全文获取类型
收费全文 | 68401篇 |
免费 | 3309篇 |
国内免费 | 701篇 |
专业分类
林业 | 3083篇 |
农学 | 2424篇 |
基础科学 | 885篇 |
6767篇 | |
综合类 | 17178篇 |
农作物 | 2808篇 |
水产渔业 | 2565篇 |
畜牧兽医 | 31983篇 |
园艺 | 977篇 |
植物保护 | 3741篇 |
出版年
2020年 | 615篇 |
2018年 | 700篇 |
2017年 | 851篇 |
2016年 | 768篇 |
2015年 | 769篇 |
2014年 | 851篇 |
2013年 | 2215篇 |
2012年 | 1677篇 |
2011年 | 1891篇 |
2010年 | 1334篇 |
2009年 | 1248篇 |
2008年 | 1793篇 |
2007年 | 1676篇 |
2006年 | 1646篇 |
2005年 | 1611篇 |
2004年 | 1428篇 |
2003年 | 1524篇 |
2002年 | 1567篇 |
2001年 | 1695篇 |
2000年 | 1608篇 |
1999年 | 1288篇 |
1995年 | 648篇 |
1994年 | 625篇 |
1992年 | 1350篇 |
1991年 | 1440篇 |
1990年 | 1469篇 |
1989年 | 1512篇 |
1988年 | 1418篇 |
1987年 | 1367篇 |
1986年 | 1404篇 |
1985年 | 1379篇 |
1984年 | 1151篇 |
1983年 | 1006篇 |
1982年 | 718篇 |
1981年 | 691篇 |
1980年 | 645篇 |
1979年 | 1127篇 |
1978年 | 916篇 |
1977年 | 819篇 |
1976年 | 768篇 |
1975年 | 860篇 |
1974年 | 1126篇 |
1973年 | 1063篇 |
1972年 | 1128篇 |
1971年 | 1080篇 |
1970年 | 1022篇 |
1969年 | 874篇 |
1968年 | 710篇 |
1967年 | 844篇 |
1966年 | 693篇 |
排序方式: 共有10000条查询结果,搜索用时 156 毫秒
991.
Mud crab larvae, Rhithropanopeus harrisii were exposed to tributyltin oxide, (TBTO) (0.5, 1, 5, 10, 15, or 25 μg l?1) or tributyltin sulfide (TBTS) (0.5, 1, 5, 20, 30, or 50 μg l?1). Differential survival was observed in TBTO concentrations equal to or greater than 10 gg l?1 and in TBTS concentrations equal to or greater than 20 μg l?1 Growth and development rate were assessed as sublethal indices of stress. Of the two, development rate was more sensitive. All groups exposed to either compound had slower development rates than controls. Growth of the larvae, as shown by weight gain, was slightly increased in low exposures (a hormetic response) and decreased significantly in higher concentrations. The mean weight of the 50 μg l?1 TBTS group showed the largest decrease, to 57% of the control. The hormesis response was further examined by measuring daily growth of the zoeae. All groups exposed to tributyltin compounds showed an initial growth lag. The hormesis response occurred in the fourth zoeal stage or megalops. While R. harrisii is relatively tolerant to the acute toxicity of tributyltin compounds compared to other marine crustaceans tested, sublethal responses occur in very low concentrations. This points to the need for analyses to characterize tributyltin environmental concentrations. 相似文献
992.
The major element compositions of three soil profiles from the north-east of Scotland have been determined and from them losses and gains of major elements have been calculated. Eluvial/illuvial coefficients are also calculated and used to measure the intensities of soil processes. This information forms the basis of a discussion on the development of the profiles. The main process involves the decomposition of minerals, especially in the upper horizons, the loss of constituents to drainage and the translocation ofa few to the lower horizons. 相似文献
993.
994.
The depletion of hypolimnetic DO and the upper depth boundary of anoxia for four different years (1978, 1979, 1980 and 1981), and the accumulation of sulfide for a single year (1981), are documented for ionically enriched hypereutrophic Onondaga Lake, NY, USA. The depletion rate, represented as the areal hypolimnetic oxygen deficit (AHOD, g m?2 day?1), was extremely high (1.2–2.7 g m?2 day?1), The large differences in the rate within individual years and from year to year were largely a result of differences in attendant vertical mixing (parameterized as the hypolimnetic heating rate). The entire hypolimnion (depth interval from 11 to 20 m) was without O2 by late June of all 4 yr; anoxia was observed above the hypolimnion on some occasions when secondary stratification occurred. Sulfide accumulated progressively in the hypolimnion in 1981 following the onset of anoxia to a volume weighted concentration of I1 mg L?1. The ionic discharge from an adjoining alkali manufacturer exacerbated the problem of limited O2 resources of the hypolimnion by: (1) decreasing vertical mixing, (2) prolonging the duration of stratification, (3) causing abbreviated turnovers, and (4) encouraging increased rates of phytoplankton settling. 相似文献
995.
J. A. A. R. Schuurkes I. C. C. Heck P. L. G. M. Hesen R. S. E. W. Leuven J. G. M. Roelofs 《Water, air, and soil pollution》1986,31(1-2):267-272
In a greenhouse, seven identical mini-ecosystems, simulating soft water ponds, were exposed to different types of artificial rain water. The effects of rain water containing H2SO4 and nitrate, and rain water containing ammonium sulphate on water quality and vegetation were studied and compared. Causal relations were established between rain water quality, water chemistry and changes in floristic composition. Ammonium sulphate deposition, particularly, strongly affected water quality and vegetation development. Although ammonium sulphate deposition was only slightly acid, due to nitrification it acted as an important acid source, causing acidification to pH=3.8. Under acidified conditions, ammonium sulphate deposition lead to a luxuriant growth ofJuncus bulbosus andAgrostis canina. In the mini-ecosystems, H2SO4 deposition with a pH of 3.5 only decreased the pH of the water to 5.1 within 1 yr. The acidification of water appeared to be coupled with changes in alkalinity, sulphate, Al, Cd, Ca, Mg, K and inorganic-N. It is concluded that in NH3-affected regions in The Netherlands, the high atmospheric deposition of ammonium sulphate probably contributes to a large extent in the acidification, eutrophication and floristic changes of oligotrophic soft waters. 相似文献
996.
997.
998.
Growth and vigor of trees show considerable variations in young teak plantations in Benin (Vertisols) as well as in Liberia (Ferralsols). Differences in growth are mainly related to topsoil acidity and the foliar Ca-status in Liberia. In Benin, waterlogging (followed by root decay) reduces the Mg-, K- and N-uptake. In addition, growth on the Vertisols is limited by a low KEX/CEC-ratio of usually <> 相似文献
999.
1000.