全文获取类型
收费全文 | 72334篇 |
免费 | 4362篇 |
国内免费 | 147篇 |
专业分类
林业 | 3001篇 |
农学 | 2598篇 |
基础科学 | 428篇 |
7950篇 | |
综合类 | 11904篇 |
农作物 | 2490篇 |
水产渔业 | 3689篇 |
畜牧兽医 | 39112篇 |
园艺 | 964篇 |
植物保护 | 4707篇 |
出版年
2018年 | 1204篇 |
2017年 | 1219篇 |
2016年 | 1104篇 |
2015年 | 1038篇 |
2014年 | 1117篇 |
2013年 | 3083篇 |
2012年 | 2108篇 |
2011年 | 2539篇 |
2010年 | 1714篇 |
2009年 | 1657篇 |
2008年 | 2358篇 |
2007年 | 2326篇 |
2006年 | 2128篇 |
2005年 | 2017篇 |
2004年 | 1856篇 |
2003年 | 1903篇 |
2002年 | 1761篇 |
2001年 | 2478篇 |
2000年 | 2397篇 |
1999年 | 1952篇 |
1998年 | 721篇 |
1997年 | 709篇 |
1996年 | 638篇 |
1995年 | 771篇 |
1994年 | 736篇 |
1993年 | 675篇 |
1992年 | 1518篇 |
1991年 | 1686篇 |
1990年 | 1523篇 |
1989年 | 1489篇 |
1988年 | 1417篇 |
1987年 | 1429篇 |
1986年 | 1413篇 |
1985年 | 1370篇 |
1984年 | 1113篇 |
1983年 | 1031篇 |
1982年 | 664篇 |
1979年 | 1022篇 |
1978年 | 787篇 |
1977年 | 760篇 |
1976年 | 756篇 |
1975年 | 780篇 |
1974年 | 938篇 |
1973年 | 902篇 |
1972年 | 859篇 |
1971年 | 788篇 |
1970年 | 825篇 |
1969年 | 795篇 |
1968年 | 699篇 |
1967年 | 699篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
利用云南29个气象站1960-2014年的日降水数据,计算集中指数(Concentration Index,CI)、降水集中指数(Precipitation Concentration Index,PCI)、降水集中度(Precipitation Concentration Degree,PCD)、降水集中期(Precipitation Concentration Period,PCP)、变差系数(Coefficient of Variation,CV)5个统计指标,分析了各指标之间的关系,并讨论了各指标的空间分布。结果表明:(1)CI介于0.62~0.71之间,平均为0.67,日降水高度集中。CI与年降水量、降水日数的相关系数分别为-0.50、-0.93,具有显著的负相关关系(α=0.05),年降水量和降水日数较少的地区,CI较大。CI与经度的相关系数为0.64,具有显著的正相关关系(α=0.05)。(2)PCI介于11.95~21.48之间,其中,3%的站点降水高度集中,52%的站点降水集中,45%的站点降水中度集中。PCD介于0.30~0.74之间,滇西北和滇东南地区的PCD较小。PCP的分析结果表明,云南降水主要集中在7、8月份。(3)CI和PCI、PCD分别关注降水的等级分布和集中程度,PCP则关注集中的时间段。PCI与PCD的空间分布情况基本上是一致的,且二者的相关系数为0.90,具有显著的正相关关系(α=0.05),故二者在一定程度上可以相互替换。采用CI、PCI和PCP 3种指标可以较好地描述降水的年内等级分布、集中程度和集中时段。(4)CV介于0.11~0.21之间,降水的年际变化随着纬度的减小、年降水量的增多呈减小的趋势,相关系数分别为0.39、-0.74;而年降水量随着海拔和纬度的升高呈下降的趋势,相关系数分别为-0.45、-0.53。 相似文献
2.
Olesya I. Zhuravleva Alexandr S. Antonov Vo Thi Dieu Trang Mikhail V. Pivkin Yuliya V. Khudyakova Vladimir A. Denisenko Roman S. Popov Natalya Y. Kim Ekaterina A. Yurchenko Andrey V. Gerasimenko Anatoly A. Udovenko Gunhild von Amsberg Sergey A. Dyshlovoy Shamil S. Afiyatullov 《Marine drugs》2021,19(1)
Seven new deoxyisoaustamide derivatives (1–7) together with known compounds (8–10) were isolated from the coral-derived fungus Penicillium dimorphosporum KMM 4689. Their structures were established using spectroscopic methods, X-ray diffraction analysis and by comparison with related known compounds. The absolute configurations of some alkaloids were determined based on CD and NOESY data as well as biogenetic considerations. The cytotoxic and neuroprotective activities of some of the isolated compounds were examined and structure-activity relationships were pointed out. New deoxyisoaustamides 4–6 at concentration of 1 µM revealed a statistical increase of PQ(paraquat)-treated Neuro-2a cell viability by 30–39%. 相似文献
3.
M. Manou J. Milgram P. Kelly S. Hoey K. Kenny S. Warde B. Kirby 《The Journal of small animal practice》2021,62(10):924-928
A 7-year-old male neutered domestic shorthair outdoor cat was referred for chronic left forelimb lameness, which had been treated with intra-articular injections of triamcinolone acetonide. A soft tissue swelling around the elbow joint, extending from the distal humerus to the proximal ulna, was surgically explored and biopsy samples obtained. Mycobacterium bovis was cultured from samples from the soft tissue and bone. The mycobacteria from the media were killed and the DNA extracted and tested on a multiplex real-time PCR for the absence of specific genes and the presence of mycobacterial genus markers. The PCR revealed bacillus Calmette-Guérin Danish Strain 1331; this was also isolated from the prescapular lymph node, muscle and bone, obtained at post mortem examination. Badgers had been vaccinated with the bacillus Calmette-Guérin vaccine SSI (Statens Serum Institute) in the area where the cat lived, in the spring and autumn of the previous year. To the authors' knowledge, this is the first report of infection with M. bovis bacillus Calmette-Guérin Danish Strain 1331 in a domestic cat, potentially associated with annual vaccination of badgers in the proximity of the cat's home. 相似文献
4.
为了分析猪源大肠杆菌中mcr-1阳性不可接合类噬菌体质粒pD72-mcr-1与blaCTX-M-55阳性可接合的F33:A-:B-质粒pD72-F33融合形成的共整合质粒pD72C的生物学特性,评估该融合质粒对mcr-1传播的潜在作用,本实验采用生长曲线测定、质粒稳定性实验、竞争性实验和接合实验,测定了融合质粒pD72C和亲本质粒pD72-F33对宿主菌的适应性、融合能力和扩散能力.结果表明:融合质粒pD72C在无抗生素的环境中能稳定传代9 d,对宿主菌有适应性优势,且比亲本质粒pD72-F33具有更好的竞争优势;该融合质粒表现出较高的融合频率和接合频率,说明该融合质粒能够扩展菌株的耐药谱并促进粘菌素耐药基因mcr-1的传播. 相似文献
5.
Dabrin A. Bégorre C. Bretier M. Dugué V. Masson M. Le Bescond C. Le Coz J. Coquery M. 《Journal of Soils and Sediments》2021,21(2):1256-1274
Journal of Soils and Sediments - Suspended particulate matter (SPM) transport through rivers is a major vector of nutrients and pollutants to continental shelf areas. To develop efficient sediment... 相似文献
6.
7.
Joanna Day David Power Rosemary Gales John Bannister Maxine P. Piggott Kerstin Bilgmann Robert Harcourt Luciano B. Beheregaray Luciana M. Möller 《水产资源保护:海洋与淡水生态系统》2021,31(6):1452-1465
- Understanding the factors driving population structure in marine mammals is needed to evaluate the impacts of previous exploitation, current anthropogenic threats, conservation status, and success of population recovery efforts.
- Sperm whales are characterized by a worldwide distribution, low genetic diversity, complex patterns of social and genetic structure that differ significantly within and between ocean basins, and a long history of being commercially whaled. In Australia, sperm whales from the (International Whaling Commission assigned) southern hemisphere ‘Division 5’ stock were very heavily exploited by whaling.
- The present study assessed the potential effects of whaling on the genetic diversity of sperm whales in Australia and the population genetic structure of these whales within a global context. A combination of historical and contemporary sperm whale samples (n = 157) were analysed across six regions, from south-eastern Australia (‘Division 6’ stock in the Pacific Ocean) to south-western Australia (‘Division 5’ stock in the Indian Ocean).
- Sperm whales sampled from the ‘Division 5’ and ‘Division 6’ stocks belong to the same population based on nuclear and mitochondrial DNA (mtDNA) analyses. Four novel sperm whale mtDNA haplotypes were identified in animals from Australian waters. Levels of genetic diversity were low in Australian sperm whales but were similar to those previously reported for populations in the Indian and Pacific Oceans.
- Given the genetic distinctiveness of sperm whales in Australian waters from other regions in the Pacific and Indian Oceans, and the lack of recovery in population numbers, further scientific studies are needed to increase our understanding of population dynamics and the effectiveness of threat management strategies in this species.
8.
Lorelei Ford Kuttichantran Subramaniam Thomas B. Waltzek Paul R. Bowser Larry Hanson 《Journal of fish diseases》2021,44(1):119-122
The channel catfish (Ictalurus punctatus, Rafinesque) ovary (CCO) cell line is the standard cell line used for channel catfish diagnostics. Next‐gen sequencing studies of a virus cultured in the CCO cells revealed mitochondrial sequences matching those of brown bullhead (Ameiurus nebulosus, Lesueur). Therefore, we systematically performed partial cytochrome oxidase 1 gene sequencing of several sources of the CCO cell line and all matched the brown bullhead and not the channel catfish. 相似文献
9.
Agroforestry Systems - Globally, forest ecosystems are shrinking and their health is declining due to a range of biotic and abiotic factors. Forest ecosystems in Pakistan... 相似文献
10.
- Many species and populations of odontocetes have modified their behaviour to take advantage of feeding opportunities provided by fishing activities, with depredation of fishing gear being the most common type of adaptation.
- The northern Adriatic Sea has been identified as an important marine mammal area because of a regular occurrence of common bottlenose dolphins. Boat surveys were conducted within a 3,000 km2 sector of the Adriatic Sea off the coast of Veneto, Italy, between April and October 2018–2019. Based on 76 days at sea, 10,711 km of navigation, and 81 h 26 min of dolphin tracking, this study contributes novel quantitative information on dolphin spatial distribution, and on their occurrence in the wake of beam trawlers, otter trawlers, and midwater pair trawlers.
- A combined generalized additive model and generalized estimation equation framework indicated that trawling—along with other physiographic, biological and anthropogenic variables—influenced dolphin distribution. In days of trawling, the chance of encountering dolphins increased by ~4.5 times (95% confidence interval 1.8–11.0) near active beam trawlers, by ~16.0 times (7.1–36.0) near otter trawlers, and by ~28.9 times (12.0–69.6) near midwater pair trawlers.
- Spatial modelling was used to create maps of predicted distribution, suggesting differences in habitat use between trawling and no-trawling days. Spatial modelling for all days identified a dolphin distribution hotspot of 832 km2, situated off the Po river delta.
- Evidence contributed by this study can be used to inform management action within one of the world's areas most heavily impacted by fishing and other human encroachment. Such management action would help enforce the European Union's Habitats Directive and Marine Strategy Framework Directive, while also informing EU's Maritime Spatial Planning.