全文获取类型
收费全文 | 2223篇 |
免费 | 119篇 |
国内免费 | 523篇 |
出版年
2024年 | 11篇 |
2023年 | 47篇 |
2022年 | 53篇 |
2021年 | 75篇 |
2020年 | 90篇 |
2019年 | 99篇 |
2018年 | 100篇 |
2017年 | 109篇 |
2016年 | 108篇 |
2015年 | 89篇 |
2014年 | 82篇 |
2013年 | 157篇 |
2012年 | 72篇 |
2011年 | 102篇 |
2010年 | 55篇 |
2009年 | 96篇 |
2008年 | 101篇 |
2007年 | 106篇 |
2006年 | 100篇 |
2005年 | 117篇 |
2004年 | 74篇 |
2003年 | 93篇 |
2002年 | 86篇 |
2001年 | 76篇 |
2000年 | 66篇 |
1999年 | 51篇 |
1998年 | 61篇 |
1997年 | 46篇 |
1996年 | 60篇 |
1995年 | 46篇 |
1994年 | 47篇 |
1993年 | 34篇 |
1992年 | 62篇 |
1991年 | 41篇 |
1990年 | 42篇 |
1989年 | 35篇 |
1988年 | 20篇 |
1987年 | 22篇 |
1986年 | 17篇 |
1985年 | 16篇 |
1984年 | 12篇 |
1983年 | 13篇 |
1982年 | 26篇 |
1981年 | 12篇 |
1980年 | 10篇 |
1979年 | 8篇 |
1978年 | 9篇 |
1977年 | 5篇 |
1971年 | 1篇 |
1958年 | 1篇 |
排序方式: 共有2865条查询结果,搜索用时 296 毫秒
51.
Within a subprogram of Integrated Monitoring (IM), understorey vegetation in Swedish natural forests was observed at fifteen reference sites over the country for twelve seasons, 1982–1993. The main task of the subprogram was to assess the impact of atmospheric deposition, mainly sulphur and nitrogen, on natural vegetation through time. The present study is focused on the variability of plant species diversity at community level and the possible impact of sulphur and nitrogen deposition. Species richness, evenness and diversity varied greatly among the sites, and between years within each site. Regarding only coniferous forests the species richness was higher in the north than in the south. But the effects of site condition and atmospheric deposition were not clarified. Changes in species diversity through time differed from site to site. No overall temporal trend was found. The atmospheric deposition of sulphur and nitrogen demonstrated a clear geographical pattern being low in the north-west and high in the south-west. Sulphur deposition declined significantly in Southern Sweden during the period. We concluded that the species diversity of understorey vegetation at the Swedish IM sites was not significantly influenced by atmospheric deposition. The changes observed are explained as natural processes. 相似文献
52.
STEFAN SLOVIK 《Global Change Biology》1996,2(5):459-477
Field data on the sulphur and cation budget of growing Norway spruce canopies (Picea abies [L.] Karst.) are summarized. They are used to test a spruce decline model capable of quantifying effects of chronic SO2 pollution on spruce forests. At ambient SO2 concentrations, acute SO2 damage is rare, but exposure to polluted air produces reversible thinning of the canopy structure with a half-time of a few years. Canopy thinning in the spruce decline model is highest (i) at elevated SO2 pollution, (ii) in the mountains, (iii) at unfertilized sites with poor K+, Mg2+ or Zn2+ supply, (iv) at low spruce litter decomposition rates, and (v) acidic, shallow soils at high annual precipitation rates in the field and vice versa. Model application using field data from Würzburg (moderate SO2 pollution, alkaline soils, no spruce decline) and from the Erzgebirge (extreme SO2 pollution, acidic soils in the mountains, massive spruce decline) predicts canopy thinning by 2–11% in Würzburg and by 45–70% in the Erzgebirge. The model also predicts different SO2-tolerance limits for Norway spruce depending on the site elevation and on the nutritional status of the needles. If needle loss of more than 25% (damage class 2) is taken to indicate ‘real damage’ exceeding natural variances, then for optimum soil conditions SO2 tolerance limits range from (27.3 ± 7.4) μg m?3 to (62.6 ± 16.5) μg m?3. For shallow and acidic soils, SO2 tolerance limits range from (22.0 ± 5.5) μg m?3 to (37.4 ± 7.5) μ m?3. These tolerance limits, which are calculated on an ecophysiological data basis for Norway spruce are close to epidemiological SO2-toIerance limits as recommended by the IUFRO, UN-ECE and WHO. The observed statistical regression slope of the plot (damaged spruce trees vs. SO2-pollution) in west Germany is confirmed by modelling (6% error). Model application to other forest trees allows deduction of the observed sequence of SO2-sensitivity: Abies > Picea > Pinus > Fagus > Quercus. Thus, acute phytotoxicity of SO2 seems not to be involved in ‘forest decline’. Chronic SO2-pollution induces massive canopy thinning of Abies alba and Picea abies only at unfavourable sites, where natural stress factors and secondary effects of SO2pollution act together to produce tree decline. 相似文献
53.
O. A. Ogunseitan 《World journal of microbiology & biotechnology》1996,12(3):251-256
A strain of Pseudomonas putida (biotype A) capable of growing on caffeine (1,3,7-trimethylxanthine) was isolated from a domestic wastewater processing operation. It used caffeine as the sole carbon source with a mean growth rate constant (k) of 0.049 h-1 (approximately 20 h per generation), whereas k for glucose utilization under similar incubation conditions was 0.31 (3.3 h per generation). The isolate contained at least two plasmids, and the increased expression of a 40 kDa protein was attributable to growth on caffeine. Degradation byproducts of caffeine metabolism by the bacterial isolate included other xanthine derivatives. The slow bacterial catabolism of caffeine in sewage has implications for the effectiveness of wastewater purification, re-use and disposal.The author is with the Laboratory for Molecular Ecology, Department of Environmental Analysis and Design, University of California at Irvine, Irvine, CA 92717-5150 U.S.A. 相似文献
54.
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
55.
L.B. Schilling 《FEMS microbiology reviews》1995,16(2-3):101-110
Abstract: Growing industrial interest in products from renewable alternative feedstocks has resulted in the creation of more industry-led federal research and development programs. The basis of this interest is introduced, followed by an overview of the various federal programs that support basic through applied research relevant to chemical products from renewable resources, and evidence of the increased coordination efforts between programs. The majority of the paper outlines a new program within the U.S. Department of Energy, the Alternative Feedstocks Program, which specifically targets chemicals from renewables which have the potential to become part of the next generation of high-volume chemical feedstocks for the manufacturing industries. The first product of the program, an iterative process technology decision analysis tool, is broadly presented using the first process development project: succinic acid from fermentable sugars. 相似文献
56.
An automatic system was installed for continuous analyses of ozone, sulphur dioxide, nitrogen monoxide and nitrogen dioxide in an experimental orchard with a canopy ofPrunus cerasifera plants in summer 1993. Air samples from three elevations (0.8 m, 1.6 m and 3 m above ground) were sequentially analyzed. Ozone concentrations above the canopy were usually higher than within the canopy; their relationships with stomatal resistance have been investigated. Sulphur dioxide levels were negligible. Nitrogen oxides showed a complex profile, with no particular trend, likely due to a reciprocal exchange between the atmosphere and the ground surface. 相似文献
57.
The Allodon River, a tributary of the Rhône, has suffered considerably from the recent expansion of human activities in the Geneva region. This study documents changes in its benthic fauna by comparing species richness before and after 1986 and by considering the possibilities of recolonization by drift.
Résumé L'Allondon, affluent du Rhône, est une rivière qui a considérablemen t souffert d'une expansion récente des activités humaines dans la région genevoise. Cette étude met en lumière l'évolution de certains éléments de la faune benthique en comparant les richesses specifiques avant et après 1986, année critique pour la macrofaune benthique du bassin genevois. Elle met en évidence les possibilités de recolonisation par dérive de certains recours de l'Allondon à partir d'affluents moins perturbés.相似文献
58.
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
59.
Richters Valda Elliott Gary Sherwin Russell P. 《In vitro cellular & developmental biology. Plant》1978,14(5):458-464
Summary The lungs of 12 mice, half of which were exposed to continuous 0.5 ppm nitrogen dioxide for 3 weeks, were explanted in culture,
and the instances of macrophage congregation were quantitated according to numbers of target cells involved, categories of
congregation from three to 11 or more, numbers of macrophages participating in each category for the total cultures, and the
influence of delaying explantation for 24 and 96 hr. A total of 9042 macrophages and 2140 epithelial and spindle target cells
were counted in the outgrowths from 306 explants. The incidence of macrophage congregation (or numbers of target cells) was
greater for the cultures from the NO2-exposed animals, both with respect to total incidences between groups (p→0), and the 0-hr (p<0.001) and 24-hr (p<0.01) culture
subgroups. In addition, the values for T3 to T6 macrophage congregation were individually and consistently greater for the exposed animal group. Postmortem interval stress
at 96 hr appeared to result in large colonies, but they were reduced greatly in number. Also the incidence of macrophage congregation
fell by 28% as compared to 0-hr and 24-hr subgroups.
Supported by Grants NHLI No. HL 17412 and EPA No. R. 800881. 相似文献
60.
Paul A. del Giorgio Alicia L. Vinocur Ruben J. Lombardo Hector G. Tell 《Hydrobiologia》1991,224(3):129-154
The phytoplankton of the River Lujan (Buenos Aires, Argentina) was studied for a period of 18 months, together with physical and chemical variables, in relation to a pollution gradient. 167 taxa were recorded within a seasonal succession characterized by dominance of diatoms with a brief summer green algae facies. A combination of several biotic indices and multivariate analysis was employed to assess the impact of pollution on the phytoplankton community. The biotic indices used were species diversity and richness, algal quotients (green algae/diatom ratio, Centrales/Pennales ratio) and the SD succession rate index. Multivariate procedures included cluster analysis and ordination by PCA of both species and samples, stepwise discriminant analysis and multiple discriminant analysis of variance (MANOVA). Results indicate that community dynamism is attenuated at the more polluted sites, concomitant with an increased predominance of a broad-tolerance algal assemblage, co-dominated by Cyclotella meneghiniana and Nitzschia stagnorum. The changes in the community structure and dynamics described herein involved alterations in the distribution and relative proportions of the algae, rather than modifications in the basic species composition. These changes may not be readily detectable by methods which over-simplify the ecological information, such as systems of indicator species and biotic indices, designed to assess the degree of pollution. The suitability of multivariate analysis and biotic indices in river phytoplankton studies is further discussed. 相似文献