全文获取类型
收费全文 | 58717篇 |
免费 | 3298篇 |
国内免费 | 37篇 |
专业分类
林业 | 3050篇 |
农学 | 2213篇 |
基础科学 | 351篇 |
7425篇 | |
综合类 | 6817篇 |
农作物 | 2099篇 |
水产渔业 | 3188篇 |
畜牧兽医 | 32889篇 |
园艺 | 724篇 |
植物保护 | 3296篇 |
出版年
2019年 | 497篇 |
2018年 | 957篇 |
2017年 | 1035篇 |
2016年 | 1008篇 |
2015年 | 865篇 |
2014年 | 1011篇 |
2013年 | 2320篇 |
2012年 | 1804篇 |
2011年 | 2197篇 |
2010年 | 1437篇 |
2009年 | 1368篇 |
2008年 | 2030篇 |
2007年 | 2040篇 |
2006年 | 1932篇 |
2005年 | 1788篇 |
2004年 | 1681篇 |
2003年 | 1666篇 |
2002年 | 1538篇 |
2001年 | 2332篇 |
2000年 | 2186篇 |
1999年 | 1722篇 |
1998年 | 684篇 |
1997年 | 623篇 |
1996年 | 601篇 |
1995年 | 671篇 |
1994年 | 572篇 |
1993年 | 558篇 |
1992年 | 1262篇 |
1991年 | 1397篇 |
1990年 | 1450篇 |
1989年 | 1331篇 |
1988年 | 1243篇 |
1987年 | 1230篇 |
1986年 | 1167篇 |
1985年 | 1104篇 |
1984年 | 917篇 |
1983年 | 723篇 |
1982年 | 524篇 |
1979年 | 792篇 |
1978年 | 616篇 |
1977年 | 469篇 |
1976年 | 524篇 |
1975年 | 502篇 |
1974年 | 624篇 |
1973年 | 605篇 |
1972年 | 633篇 |
1971年 | 601篇 |
1970年 | 588篇 |
1969年 | 537篇 |
1967年 | 497篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
Weber HA Zart MK Hodges AE Molloy HM O'Brien BM Moody LA Clark AP Harris RK Overstreet JD Smith CS 《Journal of agricultural and food chemistry》2003,51(25):7352-7358
The characterization of herbal materials is a significant challenge to analytical chemists. Goldenseal (Hydrastis canadensis L.), which has been chosen for toxicity evaluation by NIEHS, is among the top 15 herbal supplements currently on the market and contains a complex mixture of indigenous components ranging from carbohydrates and amino acids to isoquinoline alkaloids. One key component of herbal supplement production is botanical authentication, which is also recommended prior to initiation of efficacy or toxicological studies. To evaluate material available to consumers, goldenseal root powder was obtained from three commercial suppliers and a strategy was developed for characterization and comparison that included Soxhlet extraction, HPLC, GC-MS, and LC-MS analyses. HPLC was used to determine the weight percentages of the goldenseal alkaloids berberine, hydrastine, and canadine in the various extract residues. Palmatine, an isoquinoline alkaloid native to Coptis spp. and other common goldenseal adulterants, was also quantitated using HPLC. GC-MS was used to identify non-alkaloid constituents in goldenseal root powder, whereas LC-MS was used to identify alkaloid components. After review of the characterization data, it was determined that alkaloid content was the best biomarker for goldenseal. A 20-min ambient extraction method for the determination of alkaloid content was also developed and used to analyze the commercial material. All three lots of purchased material contained goldenseal alkaloids hydrastinine, berberastine, tetrahydroberberastine, canadaline, berberine, hydrastine, and canadine. Material from a single supplier also contained palmatine, coptisine, and jatrorrhizine, thus indicating that the material was not pure goldenseal. Comparative data for three commercial sources of goldenseal root powder are presented. 相似文献
992.
Heat-induced aggregation of whey proteins in solutions made from two commercial whey protein concentrates (WPCs), one derived from mineral acid whey (acid WPC) and the other from cheese whey (cheese WPC), was studied using polyacrylamide gel electrophoresis (PAGE), size exclusion chromatography (SEC), and transmission electron microscopy (TEM). Heat treatment (75 degrees C) of acid WPC solutions (12.0%, w/w, pH 6.9) resulted in formation of relatively small "soluble" aggregates that were predominantly disulfide-linked. By contrast, heat treatment of the cheese WPC solutions (under the same conditions) caused formation of relatively large aggregates, containing high proportions of aggregates linked by noncovalent associations. The rate of aggregation of both beta-lactoglobulin and alpha-lactalbumin at 75 degrees C, measured as the loss of native proteins by PAGE, was higher in the cheese WPC solution than in the acid WPC solution. Cross dialysis of the two WPC solutions resulted in alteration of the mineral composition of each WPC solution and reversing their heat-induced aggregation behavior. The results demonstrated that the mineral composition is very important in controlling the aggregation behavior of WPC products. 相似文献
993.
Variations of fatty acid compositions, glycosides precursors, and lipoxygenase and glycosidase enzymatic activities were used simultaneously to differentiate for nine genetically different cultivated teas, four seasonal changes, and the affect of leaf maturity. The muscatel flavor of second-flush teas was associated with increased activities of glycosidase and several terpenes, phenolics, and aliphatic compounds bound to glycosides, whereas high levels of fatty acids and lipoxygenase activity biosynthesized more green volatiles in monsoon teas. Sequential hydrolysis of lipids and lipoxygenase-mediated reactions, during withering and rolling, showed a 3-fold increase of hexenol, hexenal, and related volatiles, but they decreased to the levels of fresh leaf during drying. However, a 4-fold increase of the floral bend of volatiles found in black tea developed due to the hydrolysis of the glycoside precursors throughout processing stages. About 45 key volatiles were monitored for flavor superiority among different clones. Various parameters affecting yield of volatiles were optimized and recommended. 相似文献
994.
Y Karagül-Yüceer M A Drake K R Cadwallader 《Journal of agricultural and food chemistry》2001,49(6):2948-2953
Application of aroma extract dilution analysis (AEDA) on the volatile components of low-, medium-, and high-heat-treated nonfat dry milks (NDM) revealed aroma-active compounds in the log(3) flavor dilution (log(3) FD) factor range of 1 to 6. The following compounds contributed the highest log(3) FD factors to overall NDM flavor: 2,5-dimethyl-4-hydroxy-3(2H)-furanone [(Furaneol), burnt sugar-like]; butanoic acid (rancid); 3-(methylthio)propanal [(methional), boiled potato-like]; o-aminoacetophenone (grape-like); delta-decalactone (sweet); (E)-4,5-epoxy-(E)-2-decenal (metallic); pentanoic acid (sweaty); 4,5-dimethyl-3-hydroxy-2(5H)-furanone [(sotolon), curry]; 3-methoxy-4-hydroxybenzaldehyde [(vanillin), vanilla]; 2-acetyl-1-pyrroline and 2-acetyl-2-thiazoline (popcorn-like); hexanoic acid (vinegar-like); phenylacetic acid (rose-like); octanoic acid (waxy); nonanal (fatty); and 1-octen-3-one (mushroom-like). The odor intensities of Furaneol, butanoic acid, methional, o-aminoacetophenone, sotolon, vanillin, (E)-4,5-epoxy-(E)-2-decenal, and phenylacetic acid were higher in high-heat-treated samples than others. However, the odor intensities of lactones, 2-acetyl-1-pyrroline, and 2-acetyl-2-thiazoline were not affected by heat treatment. Sensory evaluation results also revealed that heat-generated flavors have a major impact on the flavor profile of NDM. 相似文献
995.
Soil nutrient changes due to land use changes in Northern China: a case study in Zunhua County, Hebei Province 总被引:4,自引:0,他引:4
Abstract. Since the 1980s, land use in rural areas of China has changed greatly as the result of political initiatives. These changes have caused soil nutrient changes which are examined in this paper for Zunhua County, northern China from 1980 to 1999. The areas of farmland, grassland, and paddy decreased greatly and were replaced by increases in forest and residential land. The soils under forest in 1999 transformed from farmland in 1980 increased in organic matter by 21%, total nitrogen by 18%, available nitrogen by 65%, available phosphorus by 17% and available potassium by 17%. Similarly, in the area which was converted from farmland in 1980 to grassland in 1999, soil organic matter, total nitrogen, available nitrogen, available phosphorus, and available potassium increased by 38%, 37%, 71%, 2% and 28%, respectively. Changes from farmland to forest and grassland not only changed land cover but also improved soil fertility and probably reduced soil nutrient losses. 相似文献
996.
K. Reinink 《Euphytica》1991,54(1):83-92
Summary The genetics of nitrate content in butterhead lettuce (Lactuca sativa L.) was studied using the mean values of five parental genotypes and several generations obtained from crosses between them. One high nitrate parental genotype was chosen and four low nitrate ones. A diallel analysis showed additive genetic effects to be the major source of variation in generation means. Estimates of additive genetic effects differed significantly between experiments, indicating genotype x experiment interactions. Effects of dominance were relatively small. The size and direction of dominance varied between experiments. Reciprocal differences were of very limited size and also varied between experiments. The inheritance of nitrate content in lettuce fitted the additive-dominance genetic model. 相似文献
997.
C. G. Campbell R. B. Mehra S. K. Agrawal Y. Z. Chen A. M. Abd El Moneim H. I. T. Khawaja C. R. Yadov J. U. Tay W. A. Araya 《Euphytica》1993,73(1-2):167-175
Efforts in grasspea (Lathyrus sativus) improvement have increased since the development of lines that are very low in the neurotoxin Beta-N-oxalyl-L-alpha-beta-diamino propionic acid (ODAP); also referred to as Beta-N oxalyl-amino-L-alanine (BOAA). Many programs now address several related aspects of improvement simultaneously. These include reduced ODAP concentrations, insect and disease resistance, nitrogen fixation, agronomic practices, fodder and forage production, and components for increased yielding ability. The coordinated, multidisciplinary approach now being applied to the genetic improvement of grasspea should allow the potential of this largely neglected grain legume to be fully realized. 相似文献
998.
Integrated evaluation of different electromagnetic impacts on biological properties of soils in southern Russia 总被引:1,自引:0,他引:1
T. V. Denisova K. Sh. Kazeev S. I. Kolesnikov V. F. Val’kov 《Eurasian Soil Science》2011,44(11):1269-1273
An integral estimation of the effect of γ radiation at doses of 1, 5, 10, and 20 kGy, microwave radiation of 800 W, and an
alternating magnetic field of industrial frequency (50 Hz) on biological properties of soils in southern Russia was performed
on the basis of the integral index of the biological soil status (IIBSS). By the degree of resistance to an alternating magnetic
field estimated from the IIBSS value, the soils of southern Russia formed the following series: brown forest soil > chernozem
≥ soddy-calcareous soil ≥ gray sandy soil. By the resistance to microwave radiation estimated from the IIBSS value, the soils
formed the following series: gray sandy soil > chernozem > chestnut soil ≥ brown forest soil. 相似文献
999.
通过对比中国黄土高原地区与俄罗斯欧洲部分东南部地区森林生长状况,分析了人工林系统的异同点,探索了可持续的人工造林策略.认为必须通过重新分配大气降水而补充林下土壤水分来改善人工林的生长状况和缓解土壤干化,在中国黄土高原地区可通过调节暴雨径流来实现,而在俄罗斯可通过集中降雪和调节融雪径流实现. 相似文献
1000.
Y. Takazawa T. Nishino Y. Sasaki H. Yamashita N. Suzuki K. Tanabe Y. Shibata 《Water, air, and soil pollution》2009,202(1-4):57-67
Comprehensive survey of major rivers in the Tokyo metropolitan area was conducted for clarifying the emission sources of perfluorooctane sulfonate (PFOS) in Tokyo. PFOS was found at all sampling sites at concentrations ranging from 0.5 to 58 ng L?1; in addition to this, it was also indicated that unknown PFOS emission sources are present in the midstream of the Tama River basin. The relationship between PFOS and perfluorooctanoic acid (PFOA) was constant at a ratio of 10:3 (PFOS/PFOA) throughout the Tama River basin. The sum of daily load amounts of PFOS from Tokyo’s major rivers to Tokyo Bay reached 215 g day?1. This value corresponds to 12.8 μg day?1 per person using the sum of wastewater treatment district populations. In contrast, an estimation of PFOS contribution of domestic wastewater was also attempted, and the contribution was 1.6 μg day?1 per person. We took samples up trunk sewers in the Tama River and finally found at the highest PFOS concentration (58,000 ng L?1) from one of the wastewater of the electronic parts manufacturing facilities. 相似文献