全文获取类型
收费全文 | 10191篇 |
免费 | 797篇 |
国内免费 | 457篇 |
专业分类
林业 | 2638篇 |
农学 | 456篇 |
基础科学 | 35篇 |
1036篇 | |
综合类 | 3336篇 |
农作物 | 358篇 |
水产渔业 | 1355篇 |
畜牧兽医 | 913篇 |
园艺 | 408篇 |
植物保护 | 910篇 |
出版年
2024年 | 59篇 |
2023年 | 181篇 |
2022年 | 226篇 |
2021年 | 329篇 |
2020年 | 339篇 |
2019年 | 391篇 |
2018年 | 273篇 |
2017年 | 424篇 |
2016年 | 447篇 |
2015年 | 383篇 |
2014年 | 493篇 |
2013年 | 631篇 |
2012年 | 674篇 |
2011年 | 724篇 |
2010年 | 618篇 |
2009年 | 598篇 |
2008年 | 576篇 |
2007年 | 613篇 |
2006年 | 517篇 |
2005年 | 430篇 |
2004年 | 331篇 |
2003年 | 295篇 |
2002年 | 220篇 |
2001年 | 215篇 |
2000年 | 183篇 |
1999年 | 162篇 |
1998年 | 153篇 |
1997年 | 116篇 |
1996年 | 119篇 |
1995年 | 127篇 |
1994年 | 103篇 |
1993年 | 94篇 |
1992年 | 106篇 |
1991年 | 91篇 |
1990年 | 71篇 |
1989年 | 50篇 |
1988年 | 32篇 |
1987年 | 15篇 |
1986年 | 9篇 |
1985年 | 4篇 |
1984年 | 1篇 |
1983年 | 2篇 |
1982年 | 5篇 |
1981年 | 5篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1978年 | 3篇 |
1976年 | 2篇 |
1955年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
12.
13.
转基因植物对环境生态平衡的影响 总被引:1,自引:0,他引:1
根据近年来国内外关于转基因植物的研究进展,结合维持生态平衡的3个主要因素,即物种组成、环境因素及信息系统,论述转基因植物对环境生态平衡的影响. 相似文献
14.
15.
17.
对四川西北部高寒草甸草地的5种潜在生态草种不同生育期的营养成分进行了分析,结果表明,各草种不同生育期的粗水分、干物质、粗蛋白、粗脂肪、粗纤维、粗灰分等含量不同.其中赖草的干物质、蛋白质、粗脂肪、无氮浸出物和粗灰分含量显著高于川草Ⅰ号、川草Ⅱ号、垂穗鹅冠草和硬杆仲冰草等4种草.5种草不同生育期营养成分的分析结果表明,各草种分蘖期和孕穗期的粗水分、粗灰分、蛋白质、粗脂肪以及钙、磷含量明显高于结实期,而粗纤维、干物质和除赖草外的无氮浸出物含量则相反.表明在利用这些草种的过程中应抓好分蘖期的管理工作,以达到充分利用其饲用价值的目的. 相似文献
19.
The distribution of woody vegetation was studied in forest edges and hedgerows in a 28 km2 southern Swedish agricultural area, characterised by species-rich edge zones. The occurrence of 21 selected woody species (taxa) was related to differences in both edge structure and landscape structure. All the species studied were represented in both edge types, but a higher frequency of animal-dispersed species was found in hedgerows.Animal dispersed species were more affected by edge width and density than wind dispersed species. A higher number of wind-dispersed species were more frequent in forest edges, in hedgerows near to forest, or with a high proportion of forest within 500 m. A clear relationship was found between the number of physically connected elements in hedgerow networks and increasing frequency of occurrence for Corylus avellana, Crataegus spp., Euonymus europaeus, and Quercus robur; which indicate the ecological significance of connectedness for certain animal dispersed species. The study supports the general principle that woody species distribution and landscape structure are linked in a positive feedback loop. The results match findings from studies in other countries and are interpreted in the context of landscape processes and the ecological characteristics of woody plant species. We emphasise the importance of understanding dispersal mechanisms of woody species for the design and improvement of edge habitats in agricultural landscapes. 相似文献
20.
- 1. Three classes of habitat used by groups of fish species classified as conservation and management priorities were developed for the Gerua River (also known as the Girwa River, Karnali River) in the Ganges river basin. This river is large (mean annual discharge ca 1500 m3 s?1, up to 900 m wide), surrounded by protected lands of India and Nepal, and upstream of major diversions and river alterations.
- 2. Fish and habitat sampling was conducted at 45 sites from 2000 to 2003. Data were analysed for 2172 fish of 14 species. Species and life stages found occupying a statistically distinct subset of the river habitats were grouped to identify classes of river habitat for conservation.
- 3. Most species and life‐stage groups specialized on specific habitat conditions revealed by multivariate analyses of variance and a principal component analysis. The most numerous and diverse group (six species, 15 life stages) was associated with deep depositional habitats with sandy substrate. Two species covering three life stages were primarily oriented to erosional habitat marked by fast current velocity with pebble and cobble substrate. A third group of three species of adults and juveniles were intermediate in habitat use.
- 4. River conservation for fish faunas should maintain both erosional and depositional channel habitats with depths, substrates, and current velocity inclusive of the ranges reported. The erosional and depositional nature of the key habitats requires that rivers be maintained with flows capable of channel‐forming functions.