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   2284篇
  免费   202篇
  国内免费   427篇
测绘学   75篇
大气科学   67篇
地球物理   455篇
地质学   445篇
海洋学   917篇
天文学   4篇
综合类   133篇
自然地理   817篇
  2024年   14篇
  2023年   55篇
  2022年   119篇
  2021年   135篇
  2020年   124篇
  2019年   124篇
  2018年   98篇
  2017年   124篇
  2016年   116篇
  2015年   120篇
  2014年   151篇
  2013年   160篇
  2012年   133篇
  2011年   165篇
  2010年   114篇
  2009年   113篇
  2008年   124篇
  2007年   120篇
  2006年   111篇
  2005年   92篇
  2004年   75篇
  2003年   70篇
  2002年   71篇
  2001年   47篇
  2000年   49篇
  1999年   50篇
  1998年   41篇
  1997年   30篇
  1996年   24篇
  1995年   24篇
  1994年   21篇
  1993年   21篇
  1992年   15篇
  1991年   7篇
  1990年   9篇
  1989年   9篇
  1988年   7篇
  1987年   5篇
  1986年   2篇
  1985年   3篇
  1984年   6篇
  1983年   4篇
  1982年   6篇
  1981年   4篇
  1980年   1篇
排序方式: 共有2913条查询结果,搜索用时 15 毫秒
991.
The authors developed a model to estimate annual tree growth based on tree-ring data (Abbr. TGTRing model) derived from the trunk at 0.5, 1.3 and 2.5 m height. This model was applied to estimate the annual biomass and carbon accumulation of a plantation in Qianyanzhou Red-Soil Hill Comprehensive Development Experimental Station of CAS in Taihe County, Jiangxi Province (Abbr. Qianyanzhou). The results showed that the inflexion points of the biomass and carbon accumulation curves occur at 17 and 18 years of age, respectively, in masson pine, whilst both inflexion points occurred at 15 years in slash pine and Chinese fir. The biomass and carbon accumulation in Chinese fir proved to be greater in the last 20 years than in the other species, with 171.697 t/hm2 and 92.29 tc/hm2, respectively. masson pine, with a biomass of 133.84 t/hm2 and a carbon accumulation of 73.92 tc/hm2 was the lowest whilst slash pine was intermediate with a biomass of 147.639 t/hm2 (unturpentined) and 135.743 t/hm2 (turpentined), and a carbon accumulation of 80.18 tc/hm2 (unturpentined) and 73.72 tc/hm2 (turpentined). In 2006, the total biomass and carbon storage of the tree stratum of masson pine in Qianyanzhou was 3324.43 t and 14,156.64 tc, respectively, whilst the values for Chinese fir were 1326.97 t and 713.27 tc. For slash pine the total biomass was 14,156.64 t (unturpentined) and 13,015.97 t (turpentined), and the total carbon storage was 7 688.21 tc (unturpentined) and 7068.78 tc (turpentined). Following the shaving of slash pine for resin, the total biomass was reduced by 1140.67 t and the total carbon storage fell by 619.43 tc.  相似文献   
992.
Ships carry ballast water for better stability and to control trim. However, the discharge of ballast water near ports is known to transport invasive species from one coastal area to another. The exchange of ballast water on the high seas is supposed to reduce such invasions of exotic species. In this study, we used denaturing gradient gel electrophoresis (DGGE) to analyze the composition of the bacterial community in ballast water before and after such a mid-ocean exchange, and we also measured total bacterial counts. Our findings confirmed that the ballast water was replaced by the mid-ocean exchange, as indicated by the marked change in the composition of the bacterial community. There was also a significant decrease in bacterial abundance after the mid-ocean exchange. Finally, our findings support the incubation hypothesis, because the composition of the bacterial communities changed over time within the same ballast water.  相似文献   
993.
994.
缅甸莫苇塘红土型镍矿成矿地质条件   总被引:4,自引:0,他引:4  
马绍春  郑国龙 《云南地质》2009,28(2):166-171
莫苇塘Mwetaung山硅酸镍矿属次生风化淋滤矿物。由硅酸镍赋存在蛇纹岩化基岩的溶液中,流动的渗透水、热带气候和地形条件均有利于该矿床的形成。  相似文献   
995.
996.
997.
钢筋混凝土框架中震可修标准及简化抗震设计方法   总被引:4,自引:1,他引:4  
本文对国际上主要建筑抗震设计规范中钢筋混凝土框架可修水准的层间位移角限值进行了比较,讨论了国内的一些相关研究结果,结合中国抗震规范确定钢筋混凝土框架中震可修层间位移角限值和屋顶侧移率限值分别为1/150和1/200。采用安全系数的抗震设计表达、论述了对应于结构层间位移角基于承载力的简化抗震设计方法。最后用实例按反应谱分析和弹性时程分析验证了钢筋混凝土框架中震可修层间位移角限值的有效控制作用,初步确定了简化抗震设计方法中梁柱构件的抗震安全系数并分析了提高目前结构抗震安全度的措施。  相似文献   
998.
BOOK REVIEWS     
Book Reviewed in this article: Women, Work, and Ideology in the Third World . Haleh Afshar, ed. The Birth of the National Park Service. The Founding Years 1913–33 . Horace M. Albright Theatres of Accumulation: Studies in Asian and Latin American Urbanization . Warwick Armstrong and T.G. McGee. New Tools for Social Scientists, Advances and Applications in Research Methods . William D. Berry and Michael S. Lewis -Beck , eds . Uncertain Dimensions: Western Overseas Empires in the Twentieth Century . Raymond F. Betts. Geography of Sugar Cane . Helmut Blume. The Mines of Neptune: Minerals and Metals from the Sea . Elisabeth Mann Borgese. Geographical Dimensions of Energy . F.J. Calzonetti and B.D. Solomon, eds. Statistical Methods for Geographers . W. A. V. Clark and P. L. Hosking. In The Wake of Columbus, Islands and Controversy . John DeVorsey, Jr., and John Parker, eds. Housing, States and Localities . Peter Dickens, Simon Duncan, Mark Goodwin and Fred Gray. Physician Location and Specialty Choice . Richard L. Ernst and Donald E. Yett. Practical Ecology for Geography and Biology: Survey, Mapping, and Data Analysis . D.D. Gilbertson , M. Kent and F.B. Pyatt Large Scale Water Transfers: Emerging Environmental and Social Experiences . Genady N. Golubev and Asit K. Biswas, eds. The Nature of the Environment . Andrew Goudie. Planning for Population Change . W.T.S. Gould and R. Lawton. The Niger and its Neighbors: Environmental History and Hydrobiology. Human Use and Health Hazards of the Major West African Rivers . A.T. Grove, ed. Remote Sensing of Ice and Snow . Dorothy K. Hall and Jaroslav Martinec. Atlas of Antebellum Southern Agriculture . Sam Bowers Hilliard. Galilee Divided; The Israel-Lebanon Frontier, 1916-1984 . Frederic C. Hor . Plains Country Towns . John C. Hudson. Agricultural Geography: A Social and Economic Analysis . Brian W. Ilbery. Industry in the 1980s: Structural Change and Interdependence . U.N. Industrial Development Organization. Crabgrass Frontier: The Suburbanization of the United States . Kenneth T. Jackson. The Industrial Geography of Italy . Russell King. Studies in Linguistic Geography: The Dialects of English in Britain and Ireland . John M. Kirk, Stewart Sanderson, J.D.A. Widdowson, eds. Tourism: A Community Approach . Peter E. Murphy. Cities and Services: The Geography of Collective Consumption . Steven Pinch. An Historical Geography of Europe 1800–1914 . Norman J. G. Pounds. An Introduction to Human Ecology Research on Agricultural Systems in Southeast Asia . A. Terry Rambo and Percy E. Sajise, eds. Discovering Past Landscapes . Michael Reed, ed. Managing the Ocean: Resources, Research, Law . Jacques G. Richardson, ed. Carta fitosociologica de la transecta botanica de la Patagonia Austral . F.A. Roig , Juan Anchorema , Orlando Dollenz , A.M. Faggi and Eduardo Mandez Intermetropolitan Migration in Canada: Changing Determinants over Three Decades . R. Paul Shaw. Chinese Cities—The Growth of the Metropolis since 1949 . Victor F. Sit, ed. Urbanization in the World-Economy . Michael Timberlake, ed. The Al Business: Commercial Uses of Artificial Intelligence . Patrick H. Winston and Karen A. Prendergast, eds.  相似文献   
999.
本研究通过构建16S r DNA克隆文库,对东海陆架五个站点的沉积物样品进行群落结构及地理分布研究。结果表明:东海陆架沉积物细菌分属于13个类群,包括变形菌门(Proteobacteria)、厚壁菌门(Firmicutes)、酸杆菌门(Acidobacteria)、绿弯菌门(Chloroflexi)、放线菌门(Actinobacteria)、硝化螺旋菌门(Nitrospira)、WS3、拟杆菌门(Bacteroidetes)、螺旋体门(Spirochaetes)、OP8、浮霉菌门(Planctomycetacia)、疣微菌门(Verrucomicrobiae)和芽单胞菌门(Gemmatimonadetes),其中变形菌门(γ-变形菌纲、δ-变形菌纲)和绿弯菌门为优势菌群,Latescibacteria(WS3)、疣微菌门和芽单胞菌门分别是站点DH6、DH21和DH9的特有类群。五个站点细菌多样性从高到低排序依次为DH9DH21DH17DH6DH16,其群落结构和丰度与各站点不同沉积环境类型有关。  相似文献   
1000.
广东省高州水库水华过程中蓝藻群落的动态特征   总被引:2,自引:2,他引:2  
2009年1-4月,高州水库发生了规模较大的粘质鱼腥藻(Anabaena mucosa)水华,最高藻细胞密度达3.4×107cells/L,并在2010年的同一时期再次出现,这也是粘质鱼腥藻首次在广东省水库水体形成水华.为了解该水库蓝藻水华发生的特点,本文于2009年8月到2010年7月对高州水库进行了逐月采样,分析了...  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