全文获取类型
收费全文 | 566篇 |
免费 | 23篇 |
专业分类
林业 | 14篇 |
农学 | 15篇 |
基础科学 | 5篇 |
79篇 | |
综合类 | 142篇 |
农作物 | 29篇 |
水产渔业 | 22篇 |
畜牧兽医 | 256篇 |
园艺 | 5篇 |
植物保护 | 22篇 |
出版年
2020年 | 9篇 |
2019年 | 7篇 |
2018年 | 6篇 |
2017年 | 8篇 |
2016年 | 8篇 |
2015年 | 5篇 |
2014年 | 14篇 |
2013年 | 11篇 |
2012年 | 37篇 |
2011年 | 27篇 |
2010年 | 20篇 |
2009年 | 14篇 |
2008年 | 19篇 |
2007年 | 29篇 |
2006年 | 17篇 |
2005年 | 27篇 |
2004年 | 18篇 |
2003年 | 16篇 |
2002年 | 9篇 |
2001年 | 5篇 |
2000年 | 9篇 |
1999年 | 15篇 |
1998年 | 8篇 |
1997年 | 5篇 |
1996年 | 8篇 |
1995年 | 6篇 |
1993年 | 6篇 |
1992年 | 10篇 |
1991年 | 7篇 |
1990年 | 8篇 |
1989年 | 6篇 |
1988年 | 12篇 |
1987年 | 7篇 |
1986年 | 5篇 |
1985年 | 7篇 |
1984年 | 5篇 |
1982年 | 7篇 |
1981年 | 8篇 |
1978年 | 5篇 |
1977年 | 6篇 |
1976年 | 5篇 |
1975年 | 6篇 |
1974年 | 5篇 |
1973年 | 5篇 |
1971年 | 5篇 |
1970年 | 9篇 |
1969年 | 7篇 |
1968年 | 6篇 |
1967年 | 5篇 |
1924年 | 8篇 |
排序方式: 共有589条查询结果,搜索用时 468 毫秒
61.
Moreau JW Weber PK Martin MC Gilbert B Hutcheon ID Banfield JF 《Science (New York, N.Y.)》2007,316(5831):1600-1603
High-spatial-resolution secondary ion microprobe spectrometry, synchrotron radiation-based Fourier-transform infrared spectroscopy, and polyacrylamide gel analysis demonstrated the intimate association of proteins with spheroidal aggregates of biogenic zinc sulfide nanocrystals, an example of extracellular biomineralization. Experiments involving synthetic zinc sulfide nanoparticles and representative amino acids indicated a driving role for cysteine in rapid nanoparticle aggregation. These findings suggest that microbially derived extracellular proteins can limit the dispersal of nanoparticulate metal-bearing phases, such as the mineral products of bioremediation, that may otherwise be transported away from their source by subsurface fluid flow. 相似文献
62.
63.
64.
A new facility has recently been added at the Lerner Marine Laboratory, Bimini, Bahamas, which makes it possible for the first time to work experimentally with large elasmobranchs up to 15 feet in length. 相似文献
65.
66.
67.
A new probe for the diagnosis of myotonic muscular dystrophy 总被引:11,自引:0,他引:11
R J Bartlett M A Pericak-Vance L Yamaoka J Gilbert M Herbstreith W Y Hung J E Lee T Mohandas G Bruns C Laberge 《Science (New York, N.Y.)》1987,235(4796):1648-1650
Myotonic muscular dystrophy (DM) is the most common muscular dystrophy, affecting adults as well as children. It is inherited as an autosomal dominant trait and is characterized by variable expressivity and late age-of-onset. Linkage studies have established the locus on chromosome 19. In order to identify tightly linked probes for diagnosis as well as to define in detail the DM gene region, chromosome 19 libraries were constructed and screened for restriction fragment length polymorphisms tightly linked to DM. A genomic clone, LDR152 (D19S19), was isolated that is tightly linked to DM; recombination fraction = 0.0 (95% confidence limits 0.0-0.03); lod score, 15.4. 相似文献
68.
Colbourne JK Pfrender ME Gilbert D Thomas WK Tucker A Oakley TH Tokishita S Aerts A Arnold GJ Basu MK Bauer DJ Cáceres CE Carmel L Casola C Choi JH Detter JC Dong Q Dusheyko S Eads BD Fröhlich T Geiler-Samerotte KA Gerlach D Hatcher P Jogdeo S Krijgsveld J Kriventseva EV Kültz D Laforsch C Lindquist E Lopez J Manak JR Muller J Pangilinan J Patwardhan RP Pitluck S Pritham EJ Rechtsteiner A Rho M Rogozin IB Sakarya O Salamov A Schaack S Shapiro H Shiga Y Skalitzky C Smith Z Souvorov A Sung W 《Science (New York, N.Y.)》2011,331(6017):555-561
We describe the draft genome of the microcrustacean Daphnia pulex, which is only 200 megabases and contains at least 30,907 genes. The high gene count is a consequence of an elevated rate of gene duplication resulting in tandem gene clusters. More than a third of Daphnia's genes have no detectable homologs in any other available proteome, and the most amplified gene families are specific to the Daphnia lineage. The coexpansion of gene families interacting within metabolic pathways suggests that the maintenance of duplicated genes is not random, and the analysis of gene expression under different environmental conditions reveals that numerous paralogs acquire divergent expression patterns soon after duplication. Daphnia-specific genes, including many additional loci within sequenced regions that are otherwise devoid of annotations, are the most responsive genes to ecological challenges. 相似文献
69.
70.
Background, Aim and Scope
Grazing animals have a dominant effect on the movement and utilization of nutrients through the soil and plant system, and
thus on the fertility of pasture soils. Grazing can accelerate and alter the timing of nutrient transfers, and increase the
amount of nutrients cycled from plant to soil. Long periods, position of shade, and water resources for grazing cattle can
influence the spatial distribution of soil biochemical properties including soil organic carbon (C), total extractable inorganic
nitrogen (TEIN), and Melich 1 extractable total phosphorus (TP).
The objective of this study is to test whether cattle congregation sites typical on most Florida ranches, such as mineral
feeders, water troughs, and shade areas are more nutrient-rich and may contribute more nutrients to surface and groundwater
supply than in other pasture locations under Florida conditions.
Materials and Methods:
Baseline soil samples around and beneath three congregations sites in established (>10 yr) grazed beef cattle pastures at
the United States Department of Agriculture (USDA), Agricultural Research Service (ARS), Subtropical Agricultural Research
Station (STARS), Brooksville, FL, were collected in 2003, 2004, and 2005.
Results:
The levels of soil TEIN and TP were significantly (p ≤ 0.001) affected by the interaction of congregation sites and distance
away from the center of the sites. Mineral feeders had the highest concentration of TP followed by shades and water troughs.
The concentrations of soil TP decreased almost linearly with distance (x = meter) away from the center of the mineral feeders
(-5.24x + 55.10; R2 = 0.92; p ≤ 0.001) and the shades (-6.25x + 57.21; R2 = 0.85; p ≤ 0.001). However, the level of TP around
the water troughs (-0.25x + 16.91; R2 = 0.09) does not appear to change significantly with distance, staying close to about
13-18 mg kg-1. The levels of TEIN decreased linearly with distance away from the mineral feeders from the center of the shades.
A linear model can describe the relationship between TEIN and distance away from the center of shades: -11.3x + 78.2; R2 =
0.95; p ≤ 0.001. The shaded sites (34.25 ± 1.7 mg kg-1) had higher levels than the mineral feeders (7.22 ± 0.60 mg kg-1) or
water troughs (10.06 ± 0.8 mg kg-1) sites.
Discussion:
The higher soil TP near and around the mineral feeders can be attributed to the presence of phosphorus in the supplemental
feeds. The average level of soil TP in the mineral feeders of 34.05 ± 0.44 mg kg-1 was not high enough to be of environmental
concern. Losses of soil phosphorus by overland flow are becoming a big concern when the concentrations for soil phosphorus
exceeded 150 mg kg-1 in the upper 20 cm of soils. With TN, the shaded sites (34.25 ± 1.7 mg kg-1) had either higher levels
than the mineral feeders (7.22 ± 0.60 mg kg-1) or water trough (10.06 ± 0.80 mg kg-1) sites. Higher TEIN content at the shade
sites may have been more likely due to frequent urination of animals and lack of vegetation immediately adjacent to shades.
The lack of vegetation within and/or near the shades then had no uptake mechanism for removal of inorganic nitrogen, unlike
the heavy demand for inorganic nitrogen by bahiagrass in other areas of the pasture. An accumulation of TEIN immediately adjacent
to shades could lead to a potential point source that would be susceptible to leaching or gaseous losses to the environment.
Conclusions:
Early results of the study are suggesting that cattle congregation sites in beef cattle operations in Florida are not nutrient-rich,
therefore may not contribute more nutrients to surface and groundwater supply under Florida conditions.
Recommendations and Perspectives:
Since there is no apparent vertical build up or horizontal movement of TEIN and TP in the landscape, we can then surmise that
cattle congregation sites may be considered not a potential source of nutrients at the watershed level, at least on the sand
ridge soils in Florida. Further research is continuing, including sampling at cattle congregation sites at other locations
in north and south Florida, which started in July 2004. 相似文献