molinia caerulea and Calluna vulgaris were substantial in the former E. tetralix dominated areas. Measurements of the C/N ratio in the morlayer were between 21 and 26 under the E. tetralix stands. As the expected C/N ratio in a healthy nutrient poor ecosystem like the E. tetralix wet heathland is around 30, this indicates that the ratio is probably decreasing and, correspondingly, the probability of nitrogen leaching from the ecosystem is increasing. The morlayer pH was extremely low-between 3.03 and 3.78. This represents a pH decline since the 1960s, where pH values generally were above 4. This supports the hypothesis that the decrease in morlayer pH is the major factor explaining the disappearance of E. tetralix and that measures to increase pH should be considered as part of the recommendations for relevant future management.

A native species with invasive behaviour in coastal dunes: evidence for progressing decay and homogenization of habitat types

A new species has recently invaded coastal dune ecosystems in North West Europe. The native and expansive inland grass, Deschampsia flexuosa, progressively dominating inland heaths, has recently invaded coastal dunes in Denmark, occasionally even as a dominant species. A total of 222 coastal locations with 5,000 random sample plots have been investigated. These findings are in contrast to historical records, and D. flexuosa has never been considered belonging to coastal dune ecosystems. The occurrence of the typical inland grass in the coastal dunes is a strong indication of increase in nutrient level and that human influences may cause a native species to be invasive in new ecosystems. This could be a radical example of change in species composition due to a long lasting exceedance of critical load of nitrogen. The investigation also showed a general increase in cover of the most dominant species.

Nutritional constraints in ombrotrophic Sphagnum plants under increasing atmospheric nitrogen deposition in Europe

•  We studied the effects of increasing levels of atmospheric nitrogen (N) deposition on nutrient limitation of ombrotrophic Sphagnum plants. •  Fifteen mires in 11 European countries were selected across a natural gradient of bulk atmospheric N deposition from 0.1 to 2 g/m²  year^-1 . Nutritional constraints were assessed based on nutrient ratios of N, phosphorus (P), and potassium (K) in Sphagnum plants collected in hummocks (i.e. relatively drier microhabitats) and in lawns (i.e. relatively wetter microhabitats). •  Nutrient ratios in Sphagnum plants increased steeply at low atmospheric N input, but above a threshold of N deposition of c. 1 g/m²  year^-1 the N : P and N : K ratios tended to saturation. Increasing atmospheric N deposition was also accompanied by a reduced retention of Ca and Mg in Sphagnum plants and a decreased stem volumetric density in hummock Sphagnum plants. •  We suggest a critical load of N deposition in Europe of 1 g/m²  year^-1 above which Sphagnum plants change from being N-limited to be K + P colimited, at N : P > 30 and N : K > 3.

Physiological and growth responses of the montane bryophyte Racomitrium lanuginosum to atmospheric nitrogen deposition

•  The effects of nitrogen (N) deposition on the moss Racomitrium lanuginosum within montane heath in Scotland were investigated over 5 yr. •  Permanent field plots were sprayed with KNO₃ or NH₄ Cl solutions, at doses equivalent to 10 and 40 kg N ha^-1  yr^-1 , in 3-6 applications each summer. •  Racomitrium growth and cover were severely reduced by N addition, whilst the proportion of dead shoots greatly increased. N dose decreased inducibility of shoot nitrate reductase activity (NRA), suggesting that N saturation of Racomitrium occurred, and caused an increase in potassium leakage. At high dosage, effects of NH₄ ⁺ were more detrimental than NO₃ ^- . •  Physiological responses to N indicate that the habitat's critical load (CL) is exceeded by addition of 10 kg N ha^-1  yr^-1 . The differential toxicity of the two forms of N suggests that predominant ion type in deposition should be taken into consideration when CLs are set. In contrast to tissue N, NRA correlated well with shoot growth, and may thus be a useful biological indicator of moss condition.

A new approach to determine the total airborne N input into the soil/plant system using 15N isotope dilution (ITNI): results for agricultural areas in Central Germany

The atmospheric deposition of nitrogen (N) in the environment is of great concern due to its impact on natural ecosystems including affecting vegetation, reducing biodiversity, increasing tree growth in forests, and the eutrophication of aquatic systems. Taking into account the average annual N emission into the atmosphere in Germany of about 2 million t N (ammonia/ammonium, NOx), and assuming homogeneous distribution throughout Germany, an average N deposition of 45 kg/ha x year can be calculated. Such high atmospheric N deposition could be confirmed by N balances from long-term field experiments in Central Germany (e.g., the Static Fertilization Experiment in Bad Lauchstdt). By contrast, estimates by standard methods indicate a deposition of only about 30 kg N/ha x year. This is because the standard methods are using wet-only or bulk collectors, which fail to take into account gaseous deposition and the direct uptake of atmospheric N by aerial plant parts. Therefore, a new system was developed using 15N isotope dilution methodology to measure the actual total atmospheric N input into a soil/plant system (Integrated Total Nitrogen Input, ITNI). A soil/plant system is labeled with [15N]ammonium-[15N]nitrate and the total input of airborne N is calculated from the dilution of this tracer by N from the atmosphere. An average annual deposition of 64 +/- 11 kg/ha x year from 1994-2000 was measured with the ITNI system at the Bad Lauchstdt research farm in the dry belt of Central Germany. Measurements in 1999/2000 at three other sites in Central Germany produced deposition rates of about 60 kg/ha x year. These data clearly show that the total atmospheric N deposition into the soil/plant system determined by the newly developed ITNI system significantly exceeds that obtained from standard wet-only and bulk collectors. The higher atmospheric N depositions found closely match those postulated from the N balances of long-term agricultural field experiments.

Impacts of increased nitrogen supply on high Arctic heath: the importance of bryophytes and phosphorus availability

• This study investigates effects of nitrogen and phosphorus on high Arctic heath vegetation, particularly bryophytes. • Heath communities received factorial combinations of nitrogen (0, 10 and 50 kg ha^-1  yr^-1 ) and phosphorus (0 and 5 kg ha^-1 yr^-1 ) in five applications per growing season, for 8 yr. • Nitrogen decreased lichen cover but did not affect cover of any other functional type. However, just 10 kg ha^-1 yr^-1 increased the proportion of physiologically active bryophte shoots, and decreased their nitrate assimilation capacity. Phosphorus had greater effects, and the combination of both nutrients altered species composition. Individual bryophyte species displayed contrasting responses to fertilization, suggesting that they should not be grouped as a single functional type. • The 'critical load' of nitrogen for Arctic heath lies below 10 kg ha^-1  yr^-1 . Nitrogen and phosphorus are colimiting in this sytem, so the critical load of nitrogen will be lower where phosphorus availability is greater. Responses of vegetation to any increase in net mineralisation due to soil warming will depend on the ratio in which nitrogen and phosphorus availabilities increase. The effects of nutrient enhancement are very persistent.