全文获取类型
收费全文 | 58713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 |
7424篇 | |
综合类 | 6816篇 |
农作物 | 2099篇 |
水产渔业 | 3188篇 |
畜牧兽医 | 32888篇 |
园艺 | 724篇 |
植物保护 | 3295篇 |
出版年
2019年 | 496篇 |
2018年 | 955篇 |
2017年 | 1035篇 |
2016年 | 1007篇 |
2015年 | 865篇 |
2014年 | 1011篇 |
2013年 | 2320篇 |
2012年 | 1804篇 |
2011年 | 2199篇 |
2010年 | 1436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 | 1537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.
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
992.
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
993.
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
994.
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
995.
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
996.
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
997.
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
998.
999.
The changes in relative efficacy of two incompatible behaviors was investigated during growth of the lobster, Homarus americanus. In larval and early juvenile stages, physiological and morphological factors favor use of the escape response over defensive behavior. In large animals, defensive behavior is preferred almost exclusively to escape behavior unless the claws are lost. The interaction of escape and defensive behavior is modified by neural and morphological factors, which are dependent on the stage in the life cycle of the organism. 相似文献
1000.
Natal homing in a marine fish metapopulation 总被引:1,自引:0,他引:1
Identifying natal origins of marine fishes is challenging because of difficulties in conducting mark-recapture studies in marine systems. We used natural geochemical signatures in otoliths (ear bones) to determine natal sources in weakfish (Cynoscion regalis), an estuarine-spawning marine fish, in eastern North America. Spawning site fidelity ranged from 60 to 81%, comparable to estimates of natal homing in birds and anadromous fishes. These data were in contrast to genetic analyses of population structure in weakfish. Our findings highlight the need for consideration of spatial processes in fisheries models and have implications for the design of marine reserves in coastal regions. 相似文献