全文获取类型
收费全文 | 139篇 |
免费 | 4篇 |
专业分类
林业 | 2篇 |
农学 | 3篇 |
基础科学 | 1篇 |
30篇 | |
综合类 | 66篇 |
农作物 | 2篇 |
水产渔业 | 5篇 |
畜牧兽医 | 23篇 |
园艺 | 5篇 |
植物保护 | 6篇 |
出版年
2021年 | 3篇 |
2020年 | 2篇 |
2018年 | 2篇 |
2017年 | 5篇 |
2016年 | 2篇 |
2015年 | 1篇 |
2014年 | 3篇 |
2013年 | 4篇 |
2012年 | 7篇 |
2011年 | 7篇 |
2010年 | 1篇 |
2009年 | 1篇 |
2008年 | 8篇 |
2007年 | 4篇 |
2006年 | 2篇 |
2005年 | 3篇 |
2004年 | 7篇 |
2003年 | 6篇 |
2002年 | 5篇 |
2001年 | 4篇 |
2000年 | 6篇 |
1999年 | 3篇 |
1998年 | 1篇 |
1997年 | 1篇 |
1996年 | 1篇 |
1995年 | 4篇 |
1994年 | 1篇 |
1991年 | 2篇 |
1990年 | 1篇 |
1989年 | 1篇 |
1988年 | 1篇 |
1987年 | 5篇 |
1986年 | 2篇 |
1984年 | 2篇 |
1983年 | 2篇 |
1982年 | 2篇 |
1980年 | 2篇 |
1979年 | 4篇 |
1978年 | 2篇 |
1976年 | 3篇 |
1974年 | 1篇 |
1972年 | 5篇 |
1971年 | 4篇 |
1969年 | 2篇 |
1968年 | 2篇 |
1966年 | 2篇 |
1945年 | 2篇 |
1942年 | 1篇 |
1936年 | 1篇 |
排序方式: 共有143条查询结果,搜索用时 0 毫秒
1.
High levels of soil sodicity, resulting from intensive irrigation with saline-sodic waters, lead to an increased soil susceptibility to seal formation and to severe problems of runoff and soil erosion. The objective of this study was to investigate the efficacy of the addition of small amounts of an anionic polyacrylamide (PAM) to the irrigation water in controlling seal formation, runoff and soil erosion. Two predominantly montmorillonitic soils were studied, a grumusol (Typic Haploxerert) and a loess (Calcic Haploxeralf), having naturally occurring exchangeable sodium percentage (ESP)>12. The soils were exposed to 60 mm of simulated irrigation with commonly used tap water (TW, electrical conductivity=0.8 dS m–1; sodium adsorption ratio (SAR)=2), or saline water (SW, electrical conductivity=5.0 dS m–1; SAR>12). PAM effectiveness in controlling runoff and erosion from the sodic soils was compared with runoff and erosion levels obtained from untreated soils having low ESPs (<4). For both soils and for both water qualities and polymer concentrations in the irrigation water, PAM was efficient in controlling runoff at low ESP levels and inefficient at high ESP levels. At moderate ESP levels, PAM's efficacy in controlling runoff was inconsistent and varied with water quality and polymer concentration. Conversely, in general, soil loss originating from rill erosion, was significantly and effectively reduced in moderate and high ESP soils by addition of PAM to the irrigation water, irrespective of water quality and polymer concentration. PAM was more effective in reducing rill erosion than in reducing runoff in the moderate and high ESP samples, because the energy involved in generating runoff is much higher than that involved in rill erosion. PAM treated surface aggregates were not stable against the distructive forces leading to seal formation and runoff production; but they were stable enough to resist the hydraulic shear exerted by the runoff flow. 相似文献
2.
To pathogenic microparasites (viruses, bacteria, protozoa, or fungi), we and other mammals (living organisms at large) are little more than soft, thin-walled flasks of culture media. Almost every time we eat, brush our teeth, scrape our skin, have sex, get bitten by insects, and inhale, we are confronted with populations of microbes that are capable of colonizing the mucosa lining our orifices and alimentary tract and proliferating in fluids and cells within us. Nevertheless, we rarely get sick, much less succumb to these infections. The massive numbers of bacteria and other micro- and not-so-micro organisms that abound and replicate in our alimentary tract and cover our skin and the mucosa lining our orifices normally maintain their communities in seemingly peaceful coexistence with the somatic cells that define us. Why don't these microbes invade and proliferate in the culture media within the soft, thin-walled flask that envelops us? Why don't they cause disease and lead to our rapid demise? 相似文献
3.
4.
Klein HP Horowitz NH Levin GV Oyama VI Lederberg J Rich A Hubbard JS Hobby GL Straat PA Berdahl BJ Carle GC Brown FS Johnson RD 《Science (New York, N.Y.)》1976,194(4260):99-105
Three different types of biological experiments on samples of martian surface material ("soil") were conducted inside the Viking lander. In the carbon assimilation or pyrolytic release experiment, (14)CO(2) and (14)CO were exposed to soil in the presence of light. A small amount of gas was found to be converted into organic material. Heat treatment of a duplicate sample prevented such conversion. In the gas exchange experiment, soil was first humidified (exposed to water vapor) for 6 sols and then wet with a complex aqueous solution of metabolites. The gas above the soil was monitored by gas chromatography. A substantial amount of O(2) was detected in the first chromatogram taken 2.8 hours after humidification. Subsequent analyses revealed that significant increases in CO(2) and only small changes in N(2) had also occurred. In the labeled release experiment, soil was moistened with a solution containing several (14)C-labeled organic compounds. A substantial evolution of radioactive gas was registered but did not occur with a duplicate heat-treated sample. Alternative chemical and biological interpretations are possible for these preliminary data. The experiments are still in process, and these results so far do not allow a decision regarding the existence of life on the plonet Mars. 相似文献
5.
6.
The activity of uroporphyrinogen III cosynthetase in hemolyzates and tissue extracts from fox squirrels is much less than in similar preparations from gray squirrels. Low activity of this enzyme explains the production of large amounts of uroporphyrin I by the fox squirrel. Members of this species thus provide a small-animal model for studies of congenital erythropoietic porphyria, a hereditary disease of man and cattle which is associated with a similar partial deficiency of uroporphyrinogen III cosynthetase. 相似文献
7.
Ecology. The value of nature and the nature of value 总被引:2,自引:0,他引:2
Daily GC Söderqvist T Aniyar S Arrow K Dasgupta P Ehrlich PR Folke C Jansson A Jansson B Kautsky N Levin S Lubchenco J Mäler KG Simpson D Starrett D Tilman D Walker B 《Science (New York, N.Y.)》2000,289(5478):395-396
Ecosystems are capital assets: When properly managed, they yield a flow of vital goods and services. Relative to other forms of capital, however, ecosystems are poorly understood, scarcely monitored, and--in many important cases--undergoing rapid degradation. The process of economic valuation could greatly improve stewardship. This potential is now being realized with innovative financial instruments and institutional arrangements. 相似文献
8.
9.
10.
A mutation in the catalytic subunit of cAMP-dependent protein kinase that disrupts regulation 总被引:8,自引:0,他引:8
L R Levin J Kuret K E Johnson S Powers S Cameron T Michaeli M Wigler M J Zoller 《Science (New York, N.Y.)》1988,240(4848):68-70
A mutant catalytic subunit of adenosine 3',5'-monophosphate (cAMP)-dependent protein kinase has been isolated from Saccharomyces cerevisiae that is no longer subject to regulation yet retains its catalytic activity. Biochemical analysis of the mutant subunit indicates a 100-fold decreased affinity for the regulatory subunit. The mutant catalytic subunit exhibits approximately a threefold increase in Michaelis constant for adenosine triphosphate and peptide cosubstrates, and is essentially unchanged in its catalytic rate. The nucleotide sequence of the mutant gene contains a single nucleotide change resulting in a threonine-to-alanine substitution at amino acid 241. This residue is conserved in other serine-threonine protein kinases. These results identify this threonine as an important contact between catalytic and regulatory subunits but only a minor contact in substrate recognition. 相似文献