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   43678篇
  免费   398篇
  国内免费   476篇
安全科学   1096篇
废物处理   2398篇
环保管理   5640篇
综合类   6301篇
基础理论   11861篇
环境理论   8篇
污染及防治   10224篇
评价与监测   3428篇
社会与环境   3375篇
灾害及防治   221篇
  2023年   170篇
  2022年   358篇
  2021年   391篇
  2020年   242篇
  2019年   322篇
  2018年   1852篇
  2017年   1800篇
  2016年   1932篇
  2015年   746篇
  2014年   1021篇
  2013年   2943篇
  2012年   1611篇
  2011年   2778篇
  2010年   1914篇
  2009年   1858篇
  2008年   2402篇
  2007年   2632篇
  2006年   1418篇
  2005年   1271篇
  2004年   1252篇
  2003年   1224篇
  2002年   1199篇
  2001年   1338篇
  2000年   933篇
  1999年   564篇
  1998年   430篇
  1997年   445篇
  1996年   479篇
  1995年   552篇
  1994年   460篇
  1993年   406篇
  1992年   439篇
  1991年   419篇
  1990年   385篇
  1989年   374篇
  1988年   341篇
  1987年   293篇
  1986年   284篇
  1985年   297篇
  1984年   324篇
  1983年   310篇
  1982年   312篇
  1981年   266篇
  1980年   204篇
  1979年   220篇
  1978年   188篇
  1977年   161篇
  1975年   165篇
  1973年   198篇
  1972年   183篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
121.
Spilt notochord syndrome is an extremely rare form of spinal dysraphism characterized by a complete cleft of the spine and a persistent communication between endoderm and ectoderm. A variant of split notochord syndrome was diagnosed in a 25-week-old fetus showing a prolapsed congenital colostomy and a spinal cystic lesion. The final diagnosis included protruding colon segment, imperforate anus with a rectourethral fistula and lipomyelomeningocele. The ultrasound features of the condition and the post-natal management are discussed. The neonate was successfully treated with a posterior sagittal anorectoplasty, while the lipomyelomeningocele was resected at a later stage. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   
122.
123.
124.
Red pine (Pinus resinosa Ait.) is rare (< 15 000 mature trees) in Newfoundland and is known from only 22 locations in the central region. Red pine occupies 3 major site types in Newfoundland: 1) red pine on medium-textured sands (RP1), 2) red pine on coarse-textured glaciofluvial deposits (RP2), and 3) red pine on Folisols over bedrock (RP3). The succession of red pine site types after cutting is from red pine to Kalmia-black spruce (Picea mariana (Mill.) B.S.P.) for RP1, and to Cladonia-Kalmia-black spruce for types RP2 and RP3. Succession after fire is usually to the pre-fire type, but this depends on the severity of the fire.Although occupying a relatively poor site, red pine at 60–70 years reaches heights in excess of 18 m, dbh in excess of 40 cm, and individual tree volumes greater than 1 m3 were recorded in 75 stemanalyzed fire-killed trees. Black spruce on that same site produces less than one-third that volume in 60 years. Merchantable volume of 140–280 m3 ha-1 were recorded i.e., Canada Land Inventory (CLI) forest capability class 5 and class 4 ratings. This raises the CLI rating two capability classes if red pine were occupying these poor quality sites over black spruce. In terms of nutrition, even the best growing red pine are nitrogen (N) deficient as shown by foliar analysis. All natural stands have foliar N concentrations below 1.3% which is the critically low level shown in the literature. Immediately after fire, foliar concentrations reach this level but are usually about 1% or less. Most other nutrients are low but are within the generally reported adequate levels in testing for P, K, Ca and Mg.Fire influences soil nutrient availability as pH increases in the RP1 type. Burning temperature also affects soil pH and the understory vegetation. The RP2 type loses more N in hotter burns on this site type and more N is tied up in these ortstein hardpan soils. The pattern of regeneration following wildfire is related to slope, density, age and species mixture of the stand as well as the thickness and composition of the duff layer.  相似文献   
125.
126.
Loss of genetic diversity in Harpacticoida near offshore platforms   总被引:1,自引:0,他引:1  
Offshore oil and gas production platforms can be a source of chronic stress that could lead to sublethal impacts on resident benthic organisms. In June 1993 and January 1994, genetic diversity of Harpacticoida (Copepoda) living proximal to operating, offshore platforms in the Gulf of Mexico was estimated to test if platforms are associated with strong selective pressures. Because harpacticoids have short generation times and direct benthic development, they are suitable organisms for examining population responses. Genetic diversity was estimated by comparing restriction fragment length polymorphisms generated from mitochondrial DNA amplified by the polymerase chain reaction on individuals of five species (Cletodes sp., Enhydrosoma pericoense, Normanella sp., Robertsonia sp., and Tachidiella sp.). Populations living at Near regions (stations<50 m from a platform) had significantly less haplotype diversity than populations of the same species living at Far regions (stations>3 km from a platform). The levels of haplotype diversity exhibited by the Far populations were similar at three different platforms located hundreds of kilometers apart. The differences in haplotype diversity between Near and Far regions were the result of a higher proportion of dominant haplotypes, and a loss of less common haplotypes. Haplotypic diversity was inversely correlated with a multivariate measurement of levels of sediment contaminants. The pattern of haplotype diversity on the Gulf of Mexico continental shelf seems to consist of a uniform level of haplotype diversity, punctuated by islands of lower diversity around oil and gas platforms. The selective pressures that lead to a loss of genetic diversity may be the result of contaminants, other differences in the physico-chemical environment, or disturbance in general.  相似文献   
127.
Guzman N  Paz MX  Paz MI  Moreno NR  Nino F 《Disasters》1989,13(2):153-164
In the past 20 years a variety of indices have been suggested for measuring the severity of trauma, however none of them meets the requirement of being a simple and objective instrument that can be utilized efficiently by lay persons and providers of health services without previous experience in triaging. A new system has been proposed which meets this requirement and which has been validated with 197 injured persons at the emergency ward of the Valle University Hospital in Call, Colombia. The instrument showed a high level of concordance with the conventional triaging method and classifies victims into four categories: critical with recovery unlikely, critical but recoverable, moderately serious, and ambulatory. There is discussion on the reliability of this instrument and it is recommended that it be validated with victims from larger disasters. Regardless, it has the advantage of being specific and free of the subjectivity that characterizes other indices.  相似文献   
128.
Winer N 《Disasters》1989,13(1):1-8
Food security in Ethiopia is discussed in the context of the repeated famines and the international responses both to them and to the socialist agricultural policies being pursued by Ethiopia. Increasing concern has been expressed by the international donor community regarding the ability of Ethiopia to absorb development funds without a major shift in emphasis in agricultural policy-making. The background to Ethiopia's present vulnerability is shown both in terms of the size of the vulnerable population and in terms of the poor performance of the agricultural sector in the last decade. The author looks at the present agricultural and marketing policy reforms and questions whether they are sufficient to generate the sort of international response needed to create the level of food security that would be required to avert future famines.  相似文献   
129.
Baron RC  Etzel RA  Sanderson LM 《Disasters》1988,12(4):356-365
On 11 August 1985, a large inadvertent release occurred from a chemical manufacturing plant in Institute, West Virginia. The emission was related to overheating of a storage tank that contained chemicals for the production of aldicarb, a cholinesterase inhibiting pesticide. As a result of this emission, 136 persons were treated in five local emergency rooms, 29 of whom were hospitalized for one or more days, mostly for observation. No fatalities resulted. Interviews with treated persons and non-treated community residents confirmed that the health effects were transient and compatible with exposure to irritating vapors rather than with exposure to methyl isocyanate (a chemical used in the process) or to aldicarb. Only 5% of the treated persons and 5% of the community residents surveyed were adequately warned of the emission by the plant siren. Because persons in the nearby community are potentially vulnerable to chemical injury, we recommend the development of an improved warning system, a formal evacuation procedure, and other measures to limit exposure of persons in this area in the event of a future release.  相似文献   
130.
Oxygen and carbon isotopic composition of the aragonite of fish otoliths was measured on 175 specimens comprising 24 different species in 1989 and 1990. All specimens but two came from the northern Adriatic Sea or the northern Tyrrhenian Sea (two freshwater specimens were studied for comparison with the marine fish). The data obtained confirm the results of previous research suggesting the existence of equilibrium conditions between the otolith aragonite and ambient water with respect to ·18O(CO3 2-) values. Examination of one of the species indicated that the CaCO3 of otoliths probably accumulates continuously over time, seasonal isotopic changes being clearly visible (from a set of radial spot samples) for both oxygen and carbon isotopes. The apparent isotopic equilibrium with ambient water suggests that the 18O(H2O) of the endolymph is equal to that of seawater and considerably different from that of fish body water. In the case of 13C(CO3 2-), isotopic equilibrium with dissolved carbon species in seawater is never reached, even though the contribution of metabolic CO2 is variable among different species and even among different individuals of the same species. This rules out the possibility of using 13C(CO3 2-) values obtained from fossil otoliths for paleoenvironmental and paleobiological conclusions.  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