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   17493篇
  免费   49篇
  国内免费   6篇
林业   3684篇
农学   1347篇
基础科学   138篇
  2905篇
综合类   778篇
农作物   2147篇
水产渔业   1866篇
畜牧兽医   1623篇
园艺   1123篇
植物保护   1937篇
  2023年   4篇
  2022年   11篇
  2021年   40篇
  2020年   16篇
  2019年   17篇
  2018年   2766篇
  2017年   2721篇
  2016年   1208篇
  2015年   99篇
  2014年   46篇
  2013年   86篇
  2012年   838篇
  2011年   2182篇
  2010年   2125篇
  2009年   1279篇
  2008年   1384篇
  2007年   1647篇
  2006年   98篇
  2005年   182篇
  2004年   159篇
  2003年   221篇
  2002年   101篇
  2001年   48篇
  2000年   74篇
  1999年   26篇
  1998年   14篇
  1997年   4篇
  1996年   8篇
  1995年   4篇
  1993年   14篇
  1992年   16篇
  1991年   6篇
  1990年   8篇
  1989年   13篇
  1988年   21篇
  1987年   7篇
  1986年   10篇
  1985年   10篇
  1984年   3篇
  1983年   3篇
  1979年   3篇
  1978年   1篇
  1977年   4篇
  1972年   1篇
  1971年   1篇
  1970年   7篇
  1969年   3篇
  1968年   4篇
  1967年   2篇
  1965年   1篇
排序方式: 共有10000条查询结果,搜索用时 296 毫秒
51.
52.
53.
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   
54.
The effects of ammonia and nitrite on vigour, survival rate, moulting rate of zoea of blue swimming crab, Portunus pelagicus, were studied. A total of five nitrite-N treatments (26.67, 53.34, 106.68, 213.36, 426.72 mg/l) and a control (no nitrite-N added) were set up for the acute nitrite-N toxicity experiment; a total of five ammonia-N treatments (8.43, 16.86, 33.72, 67.44, 134.88 mg/l) and a control (no ammonia-N added) were set up for the acute ammonia-N toxicity experiment. The results showed that the vigour, survival rate and moulting rate of zoea of the blue swimming crabs exposed to over 53.34 mg/l were significantly different (P < 0.05) from the control group. The zoea LC50 values (mg/l) of nitrite-N were 179.47, 76.56, 66.70, 37.49, 25.01, 25.35, 25.34 mg/l for 24, 36, 48, 60, 72, 84, 96 h, respectively. The vigour, survival rate and moulting rate of zoea of the blue swimming crabs exposed to over 16.86 mg/l were significantly different (P < 0.05) from the control group. The zoea LC50 values (mg/l) of ammonia-N were 51.04, 39.62, 38.72, 24.43, 16.90, 13.42, 11.16 mg/l for 24, 36, 48, 60, 72, 84, 96 h, respectively. The zoeae are highly sensitive to ammonia and nitrite, and the toxicity of ammonia and nitrite on Portunus pelagicus decrease with development of this crab.  相似文献   
55.
56.
Occupational exposure to nickel oxide (NiO) is an important cause of respiratory tract cancer. Toxicity is known to be associated with the dissociated component, i.e. nickel (II) ions. To address the relationship between physicochemical properties, including solubility in artificial lysosomal fluid, of NiO and time-course changes in the pulmonary response, we conducted an intratracheal instillation study in male Fischer rats using four different well-characterized NiO products, US3352 (NiO A), NovaWireNi01 (NiO B), I small particle (NiO C), and 637130 (NiO D). The NiOs were suspended in purified water and instilled once intratracheally into male F344 rats (12 weeks old) at 0 (vehicle control), 0.67, 2, and 6 mg/kg body weight. The animals were euthanized on days 3, 28, or 91 after instillation, and blood analysis, bronchoalveolar lavage fluid (BALF) testing, and histopathological examination were performed. The most soluble product, NiO B, caused the most severe systemic toxicity, leading to a high mortality rate, but the response was transient and surviving animals recovered. The second-most-soluble material, NiO D, and the third, NiO A, caused evident pulmonary inflammation, and the responses persisted for at least 91 days with collagen proliferation. In contrast, NiO C induced barely detectable inflammation in the BALF examination, and no marked changes were noted on histopathology. These results indicate that the early phase toxic potential of NiO products, but not the persistence of pulmonary inflammation, is associated with their solubility.  相似文献   
57.
58.
59.
The methane-mitigating potency of cashew nutshell liquid (CNSL) was evaluated by investigating gas production from batch cultures using feces from Thai native ruminants that had been incubated for different periods. Feces was obtained from four Thai native cattle and four swamp buffaloes reared under practical feeding conditions at the Kasetsart University farm, Thailand. Fecal slurry from the same farm was also included in the analysis. CNSL addition successfully suppressed the methane production potential of feces from both ruminants by shifting short chain fatty acid profiles towards propionate production. Methane mitigation continued for almost 150 days, although the degree of mitigation was more apparent from Day 0 to Day 30. Bacterial and archaeal community shifts with CNSL addition were observed in feces from both ruminants; specifically, Bacteroides increased, whereas Lachnospiraceae and Ruminococcaceae decreased in feces to which CNSL was added. Fecal slurry did not show marked changes in gas production with CNSL addition. The findings showed that the addition of CNSL to the feces of ruminants native to the Southeast Asian region can suppress methane emission. Because CNSL can be easily obtained as a byproduct of the local cashew industry in this region, its on-site application might be ideal.  相似文献   
60.
The growth and morphology of rumen methanogenic archaea (15 strains of 10 species in 5 genera, including 7 strains newly isolated in the present study) and bacteria (14 species in 12 genera) were investigated using unsupplemented in vitro pure cultures and cultures supplemented with cashew nut shell liquid (CNSL) and its phenolic compound components, anti-methanogenic agents for ruminant animals. Growth of most of the methanogens tested was inhibited by CNSL and alkylphenols at different concentrations ranging from 1.56 to 12.5 μg/ml. Of the alkylphenols tested, anacardic acid exhibited the most potent growth inhibition. Three gram-negative bacterial species involved in propionate production were resistant to CNSL and alkylphenols (>50 μg/ml). All the methanogens and bacteria that were sensitive to CNSL and alkylphenols exhibited altered morphology; disruption of the cell surface was notable, possibly due to surfactant activity of the tested materials. Cells division was inhibited in some organisms, with cell elongation and unclear septum formation observed. These results indicate that CNSL and alkylphenols, particularly anacardic acid, inhibit both rumen bacteria and methanogens in a selective manner, which could help mitigate rumen methane generation.  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