首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   5566篇
  免费   227篇
  国内免费   14篇
林业   401篇
农学   233篇
基础科学   39篇
  660篇
综合类   581篇
农作物   214篇
水产渔业   218篇
畜牧兽医   3040篇
园艺   45篇
植物保护   376篇
  2019年   45篇
  2018年   68篇
  2017年   102篇
  2016年   87篇
  2015年   59篇
  2014年   82篇
  2013年   207篇
  2012年   157篇
  2011年   178篇
  2010年   112篇
  2009年   124篇
  2008年   221篇
  2007年   215篇
  2006年   185篇
  2005年   206篇
  2004年   180篇
  2003年   185篇
  2002年   174篇
  2001年   205篇
  2000年   199篇
  1999年   185篇
  1998年   54篇
  1997年   73篇
  1996年   66篇
  1995年   57篇
  1994年   47篇
  1993年   63篇
  1992年   127篇
  1991年   136篇
  1990年   135篇
  1989年   138篇
  1988年   108篇
  1987年   122篇
  1986年   103篇
  1985年   97篇
  1984年   93篇
  1983年   67篇
  1982年   39篇
  1979年   81篇
  1978年   63篇
  1977年   73篇
  1976年   46篇
  1975年   69篇
  1974年   85篇
  1973年   74篇
  1972年   75篇
  1971年   56篇
  1970年   70篇
  1969年   51篇
  1968年   44篇
排序方式: 共有5807条查询结果,搜索用时 15 毫秒
31.
Highly reactive radicals, ROO(*), were generated from 2, 2'-azobis[2-(2-imidazolin-2-yl)propane] and linoleic acid. The ROO(*) scavenging capacity of some Italian red wines was evaluated following the changes in oxygen consumption. Under the experimental conditions the time course of oxygen consumption shows two typical behaviors: trolox-like (class I) and gallic acid-like (class II). Usually the time course of wine was similar to that of gallic acid. The rate of oxygen consumption was found to decrease exponentially with the amount of wine or gallic acid added to the test solution. On this basis the capacity of red wines to scavenge peroxy radicals was expressed as content of gallic acid (S(GA)). The S(GA) values were found to be correlated to the amount of total proanthocyanidins and total polyphenols of some Italian red wines (p < 0.01). The proanthocyanidins extracted from seeds were shown to make a major contribution to the peroxy radical scavenging capacity of red wines, whereas, interestingly, the chemical class of the low molecular weight tannins reactive to vanillin did not correlate with the S(GA) values.  相似文献   
32.
Abstract. Under the Kyoto Protocol, the European Union is committed to an 8% reduction in CO2 emissions, compared to baseline (1990) levels, during the first commitment period (2008–2012). However, within the overall EU agreement, the UK is committed to a 12.5% reduction. In this paper, we estimate the carbon mitigation potential of various agricultural land-management strategies (Kyoto Article 3.4) and examine the consequences of UK and European policy options on the potential for carbon mitigation.
We show that integrated agricultural land management strategies have considerable potential for carbon mitigation. Our figures suggest the following potentials (Tg yr−1) for each scenario: animal manure, 3.7; sewage sludge, 0.3; cereal straw incorporation, 1.9; no-till farming, 3.5; agricultural extensification, 3.3; natural woodland regeneration, 3.2 and bioenergy crop production, 4.1. A realistic land-use scenario combining a number of these individual management options has a mitigation potential of 10.4 Tg C yr−1 (equivalent to about 6.6% of 1990 UK CO2-carbon emissions). An important resource for carbon mitigation in agriculture is the surplus arable land, but in order to fully exploit it, policies governing the use of surplus arable land would need to be changed. Of all options examined, bioenergy crops show the greatest potential. Bioenergy crop production also shows an indefinite mitigation potential compared to other options where the potential is infinite.
The UK will not attempt to meet its climate change commitments solely through changes in agricultural land-use, but since all sources of carbon mitigation will be important in meeting these commitments, agricultural options should be taken very seriously.  相似文献   
33.
The fat in vegetable foods--tree nuts, peanuts, sunflower seeds, avocado, and olives--can be determined volumetrically by acid digestion of the material and separation of the fat. The assay can be performed conveniently by using the equipment developed for fat determination of milk (Gerber method). The results agree well with those obtained by Soxhlet extraction. The advantages of using the Gerber method for vegetable foods are simplicity, speed, low operation cost, and elimination of the use of inflammable solvents.  相似文献   
34.
Three winter wheat varieties with differing breadmaking quality were grown at two locations in two years at 0 or 3 × 60 kg of nitrogen application. The effect of nitrogen on amount of different components of gluten proteins was determined by reverse-phase HPLC. A high amount of nitrogen led generally to a significant increase of total protein content. However, this increase was obvious only for the gluten proteins; albumins and globulins remained nearly unaffected. The effect of increased protein content on gliadin to glutenin (gli-glu) ratio was inconsistent. While increased protein content increased the gli-glu ratio in the variety Capo, the opposite was true for the variety Renan. Gli-glu ratio of the variety Lindos showed no discernible tendency. As total protein content increased, the ratio of low molecular weight (LMW) to high molecular weight (HMW) glutenins decreased consistently, i.e., in all varieties, in both years and locations. Change of LMW to HMW ratio showed a significant negative correlation to sedimentation value and bread volume. There was no consistent change in the ratio between x- and y-type HMW subunits due to fertilization, as could be shown by densitometric measurements on SDS-PAGE gels. This ratio appeared to be dependent on the genotype and has decreased with decreasing quality. The amount of x-type subunits correlated closely with sedimentation value and bread volume. These results suggest that ratio of HMW glutenins, especially x-type subunits, to total protein content could be the best early detectable parameter with high predictive value for breadmaking quality.  相似文献   
35.
36.
After far-infrared (FIR) radiation onto rice hull, a methanolic extract was prepared for the determination of antioxidant ability. After 30 min of FIR treatment, the radical scavenging activity and total phenol contents of rice hull extracts increased from 47.74 to 79.63% and from 0.12 to 0.19 mM, respectively, compared to control. Inhibition of lipid peroxidation in extracts was also increased from 41.07 to 47.96%. According to the GC-MS analysis, more phenolic compounds (p-coumaric acid, 3-vinyl-1-oxybenzene, p-hydroxybenzaldehyde, vanillin, p-hydroxybenzoic acid, and 4,7-dihydroxyvanillic acid) were detected in FIR-irradiated rice hull extract. These results indicated that FIR radiation onto rice hull could liberate and activate covalently bound phenolic compounds that have antioxidant activities.  相似文献   
37.
Current wet chemical methods for the speciation of sulphur (S) in soils are inaccurate and do not allow one to assess the S speciation of individual soil particles and colloids. X-ray microscopy and Near Edge X-ray Absorption Fine structure Spectroscopy (NEXAFS) can be used to study individual species of S at the K-adsorption edge. We have used these techniques to identify and quantify S species in bulk soil, soil particles and colloids from Oh and Bh horizons of two forested Podzols. The partitioning of soil sulphur as determined on bulk samples of the Oh horizons by X-ray spectromicroscopy agreed fairly well with the results of a conventional S speciation for the soil at Schluchsee, and reasonably well for that at Rotherdbach. The NEXAFS analyses on individual soil particles revealed that they are richer in reduced organic sulphur than the bulk soil for the Schluchsee Oh and richer in sulphate for Rotherdbach Oh. The techniques can be used reliably to separate and quantify sulphur species with different oxidation states in the soil. The combination of X-ray transmission and sulphur fluorescence images with unfocused and focused NEXAFS spectra at the K-adsorption edge of sulphur at specific microsites allowed us to compare the distribution of S species in bulk soil with that of distinct soil particles and soil colloids. Moreover, we can use it to assess the spatial distribution of different S species on soil particles on a scale of a few hundred nanometres.  相似文献   
38.
农业土壤表面不平度与土壤风蚀、行走装置的相互作用和田间土壤含水量测定分布图等方面具有极其重要的作用.本文利用激光式不平度测试仪测定犁耕耕地地面、圆盘耙地地面和驱动耙耙地地面的不平度,测定地面不平度沿着三个方向进行,即平行于耕作方向、倾斜于耕作方向和垂直于耕作方向.对每一种地面不平度的RMS值、相关长度和自相关函数进行了分析,结果表明对于短的地块,可用单一尺度来表征地表不平度的特征,地表不平度特征属于指数相关函数,RMS高度和自相关长度具有明显的相关性.对于长的地块,平整的农业地表不平度具有分形性或多尺度性,其自相关函数是一种分形过程.  相似文献   
39.
Panosteitis, an idiopathic bone disease of young dogs, was investigated in the light of a new, empirically based hypothesis governing its pathogenesis. Extensive clinical observations suggest a close relationship between the incidence of this disease and the commercialization of various protein-rich, high-calorie dog foods. The theory of an "osseous compartment syndrome" provides a hypothetical pathogenesis, which corroborates this findings. An excessive accumulation of protein causes intraosseous edema due to its osmotic effects. Because bone is a rigid compartment, this leads to an increase in intramedullary pressure and compression of blood vessels. Subsequent osseous ischemia leads to a deficient metabolic state (decreased oxygenation, inadequate influx of nutritive substances, local acidosis, decreased removal of metabolites, disruption of local biochemical processes, etc.), and a vicious circle is created due to the resulting local inflammation. The disease is aggravated by increased metabolism due to excessive physical activity. Within the context of a pilot study, clinical, radiographic, scintigraphic and thermographic examinations and a therapeutic trial with benzopyron were carried out. In addition, more modern investigative tools, including osteomyelography, magnetic resonance tomography and intraosseous pressure measurements were used to provide objective data concerning the pathogenesis of panosteitis. In most cases, clinical remission was seen within days of monotherapy with the proteolytic substance, benzopyron (Cumartrin). This finding appears to corroborate our hypothesis.  相似文献   
40.
Despite the importance of noninfectious joint diseases in equine medicine, little is known about the acute phase response which may be elicited if the local inflammatory process of noninfectious arthritis is sufficiently strong, Therefore the aim of this study was to monitor the systemic inflammatory response during experimentally-induced noninfectious arthritis by studying the dynamics in serum of the acute phase proteins serum amyloid A (SAA), haptoglobin, fibrinogen and alpha2-globulins. Twenty-four Standardbred horses, age 3-7 years, found healthy on thorough clinical, radiological, haematological and serum biochemical examination, were injected aseptically into the right midcarpal joint with amphotericin B. Blood samples were drawn before induction of arthritis (0 h), and at 8, 16, 24, 36 and 48 h postinduction and then on Days 3, 4, 5 and 15 postinduction. All horses developed lameness with joint effusion and joint heat as well as increased respiratory rate, heart rate and body temperature. The lameness started to decline after 24-36 h and, in most animals, systemic signs disappeared on Day 2 postinjection. The concentration of the acute phase proteins increased following induction of arthritis. The SAA concentrations were higher than baseline concentrations from 16 h postinduction and were maximal at 36-48 h (227 times baseline concentration). The haptoglobin concentrations were higher than baseline concentrations from 24 h and were maximal at 48-96 h (1.14 times baseline concentration). The maximal concentrations of fibrinogen were seen between 36-72 h postinjection and increased on average 0.87 times from baseline concentrations. The fibrinogen concentrations were higher than baseline concentrations from 24 h postinjection. Alpha2-globulins concentrations showed a minor increase and increased 0.55 times from baseline concentrations. The markers had returned to baseline concentrations by Day 15. Our results demonstrate that amphotericin B-induced arthritis in a single joint gives rise to a systemic acute phase response measurable as increased concentrations in serum SAA, haptoglobin, fibrinogen and alpha2-globulins during the first 2 weeks of the condition and, thereby, that such an increase need not be indicative of infectious arthritis. Further research should be aimed at determining whether chronic noninfectious arthritis in the horse gives rise to increased acute phase protein concentrations in serum.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号