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   33215篇
  免费   392篇
  国内免费   483篇
安全科学   1121篇
废物处理   1528篇
环保管理   4470篇
综合类   5320篇
基础理论   8626篇
环境理论   25篇
污染及防治   8663篇
评价与监测   2250篇
社会与环境   1883篇
灾害及防治   204篇
  2023年   146篇
  2022年   337篇
  2021年   340篇
  2020年   255篇
  2019年   311篇
  2018年   511篇
  2017年   536篇
  2016年   805篇
  2015年   618篇
  2014年   939篇
  2013年   2795篇
  2012年   1167篇
  2011年   1564篇
  2010年   1256篇
  2009年   1301篇
  2008年   1557篇
  2007年   1645篇
  2006年   1370篇
  2005年   1197篇
  2004年   1080篇
  2003年   1177篇
  2002年   1037篇
  2001年   1288篇
  2000年   927篇
  1999年   556篇
  1998年   397篇
  1997年   394篇
  1996年   407篇
  1995年   469篇
  1994年   501篇
  1993年   397篇
  1992年   418篇
  1991年   380篇
  1990年   410篇
  1989年   368篇
  1988年   325篇
  1987年   304篇
  1986年   252篇
  1985年   273篇
  1984年   292篇
  1983年   286篇
  1982年   283篇
  1981年   262篇
  1980年   196篇
  1979年   219篇
  1978年   200篇
  1977年   140篇
  1975年   149篇
  1972年   140篇
  1971年   136篇
排序方式: 共有10000条查询结果,搜索用时 593 毫秒
261.
Supplementing the nutrient requirement of crops through organic manures plays a key role in sustaining soil fertility, and crop productivity and reducing use of fossil fuels. Field experiments were conducted for 2 years at two different locations (i.e. Lucknow and Pantnagar) in Uttar Pradesh, India. The objectives of the study were to assess the herb and essential oil yields of Japanese mint (Mentha arvensis cv. Hy 77), and its nutrient accumulation under single and combined applications of organic manures and inorganic fertilizers (NPK). Changes in physical and chemical characteristics of the soils (Fluvisols, Mollisols) were also determined. Eight treatments comprising different combinations of NPK through inorganic fertilizers and farm yard manure (FYM) were compared. The distilled waste of mint after extraction of essential oil was recycled to soils in the plots to supplement the nutritional requirement of the succeeding mustard crop (Brassica juncea cv. Pusa Bold). Herb and essential oil yield of mint were significantly higher with combined application of organic and inorganic sources of nutrients as compared to single applications. Accumulation of N and P was at par under full inorganic and combined supply whereas, K accumulation was higher with the former. Soil organic C and pH after harvest of mint did not significantly differ among the treatments, but the level of mineralizable N, Olsen-P and NH4OAc extractable K were higher in soil with integrated supply of nutrients. Significant increase in soil water stable aggregates, organic C, available NPK and microbial biomass, and decrease in soil bulk density were observed with waste recycling over fertilizer application. These benefits were reflected in the seed and stubble yield of mustard which succeeded mint. This study indicates that combined application of inorganic fertilizers with organics helps in increasing the availability of nutrients and crop yield and provides a significant effect to the succeeding crop. Similarly, recycling crop residues reduces the need for fossil fuel based fertilizer, and helps in sustaining and restoring soil fertility in terms of available nutrients and major physical and chemical characteristics of the soil.  相似文献   
262.
Nectar acquisition in the honeybee Apis mellifera is a partitioned task in which foragers gather nectar and bring it to the hive, where nest mates unload via trophallaxis (i.e. mouth-to-mouth transfer) the collected food for further storage. Because forager mates exploit different feeding places simultaneously, this study addresses the question of whether nectar unloading interactions between foragers and hive-bees are established randomly, as it is commonly assumed. Two groups of foragers were trained to exploit a different scented food source for 5 days. We recorded their trophallaxes with hive-mates, marking the latter ones according to the forager group they were unloading. We found non-random probabilities for the occurrence of trophallaxes between experimental foragers and hive-bees, instead, we found that trophallactic interactions were more likely to involve groups of individuals which had formerly interacted orally. We propose that olfactory cues present in the transferred nectar promoted the observed bias, and we discuss this bias in the context of the organization of nectar acquisition: a partitioned task carried out in a decentralized insect society.  相似文献   
263.
The biological activity of the Matsucoccus spp. sex pheromones and diene modified analogs has been tested in forests of Israel and Portugal in order to explore the structure-activity relationship of the pine bast scale pheromone/kairomone system. The response of the adult predatory bugs, Elatophilus hebraicus and E. crassicornis and of the brown lacewing, Hemerobius stigma is more selective than that of the conspecific Matsucoccus males. The removal of the terminal methyl group from the diene terminus of both pheromones 1 and 2 eliminates all kairomonal activity but retains moderate pheromonal activity. Addition of a methyl group to the diene terminus of pheromones 1 and 2 sustains full pheromonal and kairomonal activities of the Elatophilus spp. but eliminates entirely the kairomonal activity of H. stigma. Subtle designed alterations in the structure of the diene group, typical of all Matsucoccus pheromones, change the mode of the kairomonal activity markedly.  相似文献   
264.
265.
266.
267.
268.
Exencephaly was diagnosed at 17 weeks in a 27-year-old primigravida with abnormalities of the hands and a family history suggestive of autosomal dominant brachydactyly and clinodactyly. In this family there was also a history of ‘anencephaly’. To our knowledge, this is the first report on the association of exencephaly and autosomal dominant brachydactyly. As the relationship between hand and cranial anomalies is well established, we suggest that this association in our case could be due to a defect in the same gene.  相似文献   
269.
270.
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