全文获取类型
收费全文 | 9135篇 |
免费 | 1665篇 |
国内免费 | 2199篇 |
专业分类
测绘学 | 464篇 |
大气科学 | 1897篇 |
地球物理 | 2493篇 |
地质学 | 4585篇 |
海洋学 | 910篇 |
天文学 | 513篇 |
综合类 | 934篇 |
自然地理 | 1203篇 |
出版年
2024年 | 40篇 |
2023年 | 130篇 |
2022年 | 402篇 |
2021年 | 446篇 |
2020年 | 379篇 |
2019年 | 490篇 |
2018年 | 539篇 |
2017年 | 470篇 |
2016年 | 556篇 |
2015年 | 456篇 |
2014年 | 562篇 |
2013年 | 515篇 |
2012年 | 486篇 |
2011年 | 569篇 |
2010年 | 532篇 |
2009年 | 499篇 |
2008年 | 460篇 |
2007年 | 473篇 |
2006年 | 356篇 |
2005年 | 293篇 |
2004年 | 284篇 |
2003年 | 277篇 |
2002年 | 284篇 |
2001年 | 258篇 |
2000年 | 304篇 |
1999年 | 389篇 |
1998年 | 328篇 |
1997年 | 336篇 |
1996年 | 275篇 |
1995年 | 241篇 |
1994年 | 287篇 |
1993年 | 215篇 |
1992年 | 166篇 |
1991年 | 128篇 |
1990年 | 103篇 |
1989年 | 76篇 |
1988年 | 98篇 |
1987年 | 50篇 |
1986年 | 44篇 |
1985年 | 40篇 |
1984年 | 35篇 |
1983年 | 32篇 |
1982年 | 35篇 |
1981年 | 27篇 |
1980年 | 7篇 |
1979年 | 10篇 |
1978年 | 5篇 |
1975年 | 2篇 |
1958年 | 7篇 |
1954年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 444 毫秒
341.
Seed long-distance dispersal(LDD) events are typically rare, but are important in the population processes that determine large-scale forest changes and the persistence of species in fragmented landscapes. However, previous studies focused on species dispersed via animal-mediated LDD, and ignored those dispersed by wind. The aim of this study was to assess the effects of canopy openness, edge, seed source, and patch tree density on the LDD of seeds by wind in forest. We collected birch seeds, a typical wind-dispersed species, throughout a larch plantation. We then assessed the relationship between birch LDD and each factor that may influence LDD of seeds by wind including distance to edge, canopy openness size, distance to mature forest, and the tree density of the larch plantation. We used univariate linear regression analysis to assess the influence of those factors on birch LDD, and partial correlations to calculate the contribution of each factor to LDD. The results showed that both canopy openness and edge had significant influences on birch LDD. Specifically, a negative relationship was observed between distance to edge and birch LDD, whereas there was a positive correlation between canopy openness size and LDD. In contrast, the distance to the mature forest was not correlated with birch LDD. Our results suggest that patch tree density could potently affect the probability of LDD by wind vectors, which provides novel and revealing insights regarding the effect of fragmentation on wind dynamics. The data also provide compelling evidence for the previously undocumented effect of habitat fragmentation on wind-dispersed organisms. As such, these observations will facilitate reasonable conservation planning, which requires a detailed understanding of the mechanisms by which patch properties hamper the delivery of seeds of wind-dispersed plants to fragmented areas. 相似文献
342.
The Drake Passage is located between the Antarctic Peninsula and Tierra del Fuego in the south of South America. Surface seawater samples were collected at seven sites in the Drake Passage during the austral summer of 2012. The 16S rRNA sequences were analyzed from 187 isolated bacterial strains. Three phyla, 29 genera and 56 species were identified. The three phyla were Actinobacteria, Firmicutes and Proteobacteria; the Proteobacteria included α-Proteobacteria, β-Proteobacteria and γ-Proteobacteria. γ-Proteobacteria, Actinobacteria and Firmicutes were the dominant class or phyla in terms of quantity and species. Gram-positive bacteria (Actinobacteria and Firmicutes) accounted for 57.8% of all types identified. There were nine dominant genera, including Curtobacterium, Staphylococcus, and Halomonas, and 14 dominant species including Curtobacterium flaccumfaciens, Curtobacterium pusillum, and Staphylococcus sciuri. Of the strains identified, 87.2% were catalase positive or weakly positive. 相似文献
343.
344.
345.
346.
347.
348.
349.
Due to the continuous and intense rainfall from June 26 to 28, 2016, Xinlu Village in Ganshui Town, Qijiang District, Chongqing, experienced a unitary-slip landslide at approximately UTC+8 19:30 on June 28. This landslide disrupted the Chuan-Qian railway and damaged four residential buildings. To analyze and rehabilitate the landslide, the engineering geology, hydrological conditions, and deformation instability mechanism of this landslide were investigated and comprehensively analyzed based on an in situ survey, geophysical drilling, and a laboratory quick-shearing test. The results show that the landslide is a typical gradual progressive landslide. 相似文献
350.
In geotechnical practice, it is of considerable importance to assess the behavior of vertical–lateral coupled loading piles in multilayered soil deposits. This study deals with a semi-analytical formulation for the performance of a pile suffering from combined vertical and lateral loads. The emphasis is on quantifying the mobilization of the subgrade reaction provided by the layered soil stratums. In the proposed method, subgrade reactions, correlated with both the accumulative axial load transfer and side resistances depending upon the pile–soil interaction, are abstracted as a series of nonlinear springs in both vertical and lateral directions. On account of this, an alternative transfer matrix method is applied to characterize the pile reaction along the depth under the identified boundary conditions atop the pile; meanwhile, the condition of static equilibrium, between two specific pile segments located at the border of soil layers, is also essential. On this basis, validation of the solution is conducted by comparing with observations from experiments and predictions obtained from other existing methods. In addition, the influence of properties in shallow soil layer and the vertical load on the lateral response of the pile is also discussed. The results indicate a reduction in the lateral displacement and the maximum bending moment within the pile with the increase in the shallow soil stiffness, but a growth with the increase in vertical load due to the “P-Δ effect.” 相似文献