全文获取类型
收费全文 | 693篇 |
免费 | 2篇 |
国内免费 | 9篇 |
专业分类
系统科学 | 9篇 |
教育与普及 | 2篇 |
理论与方法论 | 22篇 |
现状及发展 | 86篇 |
研究方法 | 104篇 |
综合类 | 393篇 |
自然研究 | 88篇 |
出版年
2022年 | 1篇 |
2021年 | 4篇 |
2020年 | 2篇 |
2019年 | 2篇 |
2018年 | 3篇 |
2017年 | 6篇 |
2016年 | 9篇 |
2015年 | 4篇 |
2014年 | 4篇 |
2013年 | 11篇 |
2012年 | 43篇 |
2011年 | 148篇 |
2010年 | 28篇 |
2009年 | 4篇 |
2008年 | 51篇 |
2007年 | 53篇 |
2006年 | 44篇 |
2005年 | 56篇 |
2004年 | 60篇 |
2003年 | 53篇 |
2002年 | 45篇 |
2001年 | 2篇 |
2000年 | 5篇 |
1999年 | 6篇 |
1996年 | 3篇 |
1995年 | 2篇 |
1994年 | 2篇 |
1993年 | 1篇 |
1992年 | 2篇 |
1991年 | 1篇 |
1990年 | 4篇 |
1988年 | 3篇 |
1987年 | 2篇 |
1986年 | 4篇 |
1985年 | 7篇 |
1984年 | 6篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1980年 | 7篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1976年 | 2篇 |
1972年 | 1篇 |
1971年 | 1篇 |
1969年 | 1篇 |
1965年 | 3篇 |
1963年 | 3篇 |
排序方式: 共有704条查询结果,搜索用时 0 毫秒
701.
Stream denitrification across biomes and its response to anthropogenic nitrate loading 总被引:11,自引:0,他引:11
Mulholland PJ Helton AM Poole GC Hall RO Hamilton SK Peterson BJ Tank JL Ashkenas LR Cooper LW Dahm CN Dodds WK Findlay SE Gregory SV Grimm NB Johnson SL McDowell WH Meyer JL Valett HM Webster JR Arango CP Beaulieu JJ Bernot MJ Burgin AJ Crenshaw CL Johnson LT Niederlehner BR O'Brien JM Potter JD Sheibley RW Sobota DJ Thomas SM 《Nature》2008,452(7184):202-205
Anthropogenic addition of bioavailable nitrogen to the biosphere is increasing and terrestrial ecosystems are becoming increasingly nitrogen-saturated, causing more bioavailable nitrogen to enter groundwater and surface waters. Large-scale nitrogen budgets show that an average of about 20-25 per cent of the nitrogen added to the biosphere is exported from rivers to the ocean or inland basins, indicating that substantial sinks for nitrogen must exist in the landscape. Streams and rivers may themselves be important sinks for bioavailable nitrogen owing to their hydrological connections with terrestrial systems, high rates of biological activity, and streambed sediment environments that favour microbial denitrification. Here we present data from nitrogen stable isotope tracer experiments across 72 streams and 8 regions representing several biomes. We show that total biotic uptake and denitrification of nitrate increase with stream nitrate concentration, but that the efficiency of biotic uptake and denitrification declines as concentration increases, reducing the proportion of in-stream nitrate that is removed from transport. Our data suggest that the total uptake of nitrate is related to ecosystem photosynthesis and that denitrification is related to ecosystem respiration. In addition, we use a stream network model to demonstrate that excess nitrate in streams elicits a disproportionate increase in the fraction of nitrate that is exported to receiving waters and reduces the relative role of small versus large streams as nitrate sinks. 相似文献
702.
Optimization of Time-Varying Parking Charges and Parking Supply in Networks with Multiple User Classes and Multiple Parking Facilities 总被引:3,自引:0,他引:3
The optimization of parking charges and parking supply over the time of a day is an important problem in the design of transportation networks. This paper presents a bilevel model to determine the opti-mal time-varying parking charges and parking supply in road networks with multiple user classes and different types of parking facilities. The upper level of the model aims to maximize the network net benefit in response to the parking charges and parking supply, whereas the lower level is a time-dependent network equilibrium problem with elastic demand. A descent-gradient-based solution algorithm is adapted to solve the model. The numerical results show that the implementation of time-varying parking charges and parking supply is useful to effectively cater to the time-varying demand with different parking needs. The model provides a powerful tool for strategically designing parking locations and evaluating various parking policies. 相似文献
703.
Hemann MT Bric A Teruya-Feldstein J Herbst A Nilsson JA Cordon-Cardo C Cleveland JL Tansey WP Lowe SW 《Nature》2005,436(7052):807-811
The c-Myc oncoprotein promotes proliferation and apoptosis, such that mutations that disable apoptotic programmes often cooperate with MYC during tumorigenesis. Here we report that two common mutant MYC alleles derived from human Burkitt's lymphoma uncouple proliferation from apoptosis and, as a result, are more effective than wild-type MYC at promoting B cell lymphomagenesis in mice. Mutant MYC proteins retain their ability to stimulate proliferation and activate p53, but are defective at promoting apoptosis due to a failure to induce the BH3-only protein Bim (a member of the B cell lymphoma 2 (Bcl2) family) and effectively inhibit Bcl2. Disruption of apoptosis through enforced expression of Bcl2, or loss of either Bim or p53 function, enables wild-type MYC to produce lymphomas as efficiently as mutant MYC. These data show how parallel apoptotic pathways act together to suppress MYC-induced transformation, and how mutant MYC proteins, by selectively disabling a p53-independent pathway, enable tumour cells to evade p53 action during lymphomagenesis. 相似文献
704.