Assessing atmospheric dry deposition via water-soluble ionic composition of roadside leaves.

This study focuses on the water-soluble ion concentrations in the washing solution of leaves of different roadside tree species at three sites in Iran to estimate the ionic composition of the dry deposition of ambient air particulates. All considered water-soluble ion concentrations were significantly higher next to the roads with high traffic density compared to the reference site with low traffic density. The PCA results showed that Ca2+, Mg2+, [Formula: see text] and [Formula: see text] originated mainly from traffic activities and geological sources, and Na+, Cl-, K+ and F- from sea salts. In addition to sea salt, K+ and F- were also originated from anthropogenic sources i.e. industrial activities, biomass burning and fluorite mining. Moreover, the concentration of the water-soluble ions depended on species and site. C. lawsoniana had significantly higher ion concentrations in its leaf washing solution compared to L. japonicum and P. brutia which indicates C. lawsoniana is the most suitable species for accumulating of atmospheric dry deposition. From our results, it can be concluded that sites with similar traffic density can have different particle loads and water-soluble ion species, and that concentrations in leaf-washing solutions depend on site conditions and species-specific leaf surface characteristics.

Biomagnetic Monitoring of Atmospheric Pollution: A Review of Magnetic Signatures from Biological Sensors.

Biomagnetic monitoring of atmospheric pollution is a growing application in the field of environmental magnetism. Particulate matter (PM) in atmospheric pollution contains readily measurable concentrations of magnetic minerals. Biological surfaces, exposed to atmospheric pollution, accumulate magnetic particles over time, providing a record of location-specific, time-integrated air quality information. This review summarizes current knowledge of biological material ("sensors") used for biomagnetic monitoring purposes. Our work addresses the following: the range of magnetic properties reported for lichens, mosses, leaves, bark, trunk wood, insects, crustaceans, mammal and human tissues; their associations with atmospheric pollutant species (PM, NOx, trace elements, PAHs); the pros and cons of biomagnetic monitoring of atmospheric pollution; current challenges for large-scale implementation of biomagnetic monitoring; and future perspectives. A summary table is presented, with the aim of aiding researchers and policy makers in selecting the most suitable biological sensor for their intended biomagnetic monitoring purpose.

Nitrogen saturation and net ecosystem production.

Magnani et al. found that net carbon (C) sequestration of temperate and boreal forests is clearly driven by nitrogen (N) deposition. From the positive relationship between average net ecosystem production (NEP) and wet N deposition, the authors further conclude that "no signs of N saturation were apparent" in the studied forests and that this is "casting doubts on the risk of widespread ecosystem nitrogen saturation". Nitrogen additions can clearly alter net ecosystem production, but net ecosystem production cannot be used as an indicator of N saturation.

The influence of plant species, leaf morphology, height and season on PM capture efficiency in living wall systems.

Green infrastructure (GI) is already known to be a suitable way to enhance air quality in urban environments. Living wall systems (LWS) can be implemented in locations where other forms of GI, such as trees or hedges, are not suitable. However, much debate remains about the variables that influence their particulate matter (PM) accumulation efficiency. This study attempts to clarify which plant species are relatively the most efficient in capturing PM and which traits are decisive when it comes to the implementation of a LWS. We investigated 11 plant species commonly used on living walls, located close to train tracks and roads. PM accumulation on leaves was quantified by magnetic analysis (Saturation Isothermal Remanent Magnetization (SIRM)). Several leaf morphological variables that could potentially influence PM capture were assessed, as well as the Wall Leaf Area Index. A wide range in SIRM values (2.74-417 µA) was found between all species. Differences in SIRM could be attributed to one of the morphological parameters, namely SLA (specific leaf area). This suggest that by just assessing SLA, one can estimate the PM capture efficiency of a plant species, which is extremely interesting for urban greeners. Regarding temporal variation, some species accumulated PM over the growing season, while others actually decreased in PM levels. This decrease can be attributed to rapid leaf expansion and variations in meteorology. Correct assessment of leaf age is important here; we suggest individual labeling of leaves for further studies. Highest SIRM values were found close to ground level. This suggests that, when traffic is the main pollution source, it is most effective when LWS are applied at ground level. We conclude that LWS can act as local sinks for PM, provided that species are selected correctly and systems are applied according to the state of the art.

The magnetic signal from trunk bark of urban trees catches the variation in particulate matter exposure within and across six European cities.

Biomagnetic monitoring increasingly is applied to assess particulate matter (PM) concentrations, mainly using plant leaves sampled in small geographical area and from a limited number of species. Here, the potential of magnetic analysis of urban tree trunk bark to discriminate between PM exposure levels was evaluated and bark magnetic variation was investigated at different spatial scales. Trunk bark was sampled from 684 urban trees of 39 genera in 173 urban green areas across six European cities. Samples were analysed magnetically for the Saturation isothermal remanent magnetisation (SIRM). The bark SIRM reflected well the PM exposure level at city and local scale, as the bark SIRM (i) differed between the cities in accordance with the mean atmospheric PM concentrations and (ii) increased with the cover of roads and industrial area around the trees. Furthermore, with increasing tree circumferences, the SIRM values increased, as a reflection of a tree age effect related to PM accumulation over time. Moreover, bark SIRM was higher at the side of the trunk facing the prevailing wind direction. Significant relationships between SIRM of different genera validate the possibility to combine bark SIRM from different genera to improve sampling resolution and coverage in biomagnetic studies. Thus, the SIRM signal of trunk bark from urban trees is a reliable proxy for atmospheric coarse to fine PM exposure in areas dominated by one PM source, as long as variation caused by genus, circumference and trunk side is taken into account.

The Greenhouse Phyllosphere Microbiome and Associations with Introduced Bumblebees and Predatory Mites.

Greenhouses are highly productive environments in which conditions are regulated to optimize plant growth. The enclosed character of greenhouses usually results in reduced microbial diversity, while it is known that a diverse microbiome is important for plant health. Therefore, we explored the phyllosphere microbiome of tomatoes and strawberries grown in greenhouses. We observed that the microbiome of both crops was low in diversity and abundance and varied considerably over time and space. Interestingly, the core taxa of tomatoes were Snodgrasella and Gilliamella, genera typically associated with bumblebees. The same amplicon sequence variants (ASVs) were found on reared bumblebees, indicating that the bumblebees, present in the sampled greenhouses to pollinate flowers, had introduced and dispersed these bacteria in the greenhouses. Overall, we found that 80% of plants contained bumblebee-associated taxa, and on these plants, bumblebee-associated reads accounted for up to a quarter of the reads on tomatoes and a tenth of the reads on strawberries. Furthermore, predatory mites had been introduced for the control of spider mites. Their microbiome was composed of a diverse set of bacteria, which varied between batches ordered at different times. Still, identical ASVs were found on mites and crops, and these belonged to the genera Sphingomonas, Staphylococcus, Methylobacterium, and Pseudomonas. These new insights should now be further explored and utilized to diversify ecosystems that are characterized by low diversity and abundancy of microbes. IMPORTANCE Greenhouses, though highly effective agricultural environments, are characterized by reduced sources of bacterial diversity and means of dispersal compared to more natural settings. As it is known that plant health and productivity are affected by associated bacteria, improving our knowledge on the bacterial communities on greenhouse crops is key to further innovate in horticulture. Our findings show that tomato and strawberry crops cultivated in greenhouses harbor poor and variable bacterial communities. Furthermore, commonly implemented biological solutions (i.e., those based on living organisms such as bumblebees and predatory mites) are important sources and means of dispersal of bacteria in greenhouses. This study shows that there is great potential in using these biological solutions to enrich the greenhouse microbiome by introducing and dispersing microbes which have beneficial effects on crop production and protection, provided that the dispersed microbes have a beneficial function.

Bacterial Succession and Community Dynamics of the Emerging Leaf Phyllosphere in Spring.

Every year, deciduous trees shed their leaves, and when new leaves emerge next spring, they establish a characteristic bacterial leaf community. In this exploratory study, we assessed the bacterial phyllosphere (aboveground plant surfaces) of eight London plane trees (Platanus × acerifolia) in Antwerp and Milan by sampling weekly during leaf emergence and expansion. We sampled the surfaces of different tree compartments: leaves, leaf buds, branches, and trunk, for up to 6 weeks. Phyllosphere community composition was most strongly determined by tree compartment. Only the communities on the emerging leaves showed changing dynamics over time. The rate of change in the leaf phyllosphere composition, expressed as the beta dissimilarity between consecutive time points, was very high following leaf emergence, with decreasing speed over time, indicating that these communities stabilize over time. We also identified cooccurring groups of bacteria associated with potential stages of ecological succession on the leaves and accordingly named them general cluster, early cluster, middle cluster, and late cluster. Taxa of the general cluster were not only more abundant than the others on leaves, but they were also widespread on other tree compartments. The late cluster was most pronounced in trees surrounded by trafficked urban land use. This study mainly generates hypotheses on the ecological succession on the emerging leaves of deciduous trees in urban environments and contributes to understanding the development of the tree leaf phyllosphere in spring. IMPORTANCE Improving our understanding of phyllosphere ecology is key in successfully applying bacterial biological agents or modulating the leaf microbiome in order to achieve valuable ecosystem services, such as plant protection, plant growth, air purification, and developing a healthy human immune system. Modulation of the phyllosphere microbiome in the field works only with variable success. To improve the impact of our applications in the field, a better understanding of the ecological principles governing phyllosphere dynamics is required. This exploratory study demonstrates how the combination of different analyses of a chronosequence of bacterial communities can provide new ecological insights. With a limited number of sampled trees, we demonstrated different indications of ecological succession of bacterial communities in the leaves and observed a potential impact of intensely trafficked land use becoming apparent in the leaf bacterial communities approximately 3 weeks after leaf emergence, consisting of a separate stage in community development.

Phyllosphere bacterial communities in urban green areas throughout Europe relate to urban intensity.

The phyllosphere harbours a diverse and specific bacterial community, which influences plant health and ecosystem functioning. In this study, we investigated the impact of urban green areas connectivity and size on the composition and diversity of phyllosphere bacterial communities. Hereto, we evaluated the diversity and composition of phyllosphere bacterial communities of 233 Platanus x acerifolia and Acer pseudoplatanus trees in 77 urban green areas throughout 6 European cities. The community composition and diversity significantly differed between cities but only to a limited extent between tree species. We could show that urban intensity correlated significantly with the community composition of phyllosphere bacteria. In particular, a significant correlation was found between the relative abundances for 29 out of the 50 most abundant families and the urban intensity: the abundances of classic phyllosphere families, such as Acetobacteraceae, Planctomycetes, and Beijerinkiaceae, decreased with urban intensity (i.e. more abundant in areas with more green, lower air pollution, and lower temperature), while those related to human activities, such as Enterobacteriaceae and Bacillaceae, increased with urban intensity. The results of this study suggest that phyllosphere bacterial communities in European cities are associated with urban intensity and that effect is mediated by several combined stress factors.

Land use and soil characteristics affect soil organisms differently from above-ground assemblages.

BACKGROUND: Land-use is a major driver of changes in biodiversity worldwide, but studies have overwhelmingly focused on above-ground taxa: the effects on soil biodiversity are less well known, despite the importance of soil organisms in ecosystem functioning. We modelled data from a global biodiversity database to compare how the abundance of soil-dwelling and above-ground organisms responded to land use and soil properties. RESULTS: We found that land use affects overall abundance differently in soil and above-ground assemblages. The abundance of soil organisms was markedly lower in cropland and plantation habitats than in primary vegetation and pasture. Soil properties influenced the abundance of soil biota in ways that differed among land uses, suggesting they shape both abundance and its response to land use. CONCLUSIONS: Our results caution against assuming models or indicators derived from above-ground data can apply to soil assemblages and highlight the potential value of incorporating soil properties into biodiversity models.

Morphological and elemental characterization of leaf-deposited particulate matter from different source types: a microscopic investigation.

Particulate matter (PM) deposition on urban green enables the collection of particulate pollution from a diversity of contexts, and insight into the physico-chemical profiles of PM is key for identifying main polluting sources. This study reports on the morphological and elemental characterization of PM2-10 deposited on ivy leaves from five different environments (forest, rural, roadside, train, industry) in the region of Antwerp, Belgium. Ca. 40,000 leaf-deposited particles were thoroughly investigated by particle-based analysis using scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM/EDX) and their physico-chemical characteristics were explored for PM source apportionment purposes. The size distribution of all deposited particles was biased towards small-sized PM, with 32% of the particles smaller than 2.5 µm (PM2.5) and median diameters of 2.80-3.09 µm. The source type influenced both the particles' size and morphology (aspect ratio and shape), with roadside particles being overall the smallest in size and the most spherical. While forest and rural elemental profiles were associated with natural PM, the industry particles revealed the highest anthropogenic metal input. PM2-10 profiles for roadside and train sites were rather comparable and only distinguishable when evaluating the fine (2-2.5 µm) and coarse (2.5-10 µm) PM fractions separately, which enabled the identification of a larger contribution of combustion-derived particles (small, circular, Fe-enriched) at the roadside compared to the train. Random forest prediction model classified the source type correctly for 61-85% of the leaf-deposited PM. The still modest classification accuracy denotes the influence of regional background PM and demands for additional fingerprinting techniques to facilitate source apportionment. Nonetheless, the obtained results demonstrate the utility of leaf particle-based analysis to fingerprint and pinpoint source-specific PM, particularly when considering both the composition and size of leaf-deposited particles.

Mobile Biomonitoring of Atmospheric Pollution: A New Perspective for the Moss-Bag Approach.

In this work the potential of moving moss-bags, fixed to bicycles, to intercept particulate matter (PM) and linked metal(loid)s was tested for the first time. Seven volunteers carried three moss-bags for fifty days while commuting by bicycle in the urban area of Antwerp, Belgium. Moreover, one bike, equipped with mobile PM samplers, travelled along four routes: urban, industrial, green route and the total path, carrying three moss-bags at each route. The saturation isothermal remanent magnetization (SIRM) signal and chemical composition (assessed by HR-ICP-MS) of the moss samples indicated that the industrial route was the most polluted. Element fluxes (i.e., the ratio between element daily uptake and the specific leaf area) could discriminate among land uses; particularly, they were significantly higher in the industrial route for Ag, As, Cd and Pb; significantly lowest in the green route for As and Pb; and comparable for all accumulated elements along most urban routes. A comparison with a previous experiment carried out in the same study area using similar moss-bags at static exposure points, showed that the element fluxes were significantly higher in the mobile system. Finally, PM2.5 and PM10 masses measured along the four routes were consistent with element fluxes.

Multi-elemental profile and enviromagnetic analysis of moss transplants exposed indoors and outdoors in Italy and Belgium.

Air pollution represents one of the major concerns worldwide, fueled by the increasing urbanization and related PM production worsening air quality in open air as well as in confined environments. In the present work, exposure to atmospheric metal pollution was investigated in 20 paired indoor (I)-outdoor (O) sites located in two urban areas of Italy and Belgium, by chemical (ICP-MS) and magnetic (saturation isothermal remanent magnetization, SIRM) analyses of Hypnum cupressiforme moss exposed in bags. After 12 weeks, the elemental profiles of the moss material exposed in the two countries largely overlapped, except for some elements which specifically accumulated in Belgium (Ag, As, Cd, Mo, Pb and Sb) and in Italy (Ca, Mg, Co, Cr, Sr, Ti and U). Element concentrations were higher in moss exposed outdoors, with the Italian sites mostly showing a terrigenous footprint, and the Belgian sites mostly affected by elements of environmental concern (e.g., As, Pb, Sb). The Indoor/Outdoor ratios (mostly lower than 0.75) indicated indoor pollution as strongly affected by outdoor pollution, although specific elements could be of indoor origin or magnified in indoor environments (e.g., Al, Ag, Cd and Co). In line with the chemical analysis, the SIRM signal was significantly higher in outdoor than indoor moss material. A positive, significant correlation was observed between SIRM and several accumulated elements indicating SIRM analysis as a powerful tool to predict the level of metal pollution. Moss bags were confirmed as a useful and versatile tool to highlight metal contamination even in confined environments, an essential prerogative in the perspective of the evaluation of the total exposure risk for humans to these pollutants.

The potential of biomonitoring of air quality using leaf characteristics of white willow (Salix alba L.).

In this study, we assess the potential of white willow (Salix alba L.) as bioindicator for monitoring of air quality. Therefore, shoot biomass, specific leaf area, stomatal density, stomatal pore surface, and stomatal resistance were assessed from leaves of stem cuttings. The stem cuttings were introduced in two regions in Belgium with a relatively high and a relatively low level of air pollution, i.e., Antwerp city and Zoersel, respectively. In each of these regions, nine sampling points were selected. At each sampling point, three stem cuttings of white willow were planted in potting soil. Shoot biomass and specific leaf area were not significantly different between Antwerp city and Zoersel. Microclimatic differences between the sampling points may have been more important to plant growth than differences in air quality. However, stomatal pore surface and stomatal resistance of white willow were significantly different between Zoersel and Antwerp city. Stomatal pore surface was 20% lower in Antwerp city due to a significant reduction in both stomatal length (-11%) and stomatal width (-14%). Stomatal resistance at the adaxial leaf surface was 17% higher in Antwerp city because of the reduction in stomatal pore surface. Based on these results, we conclude that stomatal characteristics of white willow are potentially useful indicators for air quality.

Immobilized atmospheric particulate matter on leaves of 96 urban plant species.

Plants provide many ecosystem services in urban environments, including improving ambient air quality. Leaves of plants permit the deposition of particulate matter (PM) and, depending on their leaf traits, PM may be immobilized within the epicuticular wax (EW) layer, on trichomes, on hyphae of fungi, or inside stomatal cavities. In this study, leaves of 96 perennial urban plant species consisting of 45 deciduous broadleaf/needle-like trees, 32 deciduous broadleaf shrubs, 12 evergreen needle/scale-like trees, 5 evergreen broadleaf trees, and 2 climber species were investigated in June and September 2016 to determine the effectiveness of distinct leaf surfaces in PM immobilization after leaf washing treatment. The leaf surfaces were washed vigorously using a vortex shaker. The magnetizable component of accumulated and immobilized PM on the leaf surfaces was estimated using saturation isothermal remanent magnetization (SIRM) of the unwashed and washed leaves, respectively. In June, the washed leaf SIRM of deciduous (broadleaf/needle-like) tree and shrub species (n = 77) ranged between 0.1 and 13.9 µA. In September, the washed leaf SIRM of all investigated plant species (n = 96) ranged between 1.2 and 35.0 µA. Outcomes of this study indicate that leaves of Buddleja davidii, Viburnum lantana, and Sorbus intermedia showed the highest washed leaf SIRM and thus were the most effective in immobilizing PM on their leaf surfaces while leaves of Populus alba, Robinia pseudoacacia, and Abies fraseri with lowest washed leaf SIRM were the least effective. On average, more than half (i.e., 60%) of the magnetic signal still remained after vigorous washing but a large variation exists between species (9-96%). The leaf SIRM of washed leaves of deciduous broadleaf tree and shrub species was significantly higher compared to leaves of evergreen needle/scale-like species. Evidently, the magnetic signal of unwashed leaves was higher than washed ones and higher in September than in June. Leaf traits significantly influenced the magnetic signal of both washed and unwashed leaves: leaves with a high trichome density or high leaf wettability showed a higher unwashed and washed leaf SIRM compared to leaves with no trichomes or low leaf wettability. The effect of epicuticular wax structure types on leaf SIRM was indicated to be only marginally significant. Moreover, also the immobilized fraction of PM was significantly affected by trichome density and leaf wettability, thus substantiating that plant species with high trichome density and/or leaf wettability not only accumulate more PM but are also less prone to PM re-suspension than other species. In general, the results also indicate that leaf SIRM of unwashed leaves can be a good indicator to determine the effectiveness of a plant species in PM immobilization. Plant species effective in immobilizing PM on their leaf surfaces may likely improve ambient air quality when planted in urban environments. However, it is vital that leaves of these plant species (i.e., with high PM immobilization abilities) are carefully recycled as they may be polluted.

Green infrastructure and atmospheric pollution shape diversity and composition of phyllosphere bacterial communities in an urban landscape.

The microbial habitat on leaf surfaces, also called the phyllosphere, is a selective environment for bacteria, harbouring specific phyllosphere bacterial communities (PBCs). These communities influence plant health, plant-community diversity, ecosystem functioning and ecosystem services. Host plants in an urban environment accommodate different PBCs than those in non-urban environments, but previous studies did not address individual urban factors. In this study, the PBC composition and diversity of 55 London plane (Platanus x acerifolia) trees throughout an urban landscape (Antwerp, Belgium) were determined using 16S rRNA amplicon sequencing. An increasing proportion of green infrastructure in the surrounding of the trees, and subsequently decreasing proportion of anthropogenic land use, was linked with taxa loss, expressed in lower phyllosphere alpha diversity and higher abundances of typical phyllosphere bacteria such as Hymenobacter, Pseudomonas and Beijerinckia. Although air pollution exposure, as assessed by leaf magnetic analysis, did not link with alpha diversity, it correlated with shifts in PBC composition in form of turnover, an equilibrium of taxa gain and taxa loss. We found that both urban landscape composition and air pollution exposure - each in their own unique way - influence bacterial communities in the urban tree phyllosphere.

Leaf-deposited semi-volatile organic compounds (SVOCs): An exploratory study using GCxGC-TOFMS on leaf washing solutions.

Airborne particulate matter (PM) includes semi-volatile organic compounds (SVOCs), which can be deposited on vegetation matrices such as plant leaves. In alternative to air-point measurements or artificial passive substrates, leaf monitoring offers a cost-effective, time-integrating means of assessing local air quality. In this study, leaf washing solutions from ivy (Hedera hibernica) leaves exposed during one-month at different land use classes were explored via comprehensive two-dimensional gas chromatography with time-of-flight mass spectrometry (GCxGC-TOFMS). The composition of leaf-deposited SVOCs, corrected for those of unexposed leaves, was compared against routinely monitored pollutants concentrations (PM10, PM2.5, O3, NO2, SO2) measured at co-located air monitoring stations. The first study on leaf-deposited SVOCs retrieved from washing solutions, herein reported, delivered a total of 911 detected compounds. While no significant land use (rural, urban, industrial, traffic, mixed) effects were observed, increasing exposure time (from one to 28 days) resulted in a higher number and diversity of SVOCs, suggesting cumulative time-integration to be more relevant than local source variations between sites. After one day, leaf-deposited SVOCs were mainly due to alcohols, N-containing compounds, carboxylic acids, esters and lactones, while ketones, diketones and hydrocarbons compounds gained relevance after one week, and phenol compounds after one month. As leaf-deposited SVOCs became overall more oxidized throughout exposure time, SVOCs transformation or degradation at the leaf surface is suggested to be an important phenomenon. This study confirmed the applicability of GCxGC-TOFMS to analyze SVOCs from leaf washing solutions, further research should include validation of the methodology and comparison with atmospheric organic pollutants.

Biomonitoring of atmospheric particulate pollution via chemical composition and magnetic properties of roadside tree leaves.

Particulate matter (PM) is a main atmospheric pollution which threats human health and well-being. In this research, we chemically and magnetically analysed roadside tree leaves, collected from three tree species in two main roads (from two different cities) and a reference area, for 28 elements and the saturation isothermal remanent magnetisation. Comparison of unwashed and washed leaves revealed that deposited particles on the leaf surface contain various elements including Al, Ca, Fe, Mg, Mn, Na, Si, Ti, Ba, Co, Cr, Cu, Ni, Rb, V, Zn and Zr. Moreover, there was no significant difference between washed/unwashed leaves in Cl, K, P, S, As, Cd, Cs, Pb, Sn and Sr concentrations, which indicates tree leaves may not be a suitable biomonitor for these elements. Our results showed that site and tree species are important factors which affect atmospheric elements deposition. Among the three considered tree species, Chamaecyparis lawsoniana showed the highest potential for atmospheric particle accumulation. The PCA results revealed that Al, Fe, Ti, Co, Cr, Cu, Ni, Rb, Si, V, Zn and Zr indicated emissions from road traffic activities and soil dust; Ca, Mg and Na from sea salts and Mn and Sb from industrial activity. The biplot results showed that the site effect was much stronger than the species effect for all elements and saturation isothermal remanent magnetisation (SIRM) values. Moreover, elements from traffic, industrial activity and soil dust are significantly correlated with leaf SIRM indicating that leaf SIRM can be a suitable bioindicator of exposure to traffic-derived particles and soil dust, and not from sea salts. It is concluded that chemical composition and SIRM of urban tree leaves can serve as a good indicator of atmospheric PM pollution in Iran and anywhere else where the studied trees grow.

Edge effects on N2O, NO and CH4 fluxes in two temperate forests.

Forest ecosystems may act as sinks or sources of nitrogen (N) and carbon (C) compounds, such as the climate relevant trace gases nitrous oxide (N2O), nitric oxide (NO) and methane (CH4). Forest edges, which catch more atmospheric deposition, have become important features in European landscapes and elsewhere. Here, we implemented a fully automated measuring system, comprising static and dynamic measuring chambers determining N2O, NO and CH4 fluxes along an edge-to-interior transect in an oak (Q. robur) and a pine (P. nigra) forest in northern Belgium. Each forest was monitored during a 2-week measurement campaign with continuous measurements every 2h. NO emissions were 9-fold higher than N2O emissions. The fluxes of NO and CH4 differed between forest edge and interior, but not for N2O. This edge effect was more pronounced in the oak than in the pine forest. In the oak forest, edges emitted less NO (on average 60%) and took up more CH4 (on average 177%). This suggests that landscape structure can play a role in the atmospheric budgets of these climate relevant trace gases. Soil moisture variation between forest edge and interior was a key variable explaining the magnitude of NO and CH4 fluxes in our measurement campaign. To better understand the environmental impact of N and C trace gas fluxes from forest edges, additional and long-term measurements in other forest edges are required.

Contributing factors in foliar uptake of dissolved inorganic nitrogen at leaf level.

We investigated the influence of leaf traits, rainwater chemistry, and pedospheric nitrogen (N) fertilisation on the aqueous uptake of inorganic N by physiologically active tree leaves. Leaves of juvenile silver birch and European beech trees, supplied with NH4NO3 to the soil at rates from 0 to 200 kg N ha(-1)y(-1), were individually exposed to 100 µl of artificial rainwater containing (15)NH4(+) or (15)NO3(-) at two concentration levels for one hour. In the next vegetative period, the experiment was repeated with NH4(+) at the highest concentration only. The N form and the N concentration in the applied rainwater and, to a lesser extent, the pedospheric N treatment and the leaf traits affected the aqueous foliar N uptake. The foliar uptake of NH4(+) by birch increased when leaves were more wettable. High leaf N concentration and leaf mass per area enhanced the foliar N uptake, and NO3(-) uptake in particular, by birch. Variation in the foliar N uptake by the beech trees could not be explained by the leaf traits considered. In the first experiment, N fertilisation stimulated the foliar N uptake in both species, which was on average 1.42-1.78 times higher at the highest soil N dose than at the zero dose. However, data variability was high and the effect was not appreciable in the second experiment. Our data suggest that next to rainwater chemistry (N form and concentration) also forest N status could play a role in the partitioning of N entering the ecosystem through the soil and the canopy. Models of canopy uptake of aqueous N at the leaf level should take account of leaf traits such as wettability and N concentration.