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排序方式: 共有324条查询结果,搜索用时 15 毫秒
61.
Jan Willem Erisman Nelleke Domburg Wim de Vries Hans Kros Bronno de Haan Kaj Sanders 《中国科学C辑(英文版)》2005,48(Z2)
The Netherlands is "well known" for its nitrogen problems; it has one of the highest reactive nitrogen (Nr) emission densities in the world. It is a small country at the delta of several large European rivers. Ever since the industrial revolution, there has been a growing excess of nutrients and related emissions into the atmosphere (ammonia, nitrogen oxides and nitrous oxide)and into groundwater and surface water (nitrate), leading to a large range of cascading environmental impacts. Vehicular traffic, sewage and animal husbandry are the main sources of oxidized and reduced forms of Nr. This paper provides an overview of the origin and fate of nitrogen in the Netherlands, the various reported impacts of nitrogen, the Dutch and European policies to reduce nitrogen emissions and related impacts. In addition, ways are presented to go forward to potentially solve the problems in a European perspective. Solutions include the improvement of nitrogen efficiencies in different systems, technological options and education. 相似文献
62.
63.
Cerebrospinal fluid levels of insulin,leptin, and agouti‐related protein in relation to BMI in pregnant women
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64.
Six nests of white-tailed eagles in the southernmost Greenland were watched during the nestling period and food items recorded photographically by remote controlled cameras. The species composition of the food was representative even for the adult eagles' diet. Fish made up a major part of 90%, various bird species and arctic fox pups the remaining 10%. The total intake of an eaglet amounted lo 50 kg from hatching to fledging. The daily requirements of fledglings, some 800 g fish food, are equal to previously reported values for older birds. The rate of food consumption of eaglets is constant through most of the nestling period. 相似文献
65.
Kaj Mrtensson 《Biotechnology and bioengineering》1974,16(12):1567-1587
Kinetic studies on the parameters influencing the potential industrial application of an immobilized two-enzyme system of β-amylase and pullulanase for conversion of starch to a product with high maltose content, have been performed. The apparent Michaelis constant, the apparent product inhibitor constant, and the activation energy have been determined for the immobilized preparation and compared to the values for the corresponding soluble enzyme system. The catalytic activity of the immobilized enzymes was studied in a plug-flow reactor and a continuous feed stirred tank reactor. Mathematical models for these reactors have been formulated and adapted to fit the experimental data. Comparisons of the reactor efficiencies were made and the conditions were found to be such as to favor the plug-flow reactor. Results on operational stability tests at different temperatures and substrate concentrations are given. 相似文献
66.
Seasonal distribution of nitrifying bacteria and rates of nitrification in coastal marine sediments 总被引:4,自引:0,他引:4
The distribution of nitrification potential (NP) with depth in sediment and season was investigated in a shallow sandy sediment
(0.5 m water) and a deeper muddy sediment (17m water). In both sediments, nitrifying bacteria were present in the anoxic strata
(oxygen penetration was 5 mm below the surface). The NP at 6–8 cm depth in the sediment was 50% and 10% of the surface NP
at the sandy and muddy sediment, respectively. It is suggested that bioturbation and physical disturbance of the sediment
were the most likely reasons for this distribution. The NP increased as sediment temperature decreased. This effect was less
marked in the muddy sediment. It is concluded that during the summer, the numbers or specific activity of nitrifying bacteria
diminished for the following reasons: There was decreased O2 penetration into the sediment and increased competition for O2 by heterotrophs; there was increased competition for NH4
+ and there was inhibition by H2S. These effects counteracted the potentially higher growth rates and increased rates of NH4
+ production at the elevated summer temperatures. The potential nitrification rates in the upper 1 cm, which were measured
at 22°C, were converted to calculated rates at the in situ temperature (Q10=2.5) and in situ oxygen penetration. These calculated rates were shown to closely resemble the measured in situ rates of
nitrification. The relationship between the in situ rates of nitrification and the nitrification potential is discussed. 相似文献
67.
The problem of respiratory acclimatization 总被引:1,自引:0,他引:1
Kaj Berg 《Hydrobiologia》1953,5(4):331-350
68.
Finn Bengtsson Mogens Bugge Kaj H. Johansen† Roger F. Butterworth‡ 《Journal of neurochemistry》1991,56(3):1069-1074
Regional and whole-brain tryptophan-hydroxylating activity and serotonin turnover were investigated in portacaval shunted (PCS) rats using an in vivo decarboxylase inhibition assay. To saturate tryptophan hydroxylation with amino acid substrate, rats were administered a high dose of tryptophan 1 h prior to analysis of brain tryptophan, 5-hydroxytryptophan, serotonin, and 5-hydroxyindoleacetic acid. The analysis revealed, as expected, higher brain concentrations of tryptophan and 5-hydroxyindoles and increased serotonin synthesis rate in PCS rats as compared with shamoperated controls. Saturating levels of brain tryptophan were achieved in both PCS and sham animals after exogenous tryptophan administration. The tryptophan load resulted in increased brain serotonin turnover in all regions and in whole brain compared with rats that did not receive a tryptophan load. Tryptophan-loaded PCS rats showed increased brain serotonin turnover compared with tryptophan-loaded sham rats. Regionally, this supranormal tryptophan-hydroxylating activity was most pronounced in the mesencephalon-pons followed by the cortex. It is concluded that, at least in the PCS rat, brain tryptophan hydroxylation is an inducible process. Since it is known that brain tissue from PCS rats undergoes a redox shift toward a reduced state and that the essential cofactor tetrahydrobiopterin is active in tryptophan hydroxylation only when present in its reduced form, it is hypothesized that this is the reason for the supranormal tryptophan-hydroxylating activity displayed by the PCS rats. The hypothesis further suggests that alterations in tetrahydrobiopterin availability may serve as a mechanism by which brain tryptophan hydroxylation, and therefore serotonin turnover, can be regulated with high sensitivity in vivo. 相似文献
69.
Epiphyte shading: Its role in resulting depth distribution of submerged aquatic macrophytes 总被引:1,自引:0,他引:1
Kaj Sand-Jensen 《Folia Geobotanica》1990,25(3):315-320
The biomass of epiphytic algae increased more than the biomass of phytoplankton with increased nutrient availability (N, P, Si) in four lakes. Large epiphyte biomasses on the submerged macrophyte,Littorella uniflora, reduce light attenuation more than phytoplankton did. High values of shading by epiphytes correspond with a reduction in the maximum depth penetration ofLittorella by more than 1 meter. 相似文献
70.