全文获取类型
收费全文 | 20590篇 |
免费 | 234篇 |
国内免费 | 121篇 |
专业分类
安全科学 | 520篇 |
废物处理 | 888篇 |
环保管理 | 2758篇 |
综合类 | 3383篇 |
基础理论 | 5813篇 |
环境理论 | 10篇 |
污染及防治 | 5305篇 |
评价与监测 | 1278篇 |
社会与环境 | 858篇 |
灾害及防治 | 132篇 |
出版年
2021年 | 136篇 |
2019年 | 127篇 |
2018年 | 234篇 |
2017年 | 251篇 |
2016年 | 401篇 |
2015年 | 306篇 |
2014年 | 467篇 |
2013年 | 1598篇 |
2012年 | 573篇 |
2011年 | 815篇 |
2010年 | 666篇 |
2009年 | 661篇 |
2008年 | 829篇 |
2007年 | 863篇 |
2006年 | 765篇 |
2005年 | 662篇 |
2004年 | 648篇 |
2003年 | 628篇 |
2002年 | 609篇 |
2001年 | 763篇 |
2000年 | 574篇 |
1999年 | 325篇 |
1998年 | 261篇 |
1997年 | 278篇 |
1996年 | 279篇 |
1995年 | 331篇 |
1994年 | 302篇 |
1993年 | 284篇 |
1992年 | 277篇 |
1991年 | 278篇 |
1990年 | 294篇 |
1989年 | 277篇 |
1988年 | 249篇 |
1987年 | 240篇 |
1986年 | 222篇 |
1985年 | 200篇 |
1984年 | 255篇 |
1983年 | 210篇 |
1982年 | 271篇 |
1981年 | 219篇 |
1980年 | 184篇 |
1979年 | 201篇 |
1978年 | 184篇 |
1977年 | 153篇 |
1976年 | 138篇 |
1975年 | 143篇 |
1974年 | 159篇 |
1973年 | 162篇 |
1972年 | 148篇 |
1971年 | 146篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
131.
Joe Leigh Simpson James L. Mills George G. Rhoads George C. Cunningham Howard J. Hoffman Mary R. Conley 《黑龙江环境通报》1991,11(8):641-648
No clear answer concerning whether multivitamin/folate supplementation prevents neural tube defects (NTDs) is provided by three studies in the United States. All these studies are occurrence in nature, no recurrence studies having been conducted. The Atlanta Birth Defects Study is subject to pronounced memory and recall biases, the length between event and interview being as long as 16 years. In a second study (Boston University), objections can be raised to certain aspects of the experimental design, and the claim that 22 per cent of women started vitamins sufficiently early after pregnancy diagnosis to influence NTD formation is suspicious. Our NICHD case control study of 541 women in California and Illinois revealed no evidence for multivitamins or folic acid preventing NTDs. U.S. public policy-makers face difficulties in applying results of recurrence or occurrence studies in high-risk areas to low-risk areas in the U.S. 相似文献
132.
133.
The influence of source and timing of nitrogen fertilizers on yield and nitrogen use efficiency of four sweet potato cultivars 总被引:2,自引:0,他引:2
R. O. Ankumah V. Khan K. Mwamba K. Kpomblekou-A 《Agriculture, ecosystems & environment》2003,100(2-3):201
Nitrogen (N) availability is a very important determinant of crop yield. It is also one of the most expensive inputs in crop production. Thus, the efficient management of N by farmers with limited resource is a very important part of successful soil and crop management system. A field study was conducted on a Norfolk sandy loam soil at the George Washington Carver Agricultural Experiment Station, Tuskegee, AL, to evaluate the influence of fertilizer source and timing on the yield of four sweet potato [Ipomoea batatas (L.)] cultivars. Two N sources (urea, CO(NH2)2 and ammonium nitrate, NH4NO3), four sweet potato cultivars (‘Georgia Jet’, GA-Jet; ‘TU-82-155’, TU-155; ‘TU-1892’; and ‘Rojo Blanco’, Rojo-Bl) and three times (zero, single, and split applications) were used in a factorial designed experiment. Plants were harvested after 80 and 120 days and total and marketable yields determined. Nitrogen recovery efficiency, yield efficiency and physiological efficiency were also determined. Total and marketable yields of early maturing cultivars were significantly higher than of late maturing cultivars (P<0.05). The single application of N resulted in significantly higher yield in storage roots than split application (P<0.05). Physiological efficiency values were highly correlated with total marketable yields. Recovery rates increased with time. Late maturing cultivars tended to have higher N recovery and physiological efficiency than early maturing cultivars. These results suggest that cultivar maturity group should play an important part in N fertilization recommendations for sweet potato. 相似文献
134.
R Burt M.A WilsonT.J Keck B.D Dougherty D.E StromJ.A Lindahl 《Advances in Environmental Research》2003,8(1):51-67
Long-term copper smelting in the Anaconda and Deer Lodge Valley area of Montana has resulted in an extensive area of trace element contamination. Aerial extent of contamination is generally established, but total analysis of soils does not correlate to relative degree of impact on vegetation growth. Three pedons (Beaverell, Cetrack and Judco) were analyzed by routine soil characterization methods, aqua regia microwave digestion, sequential chemical extraction, and X-ray diffraction analysis with the objective of providing a better understanding of chemical forms and potential reactivity of selected trace elements (Cd, Co, Cr, Cu, Hg, Mn, Ni, P, Pb). Surface horizons of soils are more acidic than subsoils, with pH for all horizons ranging from 4.0 to 8.7. Beaverell is the most contaminated in the upper 20 cm with the sum of total extractable (SUMTE) trace elements by microwave digestion ranging from 1836 to 3605 mg kg−1, largest H2O-soluble (WS) and exchangeable (EX) fractions (e.g. 1.6 and 9.3%, respectively), and smallest residual (RES) fraction (e.g. 14.3%). Cetrack has greater SUMTE elements than Judco, though a lower WS+EX fraction due to the effects of alkaline pH, carbonates and high P. Oxide (OX), organic matter/sulfide (OM/S), and RES fractions predominate over WS, EX, and specially-sorbed/carbonate-bound fractions (SS/CAR) for all horizons. Copper, Zn, Pb and Cd are elevated in surface over subsurface horizons in these latter fractions, indicating these elements were anthropogenic additions. X-ray data indicate that Pb, Cu, Cr and Cd partially exists as both OX and sulfide mineral forms. 相似文献
135.
在印度洋中部查戈斯礁群的大多数珊瑚死亡之后3年,对30m水深以浅珊瑚的侵蚀和恢复情况进行了研究.北部环礁15m水深以浅、中部和南部环礁>35m处的珊瑚差不多全部死亡.由于密集珊瑚丛的损失,一些礁体"表面"下降了1 5m.珊瑚的生物侵蚀情况严重,减少了三维礁体"结构"并形成松散的碎石.幼年珊瑚数量众多,尽管大部分是在侵蚀的或不稳定的基底上,并且稳定种较少.在15m的深度,礁体间鱼类丰度和多样性仍旧较高;依赖于珊瑚生存的物种减少,而一些食草动物和食碎屑动物则增加.一个新的海面温度(SST)数据集表明,平均SST自1950年以来升高了0.65℃.造成查戈斯珊瑚礁死亡的临界SST是29.9℃. 相似文献
136.
R.H. Field S. Benke K. Bdonyi R.B. Bradbury 《Agriculture, ecosystems & environment》2007,120(2-4):399-404
Bird use of conservation tillage-established (CT—seedbed preparation without soil inversion) crops was assessed at two adjacent sites in western Hungary, in comparison with conventionally established (mouldboard ploughed—P) fields. Bird abundance was assessed in a typical rotation of spring-sown maize and winter wheat in three consecutive winters. In the first winter, several species and the guild of seed-eating passerines were significantly more abundant on CT established plots, in both crop types. In the second winter, only starlings were more abundant on CT plots. In the third winter, only skylarks favoured CT over P plots. In combination with agri-environment scheme measures designed to provide resources for biodiversity, conservation tillage could play a role in softening the impact of agriculture on farmland birds in Hungary and elsewhere in Eastern Europe, in addition to its potential for reducing soil erosion and diffuse pollution. 相似文献
137.
J. Caspar Rüegg 《Die Naturwissenschaften》1998,85(12):575-582
2+ concentrations tropomyosin is located at the edge of the thin filament, thereby interfering with the formation of strong
actin-myosin linkages (blocked state). An increase in Ca2+ activity causes an azimuthal shift of tropomyosin around the filament (by about 30°), thereby increasing the probability
of low-force crossbridge interaction, a process which by cooperative effects induces further tropomyosin movement (by an additional
10°) which results in the open state of the filament characterized by forceful crossbridge interaction. (This mechanism may
be analogous to that in ligand-gated ion channels, where ligand binding increases the open probability of the pore.) The extent
of activation then depends on the free Ca2+ concentration and on the calcium sensitivity of the thin filament that may be affected by protein phosphorylation, crossbridge
attachment, the troponin isoform composition of the filament, and novel calcium-sensitizing drugs that act on the contractile
or regulatory proteins and thus increase the force of the heart. 相似文献
138.
139.
140.