Abstract
During an in-flight lightning strike, the relative motion between the arc channel and the aircraft causes a sweeping of the attachment point on the aircraft skin. To predict the behaviour of ...the arc channel and therefore the potential locations for further restrikes, this study aims at investigating the phenomenon with an innovative method for producing sweeping arcs based on a stationary arc and an electromagnetic launcher propelling aeronautical samples. This article focuses on characterisation of the arc channel, aiming at establishing the evolution of its physical properties during swept-stroke for various experimental conditions. Firstly, the experimental coupling of the lightning generator with the electromagnetic launcher and with a wind tunnel is described with a discussion on the representativeness of the experiment. Then, high-speed cameras and voltage and current probes are used to measure the electrical and geometrical behaviour of the electric arc channel during a swept-stroke. The shape, the length, the voltage and the power evolution of the arc channel are evaluated for different input parameters such as speed, polarity, arc current and sample length. Furthermore, the influence of these parameters on the temperature of the arc channel is studied, resorting to an optical emission spectroscopy technique. The results are presented and discussed with the objective of providing a physical insight into arc elongation phenomenology during a swept-stroke.
Hydraulic redistribution (HR) is a process by which water moves through plant roots from moist to dry soils. An experiment was conducted to quantify the influence of common mycorrhizal networks ...(CMNs) and proximity to mature HR-source trees on the water relations of surrounding seedlings. Douglas-fir (Pseudotsuga menziesii var glauca (Mirb.) Franco) seedlings were planted at four distances (0.5, 1, 2.5, and 5 m) from six mature Douglas-fir trees, either directly into soil (soil plus CMN pathway) or inside 0.5 µm mesh bags (soil-only pathway). Deuterated water was used to irrigate soil beside mature trees in order to identify different HR water pathways to surrounding seedlings. This was followed by measurements of seedling deuterium enrichment, seedling water potential, soil water potential, gravimetric soil water content, and tree root density surrounding the seedlings. There was no significantly detectable difference in the quantity of HR water transferred to seedlings having access to soil and CMN pathways or soil-only pathways of water movement. Water from the irrigation plot contributed up to 1.4% of the water of Douglas-fir seedlings. Based on the assumption that the only pathway through which seedlings could access irrigation water was through the mature trees, we estimate that as much as 21.6% of the seedling water was supplied by the nearby tree. Seedling water potential was not significantly affected either by proximity to mature trees or pathway, suggesting HR may have compensated for increasing tree competitive effects with proximity. It is also possible that the lack of difference was due to a relatively moist summer. Our results suggest that residual mature trees are potentially important for hydraulic redistribution to regenerating seedlings in harvested dry interior Douglas-fir forests.
Long-term soil age gradients are useful model systems to study how changes in nutrient limitation shape communities of plant root mutualists because they represent strong natural gradients of ...nutrient availability, particularly of nitrogen (N) and phosphorus (P). Here, we investigated changes in the dinitrogen (N₂)-fixing bacterial community composition and diversity in nodules of a single host legume (Acacia rostellifera) across the Jurien Bay chronosequence, a retrogressive 2 million-year-old sequence of coastal dunes representing an exceptionally strong natural soil fertility gradient. We collected nodules from plants grown in soils from five chronosequence stages ranging from very young (10s of years; associated with strong N limitation for plant growth) to very old (> 2,000,000 years; associated with strong P limitation), and sequenced the nifH gene in root nodules to determine the composition and diversity of N₂-fixing bacterial symbionts. A total of 335 unique nifH gene operational taxonomic units (OTUs) were identified. Community composition of N₂-fixing bacteria within nodules, but not diversity, changed with increasing soil age. These changes were attributed to pedogenesis-driven shifts in edaphic conditions, specifically pH, exchangeable manganese, resin-extractable phosphate, nitrate and nitrification rate. A large number of common N₂-fixing bacteria genera (e.g. Bradyrhizobium, Ensifer, Mesorhizobium and Rhizobium) belonging to the Rhizobiaceae family (α-comprised 70% of all raw sequences and were present in all nodules. However, the oldest soils, which show some of the lowest soil P availability ever recorded, harboured the largest proportion of unclassified OTUs, suggesting a unique set of N₂-fixing bacteria adapted to extreme P limitation. Our results show that N₂-fixing bacterial composition varies strongly during long-term ecosystem development, even within the same host, and therefore rhizobia show strong edaphic preferences.
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BFBNIB, EMUNI, FZAB, GEOZS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NMLJ, NUK, OBVAL, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
Long-term soil age gradients are useful model systems to study how changes in nutrient limitation shape communities of plant root mutualists because they represent strong natural gradients of ...nutrient availability, particularly of nitrogen (N) and phosphorus (P). Here, we investigated changes in the dinitrogen (N
)-fixing bacterial community composition and diversity in nodules of a single host legume (Acacia rostellifera) across the Jurien Bay chronosequence, a retrogressive 2 million-year-old sequence of coastal dunes representing an exceptionally strong natural soil fertility gradient. We collected nodules from plants grown in soils from five chronosequence stages ranging from very young (10s of years; associated with strong N limitation for plant growth) to very old (> 2,000,000 years; associated with strong P limitation), and sequenced the nifH gene in root nodules to determine the composition and diversity of N
-fixing bacterial symbionts. A total of 335 unique nifH gene operational taxonomic units (OTUs) were identified. Community composition of N
-fixing bacteria within nodules, but not diversity, changed with increasing soil age. These changes were attributed to pedogenesis-driven shifts in edaphic conditions, specifically pH, exchangeable manganese, resin-extractable phosphate, nitrate and nitrification rate. A large number of common N
-fixing bacteria genera (e.g. Bradyrhizobium, Ensifer, Mesorhizobium and Rhizobium) belonging to the Rhizobiaceae family (α-proteobacteria) comprised 70% of all raw sequences and were present in all nodules. However, the oldest soils, which show some of the lowest soil P availability ever recorded, harboured the largest proportion of unclassified OTUs, suggesting a unique set of N
-fixing bacteria adapted to extreme P limitation. Our results show that N
-fixing bacterial composition varies strongly during long-term ecosystem development, even within the same host, and therefore rhizobia show strong edaphic preferences.
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BFBNIB, EMUNI, FZAB, GEOZS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NMLJ, NUK, OBVAL, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
Greater understanding of positive interspecific interactions in nutrient‐poor soils is a priority, particularly in phosphorus (P)‐limited ecosystems where plants with contrasting nutrient‐acquiring ...strategies co‐occur. It is also relevant to agro‐ecosystems, since global P stocks are being depleted. In this study, we assess positive interactions between sympatric plants with contrasting nutrient‐acquiring strategies from highly P‐impoverished soils from the biodiversity hotspot of south‐western Australia. Four plant species (Banksia menziesii, Eucalyptus marginata, Verticordia nitens and Melaleuca preissiana) that are non‐mycorrhizal (cluster‐rooted), ectomycorrhizal (EM), arbuscular (AM) or dual AM/EM, respectively, were grown together in a specially designed ‘common garden’ microcosm with nutrient‐poor or fertilized soil, with or without root intermingling and fungal hyphae contact. We measured growth, mycorrhizal colonization, root intermingling and nutrient uptake to determine positive or negative growth patterns amongst the various plant assemblies. Growth of the AM/EM host was best when interacting with both the EM host and a non‐mycorrhizal nutrient‐mining plant with cluster roots (Banksia) in microcosms where root intermingling was not possible. Growth promotion was only seen in pots with nutrient‐poor soils, where the better growth of Melaleuca coincided with higher shoot P, manganese, calcium, iron and boron content, whereas an increase in soil nutrient status through fertilizer addition resulted in a decrease in nutrient‐sharing between co‐occurring species. Furthermore, the dual AM/EM Melaleuca exhibited enhanced EM colonization and favoured EM over AM fungi when grown beside Eucalyptus and Banksia. We surmise that mycorrhizal networks were instrumental in the variation in both mycorrhizal type and colonization levels. We conclude that complementarity of plant nutrient‐acquisition strategies can promote growth of neighbour species. The results show a synergistic effect between EM hyphal scavenging and mobilization of limiting nutrients by cluster roots. The positive and negative interactions enable coexistence to go far beyond the traditional view that plants interact mainly through resource depletion. This study improves our understanding of how root interactions could shape plant communities and promote species diversity and packing in nutrient‐impoverished habitats.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NMLJ, NUK, OILJ, PNG, SAZU, SBCE, SBMB, UL, UM, UPUK
The aim of this work is to improve the understanding of the heating of copper cathodes under the action of a nonstationary electric arc in air. An experimental method is proposed for the measurement ...of the surface temperature distribution just after a very fast arc-controlled arc extinction. The arc current intensity is about 60-70 A, and the arc duration is in the range of 2.5-5 ms. Different kinds of surface temperature distributions have been observed depending on the arc root behavior. The measurement of the temperature decrease after the arc extinction was used to estimate the cathode surface temperature just at the moment of arc extinction. Two-dimensional thermal modeling taking into account phase changes is used to estimate the power balance at the cathode surface and to propose a second estimation of the surface temperature at the point of arc extinction. The comparisons between experimental results and thermal modeling lead in the present experimental conditions to a volt equivalent at the cathode in the range 6.7-10.7 V, to a surface power density in the range0.6 × 10 9 - 2.5 × 10 9 W/m 2 and to a maximum surface temperature in the range 850°C-1300 °C .
In arid highland environments harsh conditions for vegetation establishment prevail. Plants in these environments develop different strategies to survive, including associations with fungal root ...endophytes. These associations may improve plant growth, helping plant resistance to adverse environments. The aim of this study was to determine the relationship among arbuscular mycorrhizal fungi (AMF), dark septate endophytes (DSE) and dominance degree of Poaceae in arid highlands of Argentine Puna. We studied AMF and DSE root colonization, and rhizospheric hyphal densities of dominant and subordinate grass species. Dual and positive associations between AMF and DSE prevailed. Dominant grasses had the highest levels of AMF and DSE root colonization. Dominant and intermediate grasses were associated with the highest values of rhizospheric hyphal density. In Puna highlands, the interactions among AMF, DSE and grasses suggest that fungal root endophytes may play a role in structuring grass communities by differentially improving the resistance to harsh conditions.
•Relationships between fungal root endophytes and grass dominance were studied.•Dominant grasses had the greatest AMF and DSE root colonization.•Positive associations between AMF and DSE root colonization were determined.•We emphasize the importance of AMF-DSE extraradical development on plant dominance.•Plant-fungal root endophyte associations may determine the plant community structure.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
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