全文获取类型
收费全文 | 17924篇 |
免费 | 90篇 |
国内免费 | 1篇 |
专业分类
林业 | 3808篇 |
农学 | 1337篇 |
基础科学 | 147篇 |
3139篇 | |
综合类 | 885篇 |
农作物 | 2147篇 |
水产渔业 | 1848篇 |
畜牧兽医 | 1533篇 |
园艺 | 1146篇 |
植物保护 | 2025篇 |
出版年
2023年 | 15篇 |
2022年 | 27篇 |
2021年 | 34篇 |
2020年 | 39篇 |
2019年 | 35篇 |
2018年 | 2771篇 |
2017年 | 2744篇 |
2016年 | 1217篇 |
2015年 | 102篇 |
2014年 | 66篇 |
2013年 | 81篇 |
2012年 | 894篇 |
2011年 | 2240篇 |
2010年 | 2161篇 |
2009年 | 1320篇 |
2008年 | 1413篇 |
2007年 | 1677篇 |
2006年 | 107篇 |
2005年 | 183篇 |
2004年 | 171篇 |
2003年 | 221篇 |
2002年 | 129篇 |
2001年 | 33篇 |
2000年 | 57篇 |
1999年 | 21篇 |
1998年 | 13篇 |
1997年 | 11篇 |
1996年 | 10篇 |
1995年 | 16篇 |
1994年 | 4篇 |
1993年 | 18篇 |
1992年 | 14篇 |
1991年 | 10篇 |
1990年 | 9篇 |
1989年 | 10篇 |
1988年 | 17篇 |
1987年 | 11篇 |
1985年 | 8篇 |
1984年 | 5篇 |
1983年 | 6篇 |
1981年 | 5篇 |
1979年 | 7篇 |
1977年 | 7篇 |
1976年 | 7篇 |
1974年 | 5篇 |
1973年 | 7篇 |
1972年 | 5篇 |
1970年 | 7篇 |
1968年 | 5篇 |
1962年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
We examine changing production relations in the Mexican tequila industry to explore the ways in which large industrial firms
are using “reverse leasing arrangements,” a form of contract farming, to extend their control over small agave farmers. Under
these arrangements, smallholders rent their parcels to contracting companies who bring in capital, machinery, labor, and other
agricultural inputs. Smallholders do not have access to their land, nor do they make any of the management decisions. We analyze
the factors that have led some producers to participate in reverse leasing arrangements, while allowing other producers to
continue farming independently. In addition, we look at the ways in which farmers are responding to these new production relations
and constraints and the strategies that they are using to regain control over the production process.
相似文献
Sarah BowenEmail: |
992.
Körner C Asshoff R Bignucolo O Hättenschwiler S Keel SG Peláez-Riedl S Pepin S Siegwolf RT Zotz G 《Science (New York, N.Y.)》2005,309(5739):1360-1362
Whether rising atmospheric carbon dioxide (CO2) concentrations will cause forests to grow faster and store more carbon is an open question. Using free air CO2 release in combination with a canopy crane, we found an immediate and sustained enhancement of carbon flux through 35-meter-tall temperate forest trees when exposed to elevated CO2. However, there was no overall stimulation in stem growth and leaf litter production after 4 years. Photosynthetic capacity was not reduced, leaf chemistry changes were minor, and tree species differed in their responses. Although growing vigorously, these trees did not accrete more biomass carbon in stems in response to elevated CO2, thus challenging projections of growth responses derived from tests with smaller trees. 相似文献
993.
Food quality is an important issue on the global agenda, particularly in high- and middle-income economies, but of little concern in designing Mexico’s food policy. Food policy has focused on quantity and in the case of maize, on satisfying domestic demand by supporting large commercial agriculture and importing from abroad. However, and as argued in this paper, obtaining a food staple (maize-tortilla) of quality is also an important issue for rural households and contributes to motivating continued smallholder production. Based on case studies in the rural district of Atlacomulco, in the state of Mexico, as well as in two regions of the state of Chiapas, this paper analyzes the production and consumption strategies of rural households. We focus on goals of food security and quality and note differential trends among households of varying characteristics and local contexts. We find that the motivation of small-scale producers to grow maize should be supported by Mexico’s food policy. 相似文献
994.
Samuel Adjei-Nsiah Cees Leeuwis Ken E. Giller Thom W. Kuyper 《Agriculture and Human Values》2008,25(3):389-403
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.
相似文献
Samuel Adjei-NsiahEmail: |
995.
William H. Friedland 《Agriculture and Human Values》2008,25(2):197-201
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. 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
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
996.
Neutrophil extracellular traps kill bacteria 总被引:5,自引:0,他引:5
Brinkmann V Reichard U Goosmann C Fauler B Uhlemann Y Weiss DS Weinrauch Y Zychlinsky A 《Science (New York, N.Y.)》2004,303(5663):1532-1535
Neutrophils engulf and kill bacteria when their antimicrobial granules fuse with the phagosome. Here, we describe that, upon activation, neutrophils release granule proteins and chromatin that together form extracellular fibers that bind Gram-positive and -negative bacteria. These neutrophil extracellular traps (NETs) degrade virulence factors and kill bacteria. NETs are abundant in vivo in experimental dysentery and spontaneous human appendicitis, two examples of acute inflammation. NETs appear to be a form of innate response that binds microorganisms, prevents them from spreading, and ensures a high local concentration of antimicrobial agents to degrade virulence factors and kill bacteria. 相似文献
997.
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
998.
Elucidation of the small RNA component of the transcriptome 总被引:3,自引:0,他引:3
Lu C Tej SS Luo S Haudenschild CD Meyers BC Green PJ 《Science (New York, N.Y.)》2005,309(5740):1567-1569
Small RNAs play important regulatory roles in most eukaryotes, but only a small proportion of these molecules have been identified. We sequenced more than two million small RNAs from seedlings and the inflorescence of the model plant Arabidopsis thaliana. Known and new microRNAs (miRNAs) were among the most abundant of the nonredundant set of more than 75,000 sequences, whereas more than half represented lower abundance small interfering RNAs (siRNAs) that match repetitive sequences, intergenic regions, and genes. Individual or clusters of highly regulated small RNAs were readily observed. Targets of antisense RNA or miRNA did not appear to be preferentially associated with siRNAs. Many genomic regions previously considered featureless were found to be sites of numerous small RNAs. 相似文献
999.
Networks adapt to environmental demands by switching between distinct dynamical behaviors. The activity of frontal-lobe neurons during two-interval discrimination tasks is an example of these adaptable dynamics. Subjects first perceive a stimulus, then hold it in working memory, and finally make a decision by comparing it with a second stimulus. We present a simple mutual-inhibition network model that captures all three task phases within a single framework. The model integrates both working memory and decision making because its dynamical properties are easily controlled without changing its connectivity. Mutual inhibition between nonlinear units is a useful design motif for networks that must display multiple behaviors. 相似文献
1000.
Söllner C Burghammer M Busch-Nentwich E Berger J Schwarz H Riekel C Nicolson T 《Science (New York, N.Y.)》2003,302(5643):282-286
The stone-like otoliths from the ears of teleost fishes are involved in balance and hearing and consist of calcium carbonate crystallites embedded in a protein framework. We report that a previously unknown gene, starmaker, is required in zebrafish for otolith morphogenesis. Reduction of starmaker activity by injection of modified antisense oligonucleotides causes a change in the crystal lattice structure and thus a change in otolith morphology. The expression pattern of starmaker, along with the presence of the protein on the growing otolith, suggest that the expression levels of starmaker control the shape of the otoliths. 相似文献