全文获取类型
收费全文 | 1381篇 |
免费 | 64篇 |
国内免费 | 3篇 |
专业分类
林业 | 63篇 |
农学 | 54篇 |
基础科学 | 8篇 |
181篇 | |
综合类 | 236篇 |
农作物 | 53篇 |
水产渔业 | 42篇 |
畜牧兽医 | 715篇 |
园艺 | 24篇 |
植物保护 | 72篇 |
出版年
2023年 | 8篇 |
2021年 | 17篇 |
2020年 | 28篇 |
2019年 | 25篇 |
2018年 | 25篇 |
2017年 | 28篇 |
2016年 | 22篇 |
2015年 | 19篇 |
2014年 | 19篇 |
2013年 | 69篇 |
2012年 | 50篇 |
2011年 | 55篇 |
2010年 | 43篇 |
2009年 | 26篇 |
2008年 | 53篇 |
2007年 | 53篇 |
2006年 | 51篇 |
2005年 | 61篇 |
2004年 | 47篇 |
2003年 | 53篇 |
2002年 | 71篇 |
2001年 | 41篇 |
2000年 | 28篇 |
1999年 | 28篇 |
1998年 | 13篇 |
1997年 | 14篇 |
1996年 | 11篇 |
1995年 | 15篇 |
1994年 | 10篇 |
1993年 | 9篇 |
1992年 | 20篇 |
1991年 | 28篇 |
1990年 | 23篇 |
1989年 | 23篇 |
1988年 | 16篇 |
1987年 | 22篇 |
1986年 | 19篇 |
1985年 | 16篇 |
1984年 | 13篇 |
1983年 | 10篇 |
1979年 | 13篇 |
1978年 | 14篇 |
1977年 | 11篇 |
1976年 | 16篇 |
1975年 | 12篇 |
1974年 | 14篇 |
1973年 | 12篇 |
1972年 | 10篇 |
1971年 | 8篇 |
1969年 | 8篇 |
排序方式: 共有1448条查询结果,搜索用时 62 毫秒
81.
Soil and plant samples were collected from chlorotic plantations of Grand fir (Abies grandis) and Douglas-fir (Pseudotsuga menziesii) in Winter, 1989. The soils had been amended in 1981 with an average of 300 dry Mg ha?1 of municipal sewage sludge. The sludge amendment resulted in an N application rate of approximately 8000 kg ha?1. Foliage analysis indicated a severe Mg deficiency (0.25 g kg?1 in sludge-treated vs. 0.93 g kg?1 in untreated area) might be the cause of chlorosis. No other nutrient showed concentrations in the deficient or toxic ranges. Trace metal levels in foliage were elevated significantly for Ni, Cd and Cr in sludge-treated sites, but not toxic levels. Soil samples taken to a depth of 1.4 m indicated the potential for soil acidification (up to 0.9 pH unit) in soil surface horizons. In addition, exchangeable Ca, Mg and K may have been depleted in surface horizons. Exchangeable Al and Fe were greater in the surface of sludge-treated sites. These observations and the loss of much of the nitrogen added during the sludge amendment indicated that nitrification and cation leaching was likely the mechanism for acidification and depletion of exchangeable cations. Fertilization of the plantation with MgSO4 or dolomitic limestone was carried out in Spring 1990. New foliage collected in June, 1990 was non-chlorotic and significantly higher in Mg concentration than unfertilized foliage (1.1. vs. 0.7 g kg?1, respectively). Results of this study indicate that it is important to assess the potential for initiating a nutrient deficiency due to secondary effects of sludge application in forest systems. 相似文献
82.
Plant invaders may directly or indirectly affect ecosystem resilience through their impact on soil seed banks. The invaders, and the application of control measures, change seed bank dynamics by altering the number of seeds entering and leaving the seed bank. We tested the impact of bitou bush (Chrysanthemoides monilifera ssp. rotundata), on the seed bank. We examined seed banks in heavily-invaded, sparsely-invaded and managed dunes, where bitou bush biomass had been controlled. While management of bitou bush may have reduced the density of bitou bush seeds in the soil, it did not reduce the richness of other weed species. Native tree species richness was significantly higher in seed banks of sparsely-invaded than either heavily-invaded or managed sites, perhaps indicating a permanent shift in community structure following invasion. However, remaining indices of native seed bank diversity were similar across all invasion categories, indicating that seed banks of many native species were unaffected by both invasion and management. While examination of seed banks is informative in assessing past and potential community dynamics, low similarity between the standing vegetation and seed bank at all sites indicated that many hind dune species had other storage or regeneration modes and seed banks cannot be relied upon for comprehensive dune restoration. 相似文献
83.
Claudio E. Rafanelli Gianna L. Petriconi Henry M. Papee 《Water, air, and soil pollution》1978,9(4):403-412
The dissipation of chemical elements from ash basin effluent was determined by use of neutron activation analysis of sediment, water, plant, invertebrate, and vertebrate samples taken from successive sites through the drainage system. Samples were tested for concentrations of 40 elements for a 12 mo period. Three different modes of dissipation were found. The major mechanism for removal of chemical elements from effluent was sedimentation into the benthic sediment. All biota had greater concentrations of chemicals than did water, with invertebrates having slightly higher levels than did plants and vertebrates. Ten elements were more concentrated in at least one biotic group than in the sediment. Concentrations of six elements in the drainage system exceeded the EPA recommended limits for surface waters. The observed modes of dissipation in this aquatic system emphasize the necessity for stable biotic communities present to permit cycling of chemical elements. 相似文献
84.
Heat stress adversely affects wheat production in many regions of the world and is particularly detrimental during reproductive
development. The objective of this study was to identify novel quantitative trait loci (QTL) associated with improved heat
tolerance in wheat (Triticum aestivum L.) and to confirm previous QTL results. To accomplish this, a recombinant inbred line (RIL) population was subjected to
a three-day 38°C daytime heat stress treatment during early grain-filling. At maturity, a heat susceptibility index (HSI)
was calculated from the reduction of three main spike yield components; kernel number, total kernel weight, and single kernel
weight. The HSI, as well as temperature depression (TD) of the main spike and main flag leaf during heat stress were used
as phenotypic measures of heat tolerance. QTL analysis identified 14 QTL for HSI, with individual QTL explaining from 4.5
to 19.3% of the phenotypic variance. Seven of these QTL co-localized for both TD and HSI. At all seven loci, the allele for
a cooler flag leaf or spike temperature (up to 0.81°C) was associated with greater heat tolerance, indicated by a lower HSI.
In a comparison to previous QTL results in a RIL population utilizing the same source of heat tolerance, seven genome regions
for heat tolerance were consistently detected across populations. The genetic effect of combining three of these QTL, located
on chromosomes 1B, 5A, and 6D, demonstrate the potential benefit of selecting for multiple heat tolerance alleles simultaneously.
The genome regions identified in this study serve as potential target regions for fine-mapping and development of molecular
markers for more rapid development of heat tolerant germplasm. 相似文献
85.
Bartels RA Paul A Green H Kapteyn HC Murnane MM Backus S Christov IP Liu Y Attwood D Jacobsen C 《Science (New York, N.Y.)》2002,297(5580):376-378
We present spatial coherence measurements of extreme ultraviolet (EUV) light generated through the process of high-harmonic up-conversion of a femtosecond laser. With a phase-matched hollow-fiber geometry, the generated beam was found to exhibit essentially full spatial coherence. The coherence of this laser-like EUV source was shown by recording Gabor holograms of small objects. This work demonstrates the capability to perform EUV holography with a tabletop experimental setup. Such an EUV source, with low divergence and high spatial coherence, can be used for experiments involving high-precision metrology, inspection of optical components for EUV lithography, and microscopy and holography with nanometer resolution. Furthermore, the short time duration of the EUV radiation (a few femtoseconds) will enable EUV microscopy and holography to be performed with ultrahigh time resolution. 相似文献
86.
87.
Bockelée-Morvan D Biver N Moreno R Colom P Crovisier J Gérard E Henry F Lis DC Matthews H Weaver HA Womack M Festou MC 《Science (New York, N.Y.)》2001,292(5520):1339-1343
The gas activity of comet C/1999 S4 (LINEAR) was monitored at radio wavelengths during its disruption. A runaway fragmentation of the nucleus may have begun around 18 July 2000 and proceeded until 23 July. The mass in small icy debris (=30-centimeter radius) was comparable to the mass in the large fragments seen in optical images. The mass budget after breakup suggests a small nucleus ( approximately 100- to 300-meter radius) that had been losing debris for weeks. The HNC, H2CO, H2S, and CS abundances relative to H2O measured during breakup are consistent with those obtained in other comets. However, a deficiency in CH3OH and CO is observed. 相似文献
88.
Requirement for activin A and transforming growth factor--beta 1 pro-regions in homodimer assembly 总被引:10,自引:0,他引:10
Many proteins are initially synthesized as part of a large precursor. The role of the pro-region in the biosynthesis of transforming growth factor--beta 1 (TGF-beta 1) and activin A, two structurally related disulfide-linked homodimers synthesized as large precursors, was studied. Vectors that expressed either the pro-region or the mature regions of these molecules were used in complementation experiments, only when the pro-region was coexpressed with the mature region did intracellular dimerization and secretion of biologically active homodimers occur. The pro-regions of activin A and TGF-beta 1, therefore, aid the folding, disulfide bond formation, and export of their respective homodimers. 相似文献
89.
Light-enhanced free radical formation and trypanocidal action of gentian violet (crystal violet) 总被引:2,自引:0,他引:2
R Docampo S N Moreno R P Muniz F S Cruz R P Mason 《Science (New York, N.Y.)》1983,220(4603):1292-1295
Transmission of Chagas' disease by transfusion of blood containing Trypanosoma cruzi has often been reported, and gentian violet, a triarylmethane dye, is widely used by blood banks in attempts to eliminate such transmission. In a study of intact trypanosomes, gentian violet was found to undergo a one-electron reduction to produce a carbon-centered free radical as demonstrated by electron spin resonance spectroscopy. Either reduced nicotinamide adenine dinucleotide or the reduced dinucleotide phosphate could serve as a source of reducing equivalents for the production of this free radical by homogenates of Trypanosoma cruzi. The formation of this free radical, and the trypanocidal action of gentian violet, were enhanced by light. The enhanced free radical formation may be the basic cause of the selective toxicity of gentian violet to Trypanosoma cruzi. 相似文献
90.
Wahlen M Tanaka N Henry R Deck B Zeglen J Vogel JS Southon J Shemesh A Fairbanks R Broecker W 《Science (New York, N.Y.)》1989,245(4915):286-290
Measurements of carbon-14 in small samples of methane from major biogenic sources, from biomass burning, and in "clean air" samples from both the Northern and Southern hemispheres reveal that methane from ruminants contains contemporary carbon, whereas that from wetlands, pat bogs, rice fields, and tundra is somewhat, depleted in carbon-14. Atmospheric (14)GH(4) seems to have increased from 1986 to 1987, and levels at the end of 1987 were 123.3 +/- 0.8 percent modern carbon (pMC) in the Northern Hemisphere and 120.0 +/- 0.7 pMC in the Southern Hemisphere. Model calculations of source partitioning based on the carbon-14 data, CH(4) concentrations, and delta(13)C in CH(4) indicate that 21 +/- 3% of atmospheric CH(4) was derived from fossil carbon at the end of 1987. The data also indicate that pressurized water reactors are an increasingly important source of (14)CH(4). 相似文献