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   20854篇
  免费   258篇
  国内免费   5篇
林业   3818篇
农学   1499篇
基础科学   153篇
  3557篇
综合类   1099篇
农作物   2630篇
水产渔业   2173篇
畜牧兽医   2900篇
园艺   1185篇
植物保护   2103篇
  2023年   30篇
  2022年   68篇
  2021年   168篇
  2020年   111篇
  2019年   130篇
  2018年   2840篇
  2017年   2784篇
  2016年   1304篇
  2015年   222篇
  2014年   226篇
  2013年   298篇
  2012年   1107篇
  2011年   2467篇
  2010年   2291篇
  2009年   1459篇
  2008年   1583篇
  2007年   1852篇
  2006年   307篇
  2005年   327篇
  2004年   324篇
  2003年   350篇
  2002年   207篇
  2001年   146篇
  2000年   137篇
  1999年   62篇
  1998年   32篇
  1997年   24篇
  1996年   11篇
  1995年   11篇
  1993年   27篇
  1992年   28篇
  1991年   14篇
  1990年   23篇
  1989年   23篇
  1988年   19篇
  1987年   11篇
  1986年   7篇
  1985年   5篇
  1984年   4篇
  1983年   6篇
  1980年   6篇
  1978年   11篇
  1977年   6篇
  1974年   5篇
  1972年   6篇
  1971年   3篇
  1970年   3篇
  1969年   4篇
  1968年   8篇
  1967年   3篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
61.
62.
63.
64.
This study was conducted to evaluate the effects of dietary supplementation of Barodon, an anionic alkali mineral complex, on growth, feed utilization, humoral innate immunity and disease resistance of olive flounder. A basal experimental diet was used as a control and supplemented with 0.1, 0.2, 0.3, 0.4, or 0.5% Barodon. Triplicate groups of fish (26.4 ± 0.2 g) were fed one of the diets to apparent satiation twice daily for 10 wk. The growth performance was enhanced (P < 0.05) linearly and quadratically in fish fed diets containing Barodon compared with that in fish fed the control. Feed utilization was significantly improved by Barodon supplementation. Serum lysozyme and antiprotease activities were increased quadratically in Barodon fed groups. Also, significantly higher superoxide dismutase activity was found in Barodon‐fed fish. Dietary supplementation of 0.1–0.3% Barodon resulted in significant enhancement of fish disease resistance against Streptococcus iniae. The findings in this study indicate that dietary supplementation of Barodon can enhance growth, feed utilization, innate immunity, and disease resistance of olive flounder and that the optimum level seems to be 0.1% in diets.  相似文献   
65.
66.
A new functional porous carbons (PC-WF) is prepared by activation-pyrolysis method use waste cotton fabrics (WF) as an abundant, cheap and available precursor for removal of Brilliant Crocein (BC-GR) and Cationic Red 2GL (CR-GL) from aqueous phase. The PC-WF was characterized by BET, FTIR, SEM, and XRD techniques, the surface area, total pore volume, average pore diameter was found as 1463.5 m2 g-1, 0.783 cm3 g-1 and 2.14 nm, respectively. The influences on BC-GR and CR-GL adsorption of various experimental factors such as initial concentration and temperature were investigated. Adsorption kinetics was found to be best represented by the pseudo-second order model. The adsorption capacity was 319.8 mg g-1 for BC-GR and 842.5 mg g-1 for CR-GL at 30 °C, respectively. The results indicate that for waste cotton fabrics in particular, the practical application of this process to the production of porous carbon would be possible.  相似文献   
67.
68.
69.
70.
Forest losses or gains have long been recognized as critical processes modulating the carbon flux between the biosphere and the atmosphere. Timely, accurate and spatially explicit information on forest disturbance and recovery history is required for assessing the effectiveness of existing forest management. The major objectives of our research focused on testing the mapping efficacy of the vegetation change tracker (VCT) model over a forested area in China. We used a new version of VCT algorithm built upon the Landsat time series stacks (LTSS). The LTSS consisted of yearly image acquisitions to map forest disturbance history from 1987 to 2011 over the Ning-Zhen Mountains, Jiangsu Province of east China. The LTSS consisted of TM and ETM+ scenes with different projections due to distinct data sources (Beijing remote sensing ground station and the USGS EROS Center). The validation results of the disturbance year maps showed that most spatial agreement measures ranged from 70 to 86 %, comparable with the VCT accuracies reported for many places in USA. Very low accuracies were identified in 1995 (38.3 %) and 1992 (56.2 %) in the current analysis. These resulted from the insensitivity of the VCT algorithm to detect low intensity disturbances and also from the mis-registration errors of the image pairs. Major forest disturbance types existing in our study area were identified as agricultural expansion (39.8 %), urbanization (24.9 %), forest management practice (19.3 %), and mining (12.8 %). In general, there was a gradual decreasing trend in forest cover throughout this region, caused principally by China’s economic, demographic, environmental and political policies and decisions, as well as some weather events. While VCT has largely been used to assess long term changes and trends in the USA, it has great potential for assessing landscape level change elsewhere throughout the world.  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