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   19184篇
  免费   205篇
  国内免费   172篇
安全科学   575篇
废物处理   793篇
环保管理   2794篇
综合类   2569篇
基础理论   5151篇
环境理论   5篇
污染及防治   5400篇
评价与监测   1258篇
社会与环境   877篇
灾害及防治   139篇
  2022年   137篇
  2021年   176篇
  2020年   112篇
  2019年   155篇
  2018年   271篇
  2017年   267篇
  2016年   444篇
  2015年   369篇
  2014年   524篇
  2013年   1611篇
  2012年   654篇
  2011年   836篇
  2010年   656篇
  2009年   754篇
  2008年   868篇
  2007年   934篇
  2006年   826篇
  2005年   681篇
  2004年   706篇
  2003年   663篇
  2002年   645篇
  2001年   769篇
  2000年   580篇
  1999年   326篇
  1998年   250篇
  1997年   244篇
  1996年   293篇
  1995年   279篇
  1994年   266篇
  1993年   245篇
  1992年   220篇
  1991年   193篇
  1990年   221篇
  1989年   197篇
  1988年   192篇
  1987年   182篇
  1986年   160篇
  1985年   153篇
  1984年   188篇
  1983年   184篇
  1982年   177篇
  1981年   183篇
  1980年   142篇
  1979年   172篇
  1978年   116篇
  1977年   108篇
  1976年   92篇
  1975年   96篇
  1973年   93篇
  1972年   100篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
Nitrogen mustards (HN) and sulphur mustard (SM) are potent alkylating blister inducing chemical warfare agents. Single 1.0 LD50 dose produced a progressive fall in body weight from second day onwards in all groups of mustard agents exposed animals. Histological examination of spleen, liver skin and kidney revealed significant histopathological lesions in nitrogen mustards and sulphur mustard. These lesions include granulovascular degeneration with perinuclear clumping of the cytoplasm of hepatocytes and renal parenchymal cells. Renal lesions were characterized by congestion and hemorrhage. The maximum toxic manifestation were noted in spleen and skin of HN-3 exposed mice while sulphur mustard reported maximum toxicity in liver and kidneys. The study suggests both nitrogen mustards and sulphur mustard to be extremely toxic by percutaneous route based on histopathological observation and can contributed to earlier reported free radical generation by these toxicants.  相似文献   
952.
Kéfi S  Alados CL  Chaves RC  Pueyo Y  Rietkerk M 《Ecology》2010,91(12):3739-42; discussion 3742-5
  相似文献   
953.
A study was conducted between March 2006 and February 2007 to assess the concentration of Fe, Zn, Cu, Mn, Pb, and Cr in the gills and muscles of six fish species (Chrysichthys nigrodigitatus, Clarias anguillaris, Tillapia zillii, Mormyrus rume rume, Mormyrus macrophthalmus and Mormyrus tapirus) from lotic freshwater ecosystem at Afikpo South- East Nigeria using Atomic Absorption Spectrophotometer. In all the fish species studied the concentration of metals in the gills was significantly higher than that of the muscles (p<0.05). While the highest concentration (mg l(-1)) of Fe (702.20 +/- 0.04), Zn (34.40 +/- 0.02), Cu (2.10 +/- 0.01), Mn (4.91 +/- 0.08) Pb (0.50 +/- 0.02) and Cr (1.12 +/- 0.07) were recorded in the gills of C. nigrodigitatus, that in the muscles were recorded in T. zillii (443.20 +/- 0.08, 23.30 +/- 0.06, 1.33 +/- 0.06, 3.09 +/- 0.02, 0.31 +/- 0.01 and 0.66 +/- 0.04 for Fe, Zn, Cu, Mn, Pb and Cr respectively). The lowest concentration of all the heavy metals in the gills was recorded in M. tapirus (309.00 +/- 0.07, 16.45 +/- 0.03, 0.92 +/- 0.04, 2.15 +/- 0.04, 0.21 +/- 0.01 and 0.50 +/- 0.06 mg l(-1)for Fe, Zn, Cu Mn, Pb and Cr respectively) while the lowest in the muscles was recorded in C. anguillaris [Fe (186.00 +/- 0.07), Zn (14.20 +/- 0.08), Cu (0.56 +/- 0.03), Mn (1.30 +/- 0.02), Pb (0.10 +/- 0.01) and Cr (0.28 +/- 0.04)]. The order of heavy metals concentrations in both the gills and muscles was Fe>Zn>Mn>Cu>Cr>Pb. While the concentration of Zn, Cu and Pb both in the muscles and gills of all the six fish species studied were within the WHO and FEPA prescribed limits, that of Fe(except in the muscles), Mn and Cr were above the prescribed limits thus indicating contamination of the fish species by these metals. Fe has the highest bio-concentration factor (BCF) in both tissues while the least was Cu. Periodic monitoring of these metals in both fishes and river to ensure safety is advocated.  相似文献   
954.
Entomological surveys were conducted for three consecutive years in core and buffer zone of the Dibru-Saikhowa biosphere reserve in pre monsoon, monsoon and post-monsoon seasons. A total of 52 species of mosquitoes under eleven genera have been detected. The genus Anopheles (18 species) was the predominant followed by Culex, Aedes, Mansonia, Armigeres, Mimomyia, Ochlerostatus, Malaya, Toxorhynchites, Ficalbia and Aedeomyia. The buffer zone of the forest reserve where human habitations are there exhibited the presence of maximum number of species (49 species under 10 genera) in comparison to core zone (42 species under 10 genera). In buffer zone, maximum numbers of species (38) were recorded in monsoon season followed by post- monsoon (35 species) and Pre-monsoon season (34 species). Whereas in core zone, maximum number of species were collected in post monsoon season followed by pre monsoon season and monsoon season. In Core and buffer zone, the maximum species were recorded from the ground pool habitat and slow flowing stream respectively. Among the disease vectors, the potential Japanese encephalitis vectors incriminated in India were very much prevalent. This study provides the list of available mosquito species recorded for the first time in the Dibru-Saikhowa biosphere reserve.  相似文献   
955.
Effectof exopolysaccharides (EPS) produced by a consortium of cyanobacteria on germination of three crops wheat, maize and rice was studied at different salt concentrations. Production of EPS was found to be stimulated by salts, which in turn had a significant Na+ removal capability from aqueous solution. Seed germination, vigor index and mobilization efficiency in all the three crops remarkably improved when cyanobacterial EPS was applied. While germination improved significantly by 13 to 30%, mobilization efficiency increased marginally by 1.03 to 1.1 times and vigor index increased by 1.15 to 2.4 times in these crops in response to EPS under non-saline conditions. Salinity had an inhibitory effect on seed germination of all the species showing 18 to 54% reduction. However, in the presence of EPS, the salt induced inhibition diminished to 13 to 18%. Inhibitory effect of salt on chlorophyll concentration, vigor index and mobilization efficiency of the seedlings was much less in these crops in the presence of EPS, indicating the latter's role in salt stress alleviation.  相似文献   
956.
Background In recent years, sediments have become a central topic of scientific and public discussion as an important factor for determining water quality. While the quality of surface waters in Germany has significantly improved during the past years, highly contaminated sediments still create a considerable threat to the quality of several European catchment areas. Main features Here, we summarize different concepts and methods for the assessment of sediment quality and report on some novel integrative test methods for assessing sediment toxicity including contaminant re-mobilization during simulated re-suspension events. Results and discussion Currently, different approaches for assessment of sediment pollution exist. While instrumental chemical analyses are not suitable to accurately describe sediment toxicity, combinations of biological and chemical test procedures and integrated approaches, for example weight-of-evidence studies and effect-directed analysis (EDA), have the potential to identify key contaminants. Inter-disciplinary studies combining hydrodynamic and toxicological aspects coupled to real exposure of aquatic organisms to contaminants are currently being developed. Conclusions Monitoring and assessment of sediment quality are of increasing importance, not only for national legislation but also for the implementation of the European Water Framework Directive (WFD). Integrated approaches for the determination of sediment stability play a key role in the appropriate sediment-monitoring strategies.  相似文献   
957.
The transformation of the former blast-furnace site “Hermannhütte” in Dortmund-Hoerde into commercial, residential and service parks, called PHOENIX East and PHOENIX West involves some unusual challenges concerning the water management. The retention of sediments and nutrients plays an important role. To achieve the objectives, various management measures are being used. The installation of an artificial lake in PHOENIX East, which is groundwater- and rainfed, has different requirements than the conventional purification and retention of surface water in PHOENIX West. Both conventional methods such as storm water sedimentation tank and -reservoirs as well as soil filtration, phosphates elimination, Biodyozon®-plant and biomass management ensure good water quality.  相似文献   
958.
Abstract: In a time of increasing urbanization, the fundamental value of conserving urban biodiversity remains controversial. How much of a fixed budget should be spent on conservation in urban versus nonurban landscapes? The answer should depend on the goals that drive our conservation actions, yet proponents of urban conservation often fail to specify the motivation for protecting urban biodiversity. This is an important shortcoming on several fronts, including a missed opportunity to make a stronger appeal to those who believe conservation biology should focus exclusively on more natural, wilder landscapes. We argue that urban areas do offer an important venue for conservation biology, but that we must become better at choosing and articulating our goals. We explored seven possible motivations for urban biodiversity conservation: preserving local biodiversity, creating stepping stones to nonurban habitat, understanding and facilitating responses to environmental change, conducting environmental education, providing ecosystem services, fulfilling ethical responsibilities, and improving human well‐being. To attain all these goals, challenges must be faced that are common to the urban environment, such as localized pollution, disruption of ecosystem structure, and limited availability of land. There are, however, also challenges specific only to particular goals, meaning that different goals will require different approaches and actions. This highlights the importance of specifying the motivations behind urban biodiversity conservation. If the goals are unknown, progress cannot be assessed.  相似文献   
959.
Abstract: Marine protected areas (MPAs) have been highlighted as a means toward effective conservation of coral reefs. New strategies are required to more effectively select MPA locations and increase the pace of their implementation. Many criteria exist to design MPA networks, but generally, it is recommended that networks conserve a diversity of species selected for, among other attributes, their representativeness, rarity, or endemicity. Because knowledge of species’ spatial distribution remains scarce, efficient surrogates are urgently needed. We used five different levels of habitat maps and six spatial scales of analysis to identify under which circumstances habitat data used to design MPA networks for Wallis Island provided better representation of species than random choice alone. Protected‐area site selections were derived from a rarity–complementarity algorithm. Habitat surrogacy was tested for commercial fish species, all fish species, commercially harvested invertebrates, corals, and algae species. Efficiency of habitat surrogacy varied by species group, type of habitat map, and spatial scale of analysis. Maps with the highest habitat thematic complexity provided better surrogates than simpler maps and were more robust to changes in spatial scales. Surrogates were most efficient for commercial fishes, corals, and algae but not for commercial invertebrates. Conversely, other measurements of species‐habitat associations, such as richness congruence and composition similarities provided weak results. We provide, in part, a habitat‐mapping methodology for designation of MPAs for Pacific Ocean islands that are characterized by habitat zonations similar to Wallis. Given the increasing availability and affordability of space‐borne imagery to map habitats, our approach could appreciably facilitate and improve current approaches to coral reef conservation and enhance MPA implementation.  相似文献   
960.
Abstract: The northernmost known breeding population of jaguars occurs in the municipality of Nácori Chico, Sonora, Mexico about 270 km from the United States–Mexico border and may be the source from which jaguars sighted in the United States dispersed. Since 1999 at least 11 jaguars (Panthera onca) had been illegally killed in the area due to predator control programs. We initiated a jaguar landowner‐based conservation plan in 2004. The eight participating landowners agreed to suspend predator control programs targeting jaguars and pumas (Puma concolor) only if cattle losses were compensated. A private outfitter, with the consent of landowners, initiated white‐tailed deer (Odocoileus virginianus) hunts in 2004 and agreed to pay the group of participating landowners US$1500 for every deer hunt permit sold. The funds paid to the landowners from deer hunts were sufficient to convince landowners to suspend all predator‐control efforts of jaguars and pumas. The involvement of landowners in the jaguar conservation program in northeastern Sonora is a successful, private, wildlife‐conservation initiative that provides an example for jaguar conservation efforts in northern Mexico.  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