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   1316篇
  免费   95篇
  国内免费   5篇
耳鼻咽喉   12篇
儿科学   46篇
妇产科学   23篇
基础医学   125篇
口腔科学   16篇
临床医学   203篇
内科学   253篇
皮肤病学   31篇
神经病学   49篇
特种医学   144篇
外科学   228篇
综合类   35篇
一般理论   2篇
预防医学   144篇
眼科学   12篇
药学   45篇
肿瘤学   48篇
  2023年   10篇
  2021年   30篇
  2020年   10篇
  2019年   11篇
  2018年   31篇
  2017年   25篇
  2016年   19篇
  2015年   24篇
  2014年   42篇
  2013年   70篇
  2012年   41篇
  2011年   34篇
  2010年   51篇
  2009年   35篇
  2008年   53篇
  2007年   44篇
  2006年   38篇
  2005年   48篇
  2004年   39篇
  2003年   45篇
  2002年   47篇
  2001年   36篇
  2000年   34篇
  1999年   36篇
  1998年   28篇
  1997年   38篇
  1996年   32篇
  1995年   22篇
  1994年   23篇
  1993年   16篇
  1992年   26篇
  1991年   17篇
  1990年   18篇
  1989年   23篇
  1988年   29篇
  1987年   28篇
  1986年   20篇
  1985年   25篇
  1984年   26篇
  1983年   12篇
  1982年   17篇
  1981年   10篇
  1980年   14篇
  1979年   13篇
  1978年   19篇
  1977年   12篇
  1976年   11篇
  1975年   10篇
  1973年   10篇
  1970年   8篇
排序方式: 共有1416条查询结果,搜索用时 15 毫秒
21.
22.
23.
24.
Agriculture is being challenged to provide food, and increasingly fuel, for an expanding global population. Producing bioenergy crops on marginal lands—farmland suboptimal for food crops—could help meet energy goals while minimizing competition with food production. However, the ecological costs and benefits of growing bioenergy feedstocks—primarily annual grain crops—on marginal lands have been questioned. Here we show that perennial bioenergy crops provide an alternative to annual grains that increases biodiversity of multiple taxa and sustain a variety of ecosystem functions, promoting the creation of multifunctional agricultural landscapes. We found that switchgrass and prairie plantings harbored significantly greater plant, methanotrophic bacteria, arthropod, and bird diversity than maize. Although biomass production was greater in maize, all other ecosystem services, including methane consumption, pest suppression, pollination, and conservation of grassland birds, were higher in perennial grasslands. Moreover, we found that the linkage between biodiversity and ecosystem services is dependent not only on the choice of bioenergy crop but also on its location relative to other habitats, with local landscape context as important as crop choice in determining provision of some services. Our study suggests that bioenergy policy that supports coordinated land use can diversify agricultural landscapes and sustain multiple critical ecosystem services.In agricultural landscapes, balancing the provisioning of food and energy with maintenance of biodiversity and ecosystem functions is a global challenge. To avoid impacts on food production, attention is increasingly being focused on the potential for marginal lands to support bioenergy production (1). Marginal lands, those suboptimal for food production, may consist of relatively small areas within generally productive landscapes or larger regions where conditions generally limit crop productivity. However, there is increasing recognition that these lands are already performing a variety of useful functions, and their conversion to bioenergy cropping could reduce these services. For example, in the north central United States, rising commodity prices are predicted to bring marginal croplands—including Conservation Reserve Program lands—into annual crop production with negative impacts on wildlife habitat and water quality (2, 3). With 2013 corn plantings at recent record highs (4) and new reports of grassland and wetland conversion to cropland (5, 6), this may be occurring already.An alternative to annual cropping is conversion of marginal croplands to perennial, cellulosic crops for bioenergy. Although current US biofuel production centers on grain ethanol derived from annual monocultures of maize (Zea mays), this situation could change with full implementation of the 2007 US Energy Independence and Security Act (7), which calls for increased production of cellulosic biofuels. In the Midwest United States, perennial grasses and forbs grown on marginal lands could provide up to 25% of national targets for cellulosic biofuel, with substantial greenhouse gas (GHG) benefits (8). Moreover, increasing the area of perennial cover on the landscape is predicted to positively affect a diverse array of organisms and ecological functions (911), leading to important synergies that have not yet informed the ongoing bioenergy debate. Here we provide the most comprehensive empirical evaluation of this hypothesis to date, reporting data that elucidate the impacts of different bioenergy cropping systems on a wide variety of organisms and the ecosystem functions they perform.Previous studies have examined the ability of select bioenergy crops to support specific taxa (12) or individual services such as energy production (13) or GHG mitigation (14), without consideration of the tradeoffs or synergies that can arise when considering entire suites of organisms and ecosystem functions. We report on a unique multidisciplinary study of matched sets of organisms and ecosystem services and show that perennial grass energy crops (switchgrass, Panicum virgatum, and mixed prairie plantings) synergistically enhance diversity of a variety of organisms and levels of the services they provide. We further quantify the importance of landscape context on service provisioning, suggesting that policy supporting intentional design of bioenergy landscapes could increase sustainability of both food and energy production.  相似文献   
25.
26.
A World Health Organization requirement for biological standards is that they should exhibit long-term stability at their recommended storage temperature. Thermal stability is usually predicted in accelerated thermal degradation studies, where ampoules of the lyophilized standard are stored at elevated temperatures for relatively short times before testing. To confirm the predicted thermal stability of the 2nd international standard of human interferon alpha 2b (IFN-alpha2b; 95/566), we tested the potency of the ampouled contents of this standard after 9 years storage at the customary storage temperature of -20 degrees C in comparison with ampoules of the IS which had been stored continuously at temperatures ranging from -150 degrees C to 56 degrees C. Since IFN-alpha2b potency estimates derived from the results of antiviral assays (AVA) showed high within-assay variability, we investigated a novel reporter gene assay (RGA) based on induction of secreted alkaline phosphatase (SEAP) for comparability and precision of such estimations. We show that this RGA generated comparable estimates with overall lower variation. Additionally, the SEAP conversion of p-nitrophenyl phosphate to yellow product could be followed kinetically. Absorbance readings were shown to increase with time in proportion with increasing concentration of IFN-alpha2b. When the time-dependent increments of absorbance were plotted graphically, the slopes of lines corresponded to concentration. This approach enabled single dilutions of IFN samples, identical in molecular structure to an IFN-alpha2b standard, to be used for potency estimates by interpolation of slope value against those of the standard at fixed concentrations. It appears attractive for high through-put potency testing of various R&D IFN-alpha2b samples.  相似文献   
27.
28.
29.
Dear editor,Peripheral venous blood gas(VBG)analysis is increasingly used as an alternative to arterial sampling in Emergency Departments throughout the world.[1]There are multiple advantages using peripheral venous samples for blood gas analysis-technical ease,reduced pain and fewer complications.The difference in sample site chosen for blood gas analysis between European and Australian centres has been notable for members of our author group,prompting discussion and review of the literature.  相似文献   
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