全文获取类型
收费全文 | 252814篇 |
免费 | 15531篇 |
国内免费 | 2440篇 |
专业分类
林业 | 24485篇 |
农学 | 14924篇 |
基础科学 | 3741篇 |
43238篇 | |
综合类 | 24517篇 |
农作物 | 18387篇 |
水产渔业 | 17398篇 |
畜牧兽医 | 92400篇 |
园艺 | 7684篇 |
植物保护 | 24011篇 |
出版年
2022年 | 2114篇 |
2021年 | 3309篇 |
2020年 | 3631篇 |
2019年 | 4376篇 |
2018年 | 4423篇 |
2017年 | 5080篇 |
2016年 | 5273篇 |
2015年 | 5243篇 |
2014年 | 6596篇 |
2013年 | 17345篇 |
2012年 | 7319篇 |
2011年 | 9236篇 |
2010年 | 8707篇 |
2009年 | 9008篇 |
2008年 | 8214篇 |
2007年 | 7160篇 |
2006年 | 7593篇 |
2005年 | 6563篇 |
2004年 | 6222篇 |
2003年 | 5751篇 |
2002年 | 5179篇 |
2001年 | 5942篇 |
2000年 | 5520篇 |
1999年 | 5089篇 |
1998年 | 4098篇 |
1997年 | 4142篇 |
1996年 | 3878篇 |
1995年 | 4399篇 |
1994年 | 3791篇 |
1993年 | 3513篇 |
1992年 | 3967篇 |
1991年 | 4225篇 |
1990年 | 3915篇 |
1989年 | 3845篇 |
1988年 | 3405篇 |
1987年 | 3453篇 |
1986年 | 3257篇 |
1985年 | 3616篇 |
1984年 | 3432篇 |
1983年 | 3199篇 |
1982年 | 2722篇 |
1981年 | 2645篇 |
1980年 | 2605篇 |
1979年 | 2927篇 |
1978年 | 2630篇 |
1977年 | 2448篇 |
1976年 | 2320篇 |
1975年 | 2130篇 |
1974年 | 2235篇 |
1973年 | 2180篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
31.
33.
Spatiotemporal patterns of mountain whitefish (Prosopium williamsoni) in response to a restoration of longitudinal connectivity 下载免费PDF全文
Peter M. Kiffney Ben Cram Paul L. Faulds Karl Burton Michele Koehler Thomas P. Quinn 《Ecology of Freshwater Fish》2018,27(4):1037-1053
To examine the role of longitudinal connectivity on the spatial and temporal dynamics of mountain whitefish (Prosopium williamsoni), we quantified movement and population dynamics following installation of the Landsburg Dam fishway, Cedar River, WA, USA. Mountain whitefish is widely distributed, poorly studied and not the focus of restoration. Before the fishway, mountain whitefish were not observed above the dam. Here, we focus on snorkel counts collected at reach and mesohabitat (e.g. pools) scales over 11 summers on the 20‐km above‐dam segment following restoration. A camera within the ladder provided number, size and movement timing, thereby informing on behaviour and recolonisation. Segment‐scale abundance increased following fish passage reaching an asymptote in 7 years, and mountain whitefish were detected throughout the main stem in 10 years. Annual movement through the ladder increased over time and was positively correlated with instream abundance and discharge, but negatively correlated with water temperature. About 60% of fish movements occurred in spring and early summer, potentially for foraging opportunities. Reach‐scale abundance peaked between 7 and 10 km from the dam; deep, cool (~10.6 to 11.6°C) conditions characterised these reaches. At the mesohabitat scale, mountain whitefish detection increased with depth and velocity after accounting for distance from the dam. Our results show how restoring longitudinal connectivity allowed this nontarget species to colonise newly available habitat. Their response supports the critical roles of longitudinal connectivity and environmental conditions, that manifest at different spatial scales, in dictating how freshwater fish respond to habitat disturbance. 相似文献
34.
35.
Soil protection for a sustainable future: options for a soil monitoring network for Ireland 下载免费PDF全文
The increased recognition of the importance of soil is reflected in the UN Post‐2015 Development Agenda with sustainable development goals that directly and indirectly relate to soil quality and protection. Despite a lack of legally binding legislation for soil protection, the European Commission remains committed to the objective of soil protection. However, the achievement of a legally binding framework for soil protection relies on the implementation of a soil monitoring network (SMN) that can detect changes to soil quality over time. As beneficiaries do not pay for the provision of soil information, the options for soil monitoring are limited. The use of existing data sets should be considered first. Using Ireland as an example, this research explored the opportunities for a SMN for Ireland considering three existing national data sets. The options for a SMN are considered in terms of their spatial and stratified distribution, the parameters to be measured and an economic analysis of the options proposed. This research finds that for Ireland, either a 10 or a 16 km2 grid interval stratified by land use and drainage class offers the best potential in relation to the spatial distribution of existing data sets to reflect local data at a national level. With existing data, the stratified SIS data using the 16 km2 grid offers the best value for money, with baseline costs for analysis, excluding field costs, of between €706 481 and €2.8 million. Acknowledging the impossibility of measuring all parameters with ideal frequency, this study proposes a two‐tier system for optimized monitoring frequency. Parameters must anticipate future policy requirements. Finally, the implementation of a SMN must be accompanied by standardized methods, defined thresholds and action mandates to maintain soil quality within allowable limits. 相似文献
36.
前期研究中通过对秀珍菇经低温诱导后不同发育阶段样本进行转录组测序及差异表达基因分析发现,ID为Cluster-6377.64510的基因(命名为PpFBD1)在原基形成前期的样本中高表达。为进一步了解分析其表达特性及功能,根据转录组测序结果设计特异性引物,利用RT-PCR技术从秀珍菇中克隆获得了该基因的cDNA全长。该基因由342个核苷酸组成,编码一个由113个氨基酸组成的蛋白,蛋白相对分子质量约11 ku。进化树分析显示,PpFBD1编码蛋白与糙皮侧耳hydrophobin1编码的疏水蛋白亲缘关系最近。Gene Ontology功能分析表明,PpFBD1主要参与真菌类细胞壁的合成。荧光定量RT-PCR检测显示,该基因在菌丝体经低温诱导后恢复至室温阶段(原基形成前期)表达量最高,这表明其可能参与调控秀珍菇原基形成过程。本研究结果为阐明秀珍菇原基形成机制提供参考。 相似文献
37.
38.
得天独厚的地理环境和悠久的野生食用菌历史,奠定了云南野生食用菌的重要地位与属性,云南也逐渐将野生食用菌资源纳入到生态休闲观光旅游的范畴,并成为云南旅游经济新的增长点。结合云南的野生食用菌栽培及旅游模式开发的现状,发现云南在野生食用菌生态休闲观光旅游探索上存在的问题,通过对问题的分析与解决,对云南野生食用菌的生态休闲观光旅游建设具有一定的指导意义。 相似文献
39.
40.
In a feeding trial lasting for months Halothane-positive fattening pigs were put on a diet containing different concentrations of vitamin E and selenium. After a Myostress application vitamin E and selenium were additionally injected in order to examine stress resistance (CK-test) and regenerative capacity (histological examination of Musculus longissimus dorsi) of skeletal muscle. The study revealed that neither high dietary levels of vitamin E/selenium nor a vitamin E/selenium treatment applied after stress are capable of preventing stress related myopathy nor can they influence its healing up favourably. 相似文献