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   50333篇
  免费   522篇
  国内免费   434篇
安全科学   1450篇
废物处理   1937篇
环保管理   6886篇
综合类   7779篇
基础理论   14070篇
环境理论   27篇
污染及防治   13256篇
评价与监测   3131篇
社会与环境   2400篇
灾害及防治   353篇
  2022年   351篇
  2021年   367篇
  2020年   294篇
  2019年   386篇
  2018年   680篇
  2017年   706篇
  2016年   1023篇
  2015年   855篇
  2014年   1282篇
  2013年   4037篇
  2012年   1537篇
  2011年   2179篇
  2010年   1791篇
  2009年   1826篇
  2008年   2144篇
  2007年   2309篇
  2006年   2027篇
  2005年   1690篇
  2004年   1677篇
  2003年   1604篇
  2002年   1583篇
  2001年   1978篇
  2000年   1415篇
  1999年   875篇
  1998年   664篇
  1997年   686篇
  1996年   725篇
  1995年   787篇
  1994年   717篇
  1993年   639篇
  1992年   635篇
  1991年   608篇
  1990年   603篇
  1989年   632篇
  1988年   544篇
  1987年   502篇
  1986年   455篇
  1985年   486篇
  1984年   484篇
  1983年   508篇
  1982年   513篇
  1981年   455篇
  1980年   398篇
  1979年   430篇
  1978年   366篇
  1977年   319篇
  1976年   298篇
  1975年   300篇
  1973年   307篇
  1972年   322篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
Following the renewed effort at achieving a new green revolution for Africa, emphasis has been placed on modernizing smallholder agriculture through the deployment of improved inputs especially mechanized technologies. In Ghana, the government has in the last decade emphasized the provision of subsidized mechanized ploughing services to farmers alongside a rapidly growing private sector tractor service market. While mechanized technology adoption rates have increased rapidly, the deployment of these technologies has been without critical analysis of the impacts on production patterns and local agrarian systems. This paper examines the distributional impacts of agriculture mechanization on cropping patterns and farm sizes of smallholder farmers in northern Ghana using Geographic Information Systems (GIS) techniques, and semi-structured interviews with smallholder farmers (n=60). Specifically, comparative analysis of the field sizes and cropping patterns of participant farmers prior to and after the adoption of mechanized technologies was conducted. In-depth interviews were used to contextualize the experiences of smallholder farmers toward understanding how mechanization may be impacting traditional agriculture. Our findings reveal a mechanization paradox in which farm sizes are expanding, while cropping patterns are shifting away from traditional staple crops (pearl millet and sorghum bicolor) to market-oriented crops (maize, rice and groundnuts). This transition we argue, has adverse implications on the cultural dimension of food security, the organization of social life, and climate change adaptation. We recommend a retooling of the current agricultural policy focus to ensure context sensitivity for a more robust battle against food insecurity.  相似文献   
22.
23.
24.
Journal of Material Cycles and Waste Management - The traditional Chinese medicinal plant Magnolia officinalis has a wide range of applications; including more than 200 kinds of patented Chinese...  相似文献   
25.
Journal of Material Cycles and Waste Management - The present research work focused on fabricating Biodegradable Plate (BD plate) composed of rice husk ash, bagasse and corn starch which is...  相似文献   
26.
Abstract:  Introduction of artificial light into wildlife habitat represents a rapidly expanding form of human encroachment, particularly in coastal systems. Light pollution alters the behavior of sea turtles during nesting; therefore, long-wavelength lights—low-pressure sodium vapor and bug lights—that minimize impacts on turtles are required for beach lighting in Florida (U.S.A.). We investigated the effects of these two kinds of lights on the foraging behavior of Santa Rosa beach mice (  Peromyscus polionotus leucocephalus ). We compared patch use and giving-up densities of mice for experimental food patches established along a gradient of artificial light in the field. Mice exploited fewer food patches near both types of artificial light than in areas with little light and harvested fewer seeds within patches near bug lights. Our results show that artificial light affects the behavior of terrestrial species in coastal areas and that light pollution deserves greater consideration in conservation planning.  相似文献   
27.
Introduced and cryptogenic species in Port Phillip Bay, Victoria, Australia   总被引:4,自引:0,他引:4  
Port Phillip Bay (PPB) is a large (1,930 km2), temperate embayment in southern Victoria, Australia. Extensive bay-wide surveys of PPB have occurred since 1840. In 1995/1996 the Commonwealth Scientific and Industrial Research Organization (CSIRO) Centre for Research on Introduced Marine Pests (CRIMP) undertook an intensive evaluation of the region with the aims of developing a comprehensive species list of native and introduced biota and contrasting previous bay-wide assessments with a current field survey in order to detect new incursions and discern alterations to native communities. Two methods were used to meet these aims: a re-evaluation of regional museum collections and published research in PPB to identify and determine the timing of introductions; and field surveys for benthic (infauna, epifauna and encrusting) organisms between September 1995 to March 1996. One hundred and sixty introduced (99) and cryptogenic (61) species were identified representing over 13% of the recorded species of PPB. As expected, the majority of these are concentrated around the shipping ports of Geelong and Melbourne. Invasions within PPB appear to be increasing, possibly due to an increase in modern shipping traffic and an increase in aquaculture (historically associated with incidental introductions); however the records of extensive biological surveys suggest that this may, in part, be an artefact of sampling effort. In contrast to Northern Hemisphere studies, PPB (and Southern Hemisphere introductions in general) have significantly different suites of successfully invading taxa. PPB is presented as one of the most invaded marine ecosystems in the Southern Hemisphere.Communicated by M.S. Johnson, Crawley  相似文献   
28.
During the artificial reproduction of salmonides, the fecundity can be increased either by improving the viability of spermatozoa, or by extending the time period during which a roe micropile remains open, thus allowing its fecundation. Practically, this can be achieved by the use of some fertilising techniques suitable for fish species. Here, we show that the reproduction of rainbow trout in a 1:1 solution of deuterium-depleted water and distilled water led to a significant increase in survival of roes during their embryonic development. Moreover, the addition of glucose and fructose into the deuterium-depleted fertilising solutions led to a further increase in roe survival during embryonic development. The increase in survival is mainly explained by an increase in the motility of spermatozoa.  相似文献   
29.
30.
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