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   17913篇
  免费   114篇
  国内免费   1篇
林业   3723篇
农学   1346篇
基础科学   148篇
  2947篇
综合类   964篇
农作物   2151篇
水产渔业   1989篇
畜牧兽医   1641篇
园艺   1145篇
植物保护   1974篇
  2023年   16篇
  2022年   10篇
  2021年   35篇
  2020年   52篇
  2019年   49篇
  2018年   2782篇
  2017年   2735篇
  2016年   1214篇
  2015年   103篇
  2014年   71篇
  2013年   80篇
  2012年   885篇
  2011年   2248篇
  2010年   2161篇
  2009年   1307篇
  2008年   1412篇
  2007年   1666篇
  2006年   121篇
  2005年   188篇
  2004年   200篇
  2003年   254篇
  2002年   137篇
  2001年   25篇
  2000年   45篇
  1999年   11篇
  1998年   12篇
  1997年   15篇
  1996年   7篇
  1995年   15篇
  1994年   11篇
  1993年   16篇
  1992年   12篇
  1991年   4篇
  1990年   9篇
  1989年   11篇
  1988年   15篇
  1987年   6篇
  1984年   5篇
  1982年   5篇
  1981年   6篇
  1980年   6篇
  1979年   3篇
  1978年   7篇
  1977年   12篇
  1976年   4篇
  1975年   5篇
  1973年   6篇
  1972年   4篇
  1970年   3篇
  1968年   4篇
排序方式: 共有10000条查询结果,搜索用时 765 毫秒
991.
This study reports on action research efforts that were aimed at developing institutional arrangements beneficial for soil fertility improvement. Three stages of action research are described and analyzed. We initially began by bringing stakeholders together in a platform to engage in a collaborative design of new arrangements. However, this effort was stymied mainly because conditions conducive for learning and negotiation were lacking. We then proceeded to support experimentation with alternative arrangements initiated by individual landowners and migrant farmers. The implementation of these arrangements too ran into difficulties due to intra-family dynamics and ambiguities regarding land tenure. Further investigations to find out how ambiguities could be tackled revealed that the local actors themselves had taken initiatives towards developing institutional innovations to reduce ambiguities. However, there is still considerable scope for further development of these self-organized innovations. The article ends with a reflection on inter-disciplinary action research, where it is argued that making “mistakes” is an inherent and necessary characteristic in action research that aims to address complex social issues.
Samuel Adjei-NsiahEmail:
  相似文献   
992.
The reaction to conventional agriculture and food systems has generated a host of alternative social movements in the past several decades. Many progressive agrifood researchers have researched these movements, exploring their strengths, weaknesses, and failures. Most such research is abstracted from the movements themselves. This paper proposes a new way of self-organization that, while fulfilling traditional university demands on researchers, will provide research support for progressive agrifood movements by transcending the boundaries of disciplines and individual universities.
William H. FriedlandEmail:

William H. Friedland   is Professor Emeritus at the University of California, Santa Cruz where his research continues on commodity systems, wine and grapes, the globalization of agriculture and food, and exploring ways to strengthen alternative social movements to subvert the dominant paradigm.  相似文献   
993.
Global allocation rules for patterns of biomass partitioning in seed plants   总被引:3,自引:0,他引:3  
A general allometric model has been derived to predict intraspecific and interspecific scaling relationships among seed plant leaf, stem, and root biomass. Analysis of a large compendium of standing organ biomass sampled across a broad sampling of taxa inhabiting diverse ecological habitats supports the relations predicted by the model and defines the boundary conditions for above- and below-ground biomass partitioning. These canonical biomass relations are insensitive to phyletic affiliation (conifers versus angiosperms) and variation in averaged local environmental conditions. The model thus identifies and defines the limits that have guided the diversification of seed plant biomass allocation strategies.  相似文献   
994.
Within-field variations in potential grain yield may be due to variations in plant available soil water. Different water holding capacities affect yield differently in different years depending on weather. By estimating plant-water availability in different weathers, scenarios could be created of how yield potential and thereby fertilizer demand may vary within fields. To test this, measured cereal grain yields from a dry, a wet and an intermediate year were compared with different soil moisture related variables in a Swedish arable field consisting of clayey and sandy areas. Soil water budget calculations based on weather data and maximum plant available water (PAW), estimated from soil type and rooting data, were used to assess drought. A reasonable correlation between estimated and measured soil moisture was achieved. In the dry year, drought days explained differences in yield between the clayey and the sandy soil, but yield was better explained directly by maximum PAW, elevation, clay content and soil electrical conductivity (SEC). Yield correlated significantly with SEC and elevation within the sandy soil in the dry year and within the clayey soil in the wet year, probably due to water and nitrogen limitation respectively. Dense SEC, elevation and yield data were therefore used to divide the field into management zones representing different risk levels for drought and waterlogging. These could be used as a decision support tool for site-specific N fertilization, since both drought and waterlogging affect N fertilization demand.  相似文献   
995.
Sequence variants in SLITRK1 are associated with Tourette's syndrome   总被引:1,自引:0,他引:1  
Tourette's syndrome (TS) is a genetically influenced developmental neuropsychiatric disorder characterized by chronic vocal and motor tics. We studied Slit and Trk-like 1 (SLITRK1) as a candidate gene on chromosome 13q31.1 because of its proximity to a de novo chromosomal inversion in a child with TS. Among 174 unrelated probands, we identified a frameshift mutation and two independent occurrences of the identical variant in the binding site for microRNA hsa-miR-189. These variants were absent from 3600 control chromosomes. SLITRK1 mRNA and hsa-miR-189 showed an overlapping expression pattern in brain regions previously implicated in TS. Wild-type SLITRK1, but not the frameshift mutant, enhanced dendritic growth in primary neuronal cultures. Collectively, these findings support the association of rare SLITRK1 sequence variants with TS.  相似文献   
996.
Sirt5 is a NAD-dependent protein lysine demalonylase and desuccinylase   总被引:1,自引:0,他引:1  
Du J  Zhou Y  Su X  Yu JJ  Khan S  Jiang H  Kim J  Woo J  Kim JH  Choi BH  He B  Chen W  Zhang S  Cerione RA  Auwerx J  Hao Q  Lin H 《Science (New York, N.Y.)》2011,334(6057):806-809
Silent information regulator 2 (Sir2) proteins (sirtuins) are nicotinamide adenine dinucleotide-dependent deacetylases that regulate important biological processes. Mammals have seven sirtuins, Sirt1 to Sirt7. Four of them (Sirt4 to Sirt7) have no detectable or very weak deacetylase activity. We found that Sirt5 is an efficient protein lysine desuccinylase and demalonylase in vitro. The preference for succinyl and malonyl groups was explained by the presence of an arginine residue (Arg(105)) and tyrosine residue (Tyr(102)) in the acyl pocket of Sirt5. Several mammalian proteins were identified with mass spectrometry to have succinyl or malonyl lysine modifications. Deletion of Sirt5 in mice appeared to increase the level of succinylation on carbamoyl phosphate synthase 1, which is a known target of Sirt5. Thus, protein lysine succinylation may represent a posttranslational modification that can be reversed by Sirt5 in vivo.  相似文献   
997.
Three archaeological sites on California's Channel Islands show that Paleoindians relied heavily on marine resources. The Paleocoastal sites, dated between ~12,200 and 11,200 years ago, contain numerous stemmed projectile points and crescents associated with a variety of marine and aquatic faunal remains. At site CA-SRI-512 on Santa Rosa Island, Paleocoastal peoples used such tools to capture geese, cormorants, and other birds, along with marine mammals and finfish. At Cardwell Bluffs on San Miguel Island, Paleocoastal peoples collected local chert cobbles, worked them into bifaces and projectile points, and discarded thousands of marine shells. With bifacial technologies similar to those seen in Western Pluvial Lakes Tradition assemblages of western North America, the sites provide evidence for seafaring and island colonization by Paleoindians with a diversified maritime economy.  相似文献   
998.
Analysis of the long-term Global Aerosol Climatology Project data set reveals a likely decrease of the global optical thickness of tropospheric aerosols by as much as 0.03 during the period from 1991 to 2005. This recent trend mirrors the concurrent global increase in solar radiation fluxes at Earth's surface and may have contributed to recent changes in surface climate.  相似文献   
999.
Through a discursive and organizational analysis we seek to understand the Biosafety Protocol and the place of socioeconomic regulation of agricultural biotechnology in it. The literature on the Protocol has been fairly extensive, but little of it has explored debates over socioeconomic regulation during the negotiation process or the regulatory requirements specified in the final document. This case is especially important at a time when the spread of neoliberalism is increasingly associated with deregulation, because it sheds light on the conditions under which circumvention of the market is deemed legitimate and socio-economic regulation of agricultural technology is possible. Daniel Lee Kleinman is a professor in the Department of Rural Sociology at the University of Wisconsin, Madison, where he is also affiliated with the Holtz Center for Science and Technology Studies and the Integrated Liberal Studies Program. He is the author and editor of a number of books, including Impure Cultures: University Biology and the World of Commerce (2003). Abby J. Kinchy is a PhD candidate in the Departments of Sociology and Rural Sociology at the University of Wisconsin, Madison. Her current research examines the controversies surrounding the genetic “contamination” of Mexican maize and Canadian canola.  相似文献   
1000.
Protein O-mannosylation is an essential and evolutionarily conserved post-translational modification among eukaryotes. This form of protein modification is also described in Mycobacterium tuberculosis; however, the mechanism of mannoprotein assembly remains unclear. Evaluation of differentially translocated chimeric proteins and mass spectrometry to monitor glycosylation demonstrated that specific translocation processes were required for protein O-mannosylation in M. tuberculosis. Additionally, Rv1002c, a M. tuberculosis membrane protein homolog of eukaryotic protein mannosyltransferases, was shown to catalyze the initial step of protein mannosylation. Thus, the process of protein mannosylation is conserved between M. tuberculosis and eukaryotic organisms.  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