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81.
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83.
84.
Field studies on Norway spruce trees at high altitudes 总被引:3,自引:1,他引:2
85.
The economic and environmental costs of the heavy use of chemical N fertilizers in agriculture are a global concern. Sustainability considerations mandate that alternatives to N fertilizers must be urgently sought. Biological nitrogen fixation (BNF), a microbiological process which converts atmospheric nitrogen into a plant-usable form, offers this alternative. Nitrogen-fixing systems offer an economically attractive and ecologically sound means of reducing external inputs and improving internal resources. Symbiotic systems such as that of legumes and Rhizobium can be a major source of N in most cropping systems and that of Azolla and Anabaena can be of particular value to flooded rice crop. Nitrogen fixation by associative and free-living microorganisms can also be important. However, scientific and socio-cultural constraints limit the utilization of BNF systems in agriculture. While several environmental factors that affect BNF have been studied, uncertainties still remain on how organisms respond to a given situation. In the case of legumes, ecological models that predict the likelihood and the magnitude of response to rhizobial inoculation are now becoming available. Molecular biology has made it possible to introduce choice attributes into nitrogen-fixing organisms but limited knowledge on how they interact with the environment makes it difficult to tailor organisms to order. The difficulty in detecting introduced organisms in the field is still a major obstacle to assessing the success or failure of inoculation. Production-level problems and socio-cultural factors also limit the integration of BNF systems into actual farming situations. Maximum benefit can be realized only through analysis and resolution of major constraints to BNF performance in the field and adoption and use of the technology by farmers. 相似文献
86.
Nutrient uptake relationship to root characteristics of rice 总被引:1,自引:0,他引:1
Data on root parameters and distribution are important for an improved understanding of the factors influencing nutrient uptake
by a crop. Therefore, a study was conducted on a Crowley silt loam at the Rice Research and Extension Center near Stuttgart,
Arkansas to measure root growth and N, P and K uptake by three rice (Oryza sativa L.) cultivars at active tillering (36 days after emergence (DAE)), maximum tillering (41 DAE), 1.25 cm internode elongation
(55 DAE), booting (77 DAE) and heading (88 DAE). Soil-root core samples were taken to a depth of 40 cm after plant samples
were removed, sectioned into 5 cm intervals, roots were washed from soil and root lengths, dry weights and radii were measured.
Root parameters were significantly affected by the soil depth × growth stage interaction. In addition, only root radius was
affected by cultivar. At the 0- to 5-cm soil depth, root length density ranged from 38 to 93 cm cm-3 throughout the growing season and decreased with depth to about 2 cm cm-3 in the 35- to 40-cm depth increment. The increase in root length measured with each succeeding growth stage in each soil
horizon also resulted in increased root surface area, hence providing more exposed area for nutrient uptake. About 90% of
the total root length was found in the 0- to 20-cm soil depth throughout the season. Average root radius measured in the 0-
to 5-cm and 35- to 40-cm depth increments ranged from 0.012 to 0.013 cm and 0.004 to 0.005 cm, respectively throughout the
season. Total nutrient uptake by rice differed among cultivars only during vegetative growth. Differences in total nutrient
uptake among the cultivars in the field appear to be related to absorption kinetics of the cultivars measured in a growth
chamber study.
Published with permission of the Arkansas Agricultural Experiment Station. 相似文献
87.
The Liphook Forest Fumigation Project: an overview 总被引:2,自引:1,他引:1
The aim of the Liphook Project was to assess the effects of SO2 and O3, singly and in combination, on coniferous forest ecosystems. More than 4000 trees of Scots pine (Pinus sylvestris), Norway spruce (Picea abies) and Sitka spruce (P. sitchensis) were fumigated for nearly 4 years using an open-air fumigation technique especially developed for the purpose. The technique eliminated artifacts due to chambers and enabled a variety of effects of the pollutant gases on forest ecosystems to be studied. Most symptoms of forest decline did not occur, but each species reacted in a different way to SO2 stress, providing no evidence for universal forest decline symptoms. However, some of the mechanisms hypothesized to underlie forest declines were observed as an effect of SO2 treatment, though others were not, notably any major effect of O3. The results are assessed against proposed regulatory standards (critical loads and levels) for the protection of forest ecosystems against pollution. 相似文献
88.
Blackfoot disease is a peripheral vascular disease resulting in gangrene of the lower extremities. Although extensive epidemiological
study has implicated high arsenic content in artesian well water of the endemic area bears some important connection with
the disease, the etiology of the disease is still not clarified. In this study, attention is paid to chemical speciation of
arsenic in order to find out whether the concentrations of arsenic species in urine of Blackfoot disease patients are different
from those of controls. Experimental results indicate that the total arsenic, inorganic arsenic, monomethylarsonic acid, and
other forms of arsenic in the urine of patients are significantly higher than those of the contols. The possible connection
of those arsenic species with the etiology of the disease is discussed. 相似文献
89.
S. M. Renaud H. C. Zhou D. L. Parry Luong-Van Thinh K. C. Woo 《Journal of applied phycology》1995,7(6):595-602
The effect of temperature from 10 °C to 35 °C on the growth, total lipid content, and fatty acid composition of three species of tropical marine microalgae, Isochrysis sp., Nitzschia closterium, N. paleacea (formerly frustulum), and the Tahitian Isochrysis sp. (T.ISO), was investigated.Cultures of N. closterium, Isochrysis sp. and T.ISO grew very slowly at 35 °C, while N. closterium did not grow at temperatures higher than 30 °C or lower than 20 °C. N. paleacea was low-temperature tolerant, with cells growing slowly at 10 °C. N. paleacea produced the highest percentage of lipids at 10 °C, while the other species produced maximum amounts of lipid at 20 °C. None of the species maintained high levels of polyunsaturated fatty acids (PUFAs) at high growth temperature and there was a significant inverse relationship between the percentage of PUFAs and temperature for N. paleacea. A curved relationship was found between temperature and percentage of PUFA for N. closterium and tropical Isochrysis sp., with the maximum production of PUFA at 25 °C and 20 °C, respectively. The two Nitzschia species produced higher levels of the essential fatty acid eicosapentaenoic acid [20:5(n-3)] at lower growth temperatures, but the two Isochrysis species had little change in percentage of 20:5(n-3) with temperature. Only T.ISO had the highest percentage of 22:6(n-3) at lowest growth temperature (11.4% total fatty acids at 10 °C).School of Mathematical and Physical SciencesAuthor for correspondence 相似文献
90.
A combined palynological and phytosociological methodological approach was used in the reconstruction of precultural forest vegetation in a small model area of Bílý Kí-Janík in the Moravskoslezské Beskydy mountains (northwest Slovakia). Two pollen diagrams show the predominance of Fagus sylvatica, Abies alba and Picea abies in past natural mixed forests. The present-day occurrence and distribution of forest herbs allow the differentiation of five past vegetation types with above mentioned forest climax trees covering the area before the beginning of human interference. A map of reconstructed natural vegetation is presented which can be used for revitalization of present secondary spruce plantations, heavily affected by air pollution. 相似文献