首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   725篇
  免费   9篇
  国内免费   6篇
安全科学   33篇
废物处理   20篇
环保管理   173篇
综合类   66篇
基础理论   214篇
环境理论   1篇
污染及防治   113篇
评价与监测   60篇
社会与环境   48篇
灾害及防治   12篇
  2022年   4篇
  2021年   3篇
  2020年   7篇
  2019年   8篇
  2018年   19篇
  2017年   13篇
  2016年   24篇
  2015年   12篇
  2014年   18篇
  2013年   37篇
  2012年   24篇
  2011年   44篇
  2010年   23篇
  2009年   37篇
  2008年   35篇
  2007年   32篇
  2006年   31篇
  2005年   27篇
  2004年   20篇
  2003年   21篇
  2002年   28篇
  2001年   17篇
  2000年   23篇
  1999年   14篇
  1998年   13篇
  1997年   9篇
  1996年   14篇
  1995年   18篇
  1994年   16篇
  1993年   13篇
  1992年   9篇
  1991年   7篇
  1990年   10篇
  1989年   6篇
  1988年   11篇
  1987年   7篇
  1985年   11篇
  1984年   9篇
  1983年   14篇
  1982年   8篇
  1981年   5篇
  1980年   4篇
  1979年   8篇
  1977年   3篇
  1976年   2篇
  1975年   3篇
  1974年   3篇
  1972年   4篇
  1968年   2篇
  1967年   2篇
排序方式: 共有740条查询结果,搜索用时 635 毫秒
151.
The most prevalent soft tissue tumour in children is rhabdomyosarcoma. These tumours may develop within or outside of muscle anywhere in the body and at any age. We report what is apparently the earliest case of non-cardiac rhabdomyosarcoma detected prenatally.  相似文献   
152.
 Although pathogens appear to have exerted significant selective pressure on various aspects of sociality, mechanisms of disease resistance in the social insects are poorly understood. We report here on an immune response to infection by the dampwood termite, Zootermopsis angusticollis. Nymphs immunized with an injection of 7.6×107, 7.6×105, or 7.6×104 cells/ml glutaraldehyde-killed solution of the bacterium Pseudomonas aeruginosa had significantly higher survivorship than controls following a challenge with a lethal concentration of active bacteria. Similarly, nymphs exposed to a 9×10–1 spores/ml suspension of the fungus Metarhizium anisopliae had higher survivorship than controls after a challenge with a lethal concentration of spores. Prior exposure to a pathogen thus conferred upon termites a degree of protection during a subsequent encounter with the same pathogen. This represents the first demonstration of immune function in vivo in a social insect. Received: 18 May 1999 / Accepted in revised form: 26 August 1999  相似文献   
153.
To participate in the potential market for carbon credits based on changes in the use and management of the land, one needs to identify opportunities and implement land-use based emissions reductions or sequestration projects. A key requirement of land-based carbon (C) projects is that any activity developed for generating C benefits must be additional to business-as-usual. A rule-based model was developed and used that estimates changes in land-use and subsequent carbon emissions over the next twenty years using the Eastern Panama Canal Watershed (EPCW) as a case study. These projections of changes in C stocks serve as a baseline to identify where opportunities exist for implementing projects to generate potential C credits and to position Panama to be able to participate in the emerging C market by developing a baseline under scenarios of business-as-usual and new-road development. The projections show that the highest percent change in land use for the new-road scenario compared to the business-as-usual scenario is for urban areas, and the greatest cause of C emission is from deforestation. Thus, the most effective way to reduce C emissions to the atmosphere in the EPCW is by reducing deforestation. In addition to affecting C emissions, reducing deforestation would also protect the soil and water resources of the EPCW. Yet, under the current framework of the Clean Development Mechanism (CDM), only credits arising from reforestation are allowed, which after 20 years of plantation establishment are not enough to offset the C emissions from the ongoing, albeit small, rate of deforestation in the EPCW. The study demonstrates the value of spatial regional projections of changes in land cover and C stocks: The approach helps a country identify its potential greenhouse gas (GHG) emission liabilities into the future and provides opportunity for the country to plan alternative development pathways. It could be used by potential project developers to identify which types of projects will generate the largest C benefits and provide the needed baseline against which a project is then evaluated. Spatial baselines, such as those presented here, can be used by governments to help identify development goals. The development of such a baseline, and its expansion to other vulnerable areas, well positions Panama to respond to the future market demand for C offsets. It is useful to compare the projected change in land cover under the business-as-usual scenario to the goals set by Law 21 for the year 2020. Suggested next steps for analysis includeusing the modeling approach to exploreland-use, C dynamics and management ofsecondary forests and plantations, soilC gains or losses, sources ofvariability in the land use and Cstock projections, and other ecologicalimplications and feedbacks resulting fromprojected changes in land cover.  相似文献   
154.
Usingstructuredsystemsanalysistodesignanintegratedsystemfortransportplanningandenvironmentalanalysis¥S.P.Losee;A.L.Brown(Facu...  相似文献   
155.
Interactions between period and degree of pressure have been investigated in membrane and filament systems of gill tissue, skin, and muscles of Modiolus modiolus, Mytilus edulis (Baltic and North Seas) and Branchiostoma lanceolatum (Pallas) at the ultrastructural level. The results show the importance of time for the degree of structural disintegration at constant pressure. Destruction already occurs after 2 min of low-pressure treatment at 300 atm. After 10 min continued pressure at the same level, no further structural dis-organization (qualitative or quantitative) occurs, even when the time period is prolonged. For some organelles (i.e., cilia and microvilli), better structural preservation is apparent after 30-min pressure compared to 10-min treatment at the same pressure level. This is a so-called “acclimatisation” to pressure.  相似文献   
156.
Feeding, growth and bioluminescence of the thecate heterotrophic dinoflagellate Protoperidinium huberi were measured as a function of food concentration for laboratory cultures grown on the diatom Ditylum brightwellii. Ingestion of food increased with food concentration. Maximum ingestion rates were measured at food concentrations of 600 g C l-1 and were 0.7 g C individual-1 h-1 (1.8 D. brightwelli cells individual-1 h-1). Clearance rates decreased asymptotically with increasing food concentration. Maximum clearance rates at low food concentration were ca. 23 l ind-1 h-1, which corresponds to a volume-specific clearance rate of 5.9x105 h-1. Cell size of P huberi was highly variable, with a mean diameter of 42 m, but no clear relationship between cell size and food concentration was evident. Specific growth rates increased with food concentration until maximum growth rates of 0.7 d-1 were reached at a food concentration of 400 g C l-1 (1000 cells ml-1). Food concentrations as low as 10 g C l-1 of D. brightwellii (25 cells ml-1) were able to support growth of P. huberi. The bioluminescence of P. huberi varied with its nutritional condition and growth rate. Cells held without food lost their bioluminescence capacity in a matter of days. P. huberi raised at different food concentrations showed increased bioluminescence capacity, up to food concentration that supported maximum growth rates. The bioluminescence of P. huberi varied over a diel cycle, and these rhythmic changes persisted during 48 h of continuous darkness, indicating that the rhythm was under endogenous control.  相似文献   
157.
Summary Interactions between pairs of shrews on a foraging grid were analysed to test for the effects of prior residence, food density and differences in competitive ability between individuals on the outcome.Prior residence was an important factor influencing the outcome of interactions but the degree of resident advantage varied both with food density on the grid and the difference in competitive ability between shrews.Payoff and resource holding power (R.H.P.) asymmetry effects can be implied from the influence of food density and competitive ability differences but prior residence may operate as an uncorrelated asymmetry only under certain conditions determined by combinations of the other two asymmetries.  相似文献   
158.
Measuring carbon in forests: current status and future challenges   总被引:30,自引:0,他引:30  
To accurately and precisely measure the carbon in forests is gaining global attention as countries seek to comply with agreements under the UN Framework Convention on Climate Change. Established methods for measuring carbon in forests exist, and are best based on permanent sample plots laid out in a statistically sound design. Measurements on trees in these plots can be readily converted to aboveground biomass using either biomass expansion factors or allometric regression equations. A compilation of existing root biomass data for upland forests of the world generated a significant regression equation that can be used to predict root biomass based on aboveground biomass only. Methods for measuring coarse dead wood have been tested in many forest types, but the methods could be improved if a non-destructive tool for measuring the density of dead wood was developed. Future measurements of carbon storage in forests may rely more on remote sensing data, and new remote data collection technologies are in development.  相似文献   
159.
Brown RH  Cape JN  Farmer JG 《Chemosphere》1999,38(4):795-806
Concentrations of 4 chlorinated hydrocarbons, C2H3Cl3 (1,1,1-trichloroethane), CCl4 (tetrachloromethane), C2HCl3 (trichloroethene) and C2Cl4 (tetrachloroethene) have been measured in needles of Scots pine (Pinus sylvestris L.) growing close to two industrial sites and in a rural area in northern Britain. Pentane extracts of pine needles sampled over 14 months were analysed using gas chromatography with electron capture detection. Geometric mean concentrations were not significantly different among the sites, with values (in ng g(-1) dry weight) of 7-15 for C2H3Cl3, 3.2-6.5 for CCl4, 70-240 for C2HCl3 and 11-26 for C2Cl4. There was no evidence of accumulation with needle age, but more exposed sites (e.g. those higher in the canopy) showed significantly larger concentrations. The influence of possible local sources could not be detected.  相似文献   
160.
Induction of phase I biotransformation enzymes is recognized as a hallmark response in fish exposed to coplanar PCBs. Depletions of vitamins A and E and disrupted thyroid hormone and glandular structure secondary to this induction have not yet been examined in an arctic fish species. Arctic grayling were exposed to a single oral dose of 0 (control), 10, 100 or 1000 ng 3,3',4,4'-tetrachlorobiphenyl (TCB) g(-1) bodyweight, a contaminant found in most arctic fish. After 30 and 90 days of exposure, TCB concentrations in tissues, hepatic phase I activity (as ethoxyresorufin-O-deethylase (EROD)), plasma and tissue vitamin A and E concentrations, plasma thyroid hormone levels and thyroid glandular structure were examined. Total plasma osmolality, as an indicator of overall fish health was also monitored. TCB recovery in tissues was low and extremely variable, making comparisons between intended dose groups inappropriate. Therefore, correlation analysis between actual recovered TCB concentrations and biochemical responses was employed. Hepatic EROD activity correlated strongly with liver TCB concentrations. Liver concentrations of vitamin A were altered as a function of TCB concentrations and EROD activity, but plasma vitamin A status was not affected. Vitamin E was depleted by TCB accumulation in blood and EROD induction in liver of males only at 90 days postexposure. Thyroid hormones status and glandular structure were not affected by the short duration TCB exposures used in this experiment. TCB concentrations were correlated with an elevation in plasma osmolality. Results from this experiment indicate that the vitamin status and osmoregulation of arctic grayling exposed to TCB can be compromised. Further studies of field populations exposed to this type of contaminant are warranted.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号