1,721,112 research outputs found
Do soil protozoa enhance plant growth by hormonal effects?
No full textWe investigated changes in root morphology of watercress seedlings (Lepidium sativum L.) and effects on the composition of the rhizosphere bacterial community to test the hypothesis that rhizosphere protozoa affect plant growth by a grazing-induced stimulation of plant growth-promoting rhizobacteria. The presence of Acanthamoebae (Protozoa: Amoebida) induced changes in root morphology of watercress seedlings as soon as the root protruded from the seed. The root system was greater and more branched. These changes resembled hormonal effects and were accompanied by an increase in the proportion of auxin, indolyl-3-acetic acid (IAA) producing rhizosphere bacteria. IAA did not originate from amoebal metabolism, but resulted from changes in the composition and activity of the microbial community. Therefore, amoebae affected both the functioning and turnover of rhizosphere microoganisms. We propose a new mechanism based on hormonal effects of protozoa on root growth. Protozoa function as bacteria-mediated mutualists promoting plant growth by hormonal feed-back mechanisms and nutrient effects based on nutrient release from grazed bacterial biomass, i.e. the microbial-loop. (C) 2002 Elsevier Science Ltd. All rights reserved
Substrate heterogeneity and microfauna in soil organic 'hotspots' as determinants of nitrogen capture and growth of ryegrass
No full textIn this study we simultaneously manipulated the patchiness of complex organic resources and the composition of microfaunal populations (protozoa and nematodes) in soil, to influence microbial mineralization processes and to elucidate the underlying mechanisms of nutrient acquisition from decomposing plant residues by ryegrass plants. Hotspot treatments of decreasing patchiness were established by filling laboratory microcosms with defaunated soil and adding labelled (C-13, N-15) grass residues as 1-layer, 4-layer or completely mixed within the soil. Microfaunal treatments were set up by inoculation of the soil with either protozoa or bacterivorous nematodes, a combination of both or neither (control). The microcosms were planted with surface sterile ryegrass seedlings. Growth of ryegrass plants was enhanced by both, increasing patchiness of the organic matter in soil (1-layer > 4-layer > mixed) and microfloral-microfaunal interactions (protozoa + nematodes = protozoa, nematodes > control). The presence of microfauna enhanced the decomposition of hotspot material. Protozoan grazing in particular increased the availability of N in soil and leaching water and led to a concomitant increase in plant growth. While root foraging in organic hotspots enhanced the spatial coupling of mineralization and plant uptake, microfaunal grazing increased the temporal coupling of nutrient release and plant uptake. Consequently the greatest plant biomass was found in treatments combining aggregation of organic material in patches and the presence of microfauna. (C) 2000 Elsevier Science B.V, All rights reserved
Functional stability, substrate utilisation and biological indicators of soils following environmental impacts
No full textStability of a soil property to perturbation comprises both resistance and resilience. Resistance is defined as the ability of the soil to withstand the immediate effects of perturbation, and resilience the ability of the soil to recover from perturbation. Functional stability is used here to describe the stability of a biological function to perturbation, rather than the stability of physical structure or chemical properties. The function chosen for this study was the short-term decomposition of added plant residues, and the perturbations were copper and heat stresses. Previous studies had shown that functional stability was reduced greatly in soils with experimentally reduced biodiversity. The objective of this study was to determine the relative sensitivity of functional stability and potential indicators of biological status to detect alteration of held soils by various environmental impacts. Functional stability, protozoan populations and substrate mineralisation kinetics, were measured on paired soils with: high or low plant species diversity; hydrocarbon pollution or not; extensive or intensive agricultural management practices. Substrate mineralisation kinetics were poorly related to the soil's antecedent conditions and were stimulated significantly by hydrocarbon pollution. Protozoan populations were potentially useful for detecting differences within soil type, but will require greater taxonomic input to be most useful. Functional stability particularly resistance, was able to quantify differences between and within soils. The potential development of the technique in relation to soil health is discussed. (C) 2001 Elsevier Science B.V. All rights reserved
Effects of earthworms and organic litter distribution on plant performance and aphid reproduction
No full textHuman management practices and large detritivores such as earthworms incorporate plant litter into the soil, thereby forming a heterogeneous soil environment from which plant roots extract nutrients. In a greenhouse experiment we investigated effects of earthworms and spatial distribution of N-15-labelled grass litter on plants of different functional groups [Lolium perenne (grass), Plantago lanceolata (forb), Trifolium repens (legume)]. Earthworms enhanced shoot and root growth in L. perenne and P. lanceolata and N uptake from organic litter and soil in all plant species. Litter concentrated in a patch (compared with litter mixed homogeneously into the soil) increased shoot biomass and N-15 uptake from the litter in L. perenne and enhanced root proliferation in P. lanceolata when earthworms were present. Growth of clover (T. repens) was rather independent of the presence of earthworms and organic litter distribution: nevertheless, clover took up more nitrogen in the presence of earthworms and exploited more N-15 from the added litter than the other plant species. The magnitude of the effects of earthworms and organic litter distribution differed between the plant species, indicating different responses of plants with contrasting root morphology. Aphid (Myzus persicae) reproduction was reduced on P. lanceolata in the presence of earthworms. We suggest that earthworm activity may indirectly alter plant chemistry and hence defence mechanisms against herbivores
Contrasting effects of microbial partners in the rhizosphere: interactions between Norway spruce seedlings (Picea abies Karst.), mycorrhiza (Paxillus involutus (Batsch) Fr.) and naked amoebae (Protozoa)
No full textThe importance of the soil microbial community for plant mineral nutrition and nutrient cycling has long been recognized. One of the most important interactions is the symbiosis of plants with mycorrhizas. In contrast, the effects of soil microfauna on plant performance have so far received little attention, although soil protozoa in particular, have been shown to beneficially affect plant growth. We investigated in a laboratory experiment the impact of mycorrhiza and protozoa and their interaction on plant performance. Spruce seedlings with or without the ectomycorrhizal fungus Paxillus involutus (Batsch) Fr. were grown in microcosm chambers with defaunated forest soil with naked amoebae (Acanthamoeba sp.) or without protozoa for 10 months. The presence of protozoa resulted in the development of a more complex root system by increasing root length (51%), length of fine roots (64%) and number of root tips (43%). The effects of protozoa were more pronounced in the absence of mycorrhiza. In contrast to protozoa, the presence of mycorrhiza resulted in a less complex root system, i.e. root length, length of fine roots and number of root tips were reduced by 47, 47 and 40%, respectively. Shoot height, and stem, shoot and needle mass were at a maximum in the combined treatment with both mycorrhiza and protozoa. The presence of mycorrhiza and protozoa also affected plant nutrient concentrations. In treatments with protozoa shoots of spruce seedlings contained less nitrogen, leading, e.g. to an increased C/N ratio in needles. Conversely, in treatments with mycorrhiza concentrations of phosphorus in needles were increased by a factor of almost two. Mycorrhiza and protozoa also affected rhizosphere microorganisms. Microbial biomass was reduced in the presence of mycorrhiza, mainly due to a reduction in bacterial numbers. Conversely, in the presence of protozoa the length of hyphae in the rhizosphere was reduced. It is concluded that the plant-mycorrhiza mutualism and the bacteria-mediated mutualism between plants and protozoa (microbial loop) complement each other; plant resources presumably are allocated to optimize simultaneous exploitation of both mutualistic relationships. (C) 2001 Elsevier Science B.V. All rights reserved
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Microflora, Protozoa and Nematoda in Lumbricus terrestris burrow walls: a laboratory experiment
No full textAnecic earthworms Lumbricus terrestris L. were kept in laboratory microcosms containing soil and litter from a lime (Tilia cordata) and beech (Fagus sylvatica) forest. Nutrient (mineral nitrogen and phosphorus) contents, microbial activity and densities of protozoa and nematodes were determined in burrow walls and control soils after 165 days. Well-developed burrow linings consisting of earthworm faeces were formed in the "Tilia" treatment, but not in the "Fagus" treatment, presumably because beech litter was not an adequate food resource for L. terrestris. Consequently, increases in microbial biomass, basal respiration and microbial volume in burrow walls compared to surrounding soil were significant in the "Tilia" treatment only. However, in both treatments burrow walls were strongly enriched in mineral nitrogen and phosphorus. The density and biomass of protozoa were significantly greater in burrow walls compared to the control soil. The numbers of naked amoebae increased by similar factors of 4 and 3.5 in burrow walls of the "Tilia" and "Fagus" treatment, respectively. Flagellate density increased more than tenfold in burrow walls of the "Tilia" treatment but only twofold in the "Fagus" treatment. In addition, a comparatively large ciliate population was present in burrow walls in the "Tilia" treatment. The total abundance of protozoa was significantly correlated with the contents of inorganic N and P in the samples (r=0.68 and 0.63 respectively, P<0.03) suggesting that protozoan grazing participated in the mobilisation of nutrients from microbial biomass. The total nematode density was increased in burrow walls by a factor of 3 in the "Tilia" treatment (mostly due to bacterivorous nematodes), but was not affected in the "Fagus" treatment. In both treatments density of fungivorous nematodes increased, while that of plant parasites decreased in burrow walls compared to the control soil. It is concluded that the grazing pressure of protozoa and nematodes may control the dynamics of the microbial succession in earthworm burrow walls, strongly affecting nutrient cycling processes in these microhabitats
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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