全文获取类型
收费全文 | 9545篇 |
免费 | 236篇 |
国内免费 | 14篇 |
专业分类
林业 | 1412篇 |
农学 | 241篇 |
基础科学 | 85篇 |
1538篇 | |
综合类 | 852篇 |
农作物 | 307篇 |
水产渔业 | 495篇 |
畜牧兽医 | 4052篇 |
园艺 | 260篇 |
植物保护 | 553篇 |
出版年
2024年 | 121篇 |
2023年 | 91篇 |
2022年 | 94篇 |
2021年 | 130篇 |
2020年 | 171篇 |
2019年 | 114篇 |
2018年 | 268篇 |
2017年 | 216篇 |
2016年 | 284篇 |
2015年 | 216篇 |
2014年 | 218篇 |
2013年 | 488篇 |
2012年 | 463篇 |
2011年 | 451篇 |
2010年 | 282篇 |
2009年 | 202篇 |
2008年 | 370篇 |
2007年 | 426篇 |
2006年 | 376篇 |
2005年 | 310篇 |
2004年 | 291篇 |
2003年 | 268篇 |
2002年 | 256篇 |
2001年 | 170篇 |
2000年 | 158篇 |
1999年 | 131篇 |
1995年 | 53篇 |
1992年 | 98篇 |
1991年 | 114篇 |
1990年 | 133篇 |
1989年 | 142篇 |
1988年 | 123篇 |
1987年 | 102篇 |
1986年 | 107篇 |
1985年 | 98篇 |
1984年 | 96篇 |
1983年 | 77篇 |
1979年 | 104篇 |
1978年 | 73篇 |
1977年 | 63篇 |
1975年 | 66篇 |
1974年 | 85篇 |
1973年 | 97篇 |
1972年 | 70篇 |
1971年 | 84篇 |
1970年 | 78篇 |
1969年 | 86篇 |
1968年 | 66篇 |
1967年 | 64篇 |
1965年 | 57篇 |
排序方式: 共有9795条查询结果,搜索用时 15 毫秒
41.
Landscape Ecology - Landscape archaeology has a lot to offer to landscape ecology, being an interdisciplinary approach that emphasizes the study of long-term human–environment dynamics. We... 相似文献
42.
43.
Marion A. Sonnleitner Madeleine S. Günthardt-Goerg Inga K. Bucher-Wallin Werner Attinger Sybille Reis Rainer Schulin 《Water, air, and soil pollution》2001,126(3-4):271-290
Sixteen open-top chambers, each equipped with two non-weighablegravity-drained lysimeter compartments, were used to investigate the impacts of elevated atmospheric carbon dioxide (CO2) concentration and nitrogen (N) deposition on the water relations and growth of young model forest ecosystems on two different types of soils. The same vegetation of a mixed spruce and beech overstorey and various herbs in the understorey was planted in all treatments on both soils. The soils were repacked on top of a drainage layer. Four combinations of treatments were applied in four replicates each: ambient (370 cm3 m-3) CO2 + low (7 kg N ha-1 a-1) N deposition, ambient CO2 + high(70 kg N ha-1 a-1) N deposition, elevated (590 cm3 m-3) CO2 + low N deposition, and elevated CO2 + high N deposition. After canopy closure, treatment effects on evapotranspiration and growth during the third year of study were very different for the two soils.On the acidic sandy loam, elevated CO2 enhanced growth(leaf biomass +21%, roots +27%) at reduced evapotranspiration (–9%). High N deposition increased aboveground growth even more strongly (+50%), but also increased evapotranspiration (+16%). Together, elevated CO2 and high N had a more than additive fertilizer effect on growth, while their effects on evapotranspirationcompensated. On the calcareous loamy sand, elevated CO2not only tended to enhance growth (leaf biomass +17%, roots +20%), but also increased evapotranspiration (+5%).On this soil, aboveground growth was stimulated by N only incombination with elevated CO2, but less than on the acidic soil, while evapotranspiration (–6.5%) and root growth into the subsoil (–54%) were decreased by increased N deposition at both CO2 concentrations, in contrast to the N treatments on the acidic sandy loam. The influence of the soil on the observed ecosystem responses canbe interpreted in terms of the concept of optimal resource allocation. 相似文献
44.
J. Flüchter 《Aquaculture (Amsterdam, Netherlands)》1982,27(1):83-85
Up to now the larval stages of fishes could not be reared successfully past metamorphosis when fed exclusively upon dry food. In rearing whitefish larvae, living Artemia nauplii could be replaced by shock-frozen ones (at ?196°C). This suggested that a substance essential for the whitefish larvae is present in living Artemia and preserved during rapid shock freezing. This substance seemed to be insoluble in water. It could be dissolved and extracted from the nauplii by acetone and could be transferred to a dry food. This enriched dry food offered to the whitefish larvae enabled them to grow up to and past metamorphosis successfully. 相似文献
45.
Kietzmann M Braun M Schneider M Pankow R 《Veterinary journal (London, England : 1997)》2008,178(1):115-118
Mammary glands taken at slaughter from healthy lactating cows were perfused in vitro with warmed and gassed Tyrode solution. Marbofloxacin was administered \"systemically\" via the perfusion fluid at concentrations similar to those measured in plasma following intravenous administration of 2mg/kg marbofloxacin. Samples from the perfusate were taken over a 24h period. Glandular tissue samples at different vertical distances from the teat up to the udder base were gathered from each of the four quarters after 3, 6, 12 and 24h. The marbofloxacin content of the tissue samples was analysed by high performance liquid chromatography with UV detection. The addition of marbofloxacin to the perfusion fluid produced median concentrations above the MIC90 (0.016microg/mL) against Escherichia coli at all glandular tissue sites measured after 3 and 6h with remarkable variations. Samples taken after 12 and 24h contained marbofloxacin in concentrations (median) of 0.22 (<0.05-0.32)microg/g and 0.13 (<0.05-0.16)microg/g. It is concluded that a systemic administration of marbofloxacin is well suited for the treatment of E. coli mastitis. 相似文献
46.
47.
Manfred Kerschberger Günther Franke Werner Schuricht 《Archives of Agronomy and Soil Science》2013,59(1-2):147-164
Die Gehalte für die sechs Schwermetalle Cd, Cr, Cu, Ni, Pb und Zn in landwirtschaftlich relevanten Stoffgruppen (mineralische und organische Düngestoffe, Kulturpflanzen und tierische Produkte) wurden aus Literaturdaten für die beiden statistischen Maßzahlen Mittelwert und Spannweite der Mittelwerte in Übersichtstabellen zusammengefasst. 相似文献
48.
Spolders M Sun H Wähner M Grün M Rehage J Flachowsky G 《Berliner und Münchener tier?rztliche Wochenschrift》2008,121(7-8):278-285
In a feeding trial at the Institute of Animal Nutrition of the Federal Research Institute of Animal Health (FLI) over 12 weeks with 20 cows of the German Holstein Breed the influence of different copper and zinc contents in the ration on their concentration in blood serum, liver and hair was tested. All animals received a diet based on maize- and grass silage ad libitum. The animals were divided in two groups with 10 cows each; group A received a concentrate according to their milk yield with a copper and zinc content as recommended (GfE 2001), whereas group B was offered a concentrate with roughly the double amount of copper and zinc. At the beginning and at the end of the trial a sample of blood, pigmented hair and a liver bioptate was taken from all animals to evaluate the incorporation of copper and zinc in these tissues. In serum and pigmented hair the copper concentrations did not differ between the two groups [13.4 for Group A and 12.5 micromol/L for Group B in serum respectively 6.8 (Group A) and 7.4 mg/kg DM (Group B) in pigmented hair]. Only the copper concentration in the liver was influenced by the different feeding. The higher copper content for group B resulted in a significantly higher copper concentration in the liver (506 mg/kg DM compared to 383 mg/kg DM). The liver is the best indicator organ for a sufficient copper supply. An increase in the zinc content in the ration resulted neither in higher zinc concentrations in serum (15.1 in Group B in comparison to 13.4 micromol/L for Group A) nor in higher zinc concentrations in liver (140 for Group B and 112 mg/kg DM for Group A) and pigmented hair (130 in Group A and 123 mg/kg DM in Group B). There is a significant correlation between copper intake and copper concentration in the liver (r = 0.46), whereas the correlation between zinc intake and zinc concentration in the liver is only tendencially (r = 0.23). The three tested samples serum, liver and cow hair are not qualified to reflect exactly a sufficient zinc supply. 相似文献
49.
S Gür G Türk E Demirci A Yüce M Sönmez Ş Özer EH Aksu 《Reproduction in domestic animals》2011,46(2):289-295
The aim of this study was to determine the changes in diameter of corpus luteum (CL), maternal progesterone (P) concentration, lipid peroxidation and non‐enzymatic antioxidant levels along with enzymatic antioxidant activities in pregnant ewes bearing single and twin foetuses. The ewes were selected from healthy animals that were brought to the abattoir for slaughtering. The ewes were divided into three groups: Group 1 (non‐pregnant, non‐oestrous, n = 30), Group 2 (pregnant bearing a single foetus, n = 30) and Group 3 (pregnant bearing twin foetuses, n = 12) after they were slaughtered. Pregnant ewes were in the first half of the pregnancy. The diameter of CL and P concentration of pregnant ewes bearing a single foetus or twin foetuses were found higher than that found in non‐pregnant ewes. Similarly, the P concentration of pregnant ewes bearing twin foetuses was higher than that found in pregnant ewes bearing a single foetus. Malondialdehyde (MDA) level in pregnant ewes bearing twin foetuses was higher than that found in both non‐pregnant and pregnant ewes bearing a single foetus. The serum glutathione (GSH) level and glutathione‐peroxidase (GSH‐Px) activity of pregnant ewes bearing twin foetuses were found lower than that found in non‐pregnant ewes. Additionally, the GSH‐Px activity of pregnant ewes bearing twin foetuses was found lower than that found in pregnant ewes bearing a single foetus. No significant difference was found between pregnant ewes bearing female and male foetus with respect to diameter of CL, P concentration and oxidative stress parameters. There were significant positive correlations between foetal number (0, 1, 2) and diameter of CL, P concentration, MDA level, and between P concentration and diameter of CL, MDA level. However, significant negative correlations were found between foetal number (0, 1, 2) and GSH level, GSH‐Px activity, and between P concentration and GSH‐Px activity. In conclusion, the diameter of CL enlarges, P production increases and oxidant/antioxidant balance impairs because of the gestation stress in ewes during pregnancy. 相似文献
50.
Summary In microdensitometry, based on X-ray absorption technique and a direct, electronic detector technique, it is usually not possible to observe the collimator and its alignment to the annual ring under investigation during the measurement. In this paper a method for automatic alignment of the collimator to the ring is described. The operating technique is based on an optimization of the resolution of the instrument at every annual ring while the collimator is rotated to the best position. The procedure has been in use a number of years and has proven itself to be efficient and reliable. The theory of operation for the procedure is described and its software structure is shown in a block diagram. Advantages and limitations of the procedure are discussed.The funding of this project is made by Nordbanken. The density profile in Fig. 5 has been recorded by Mr. Bo Larsson and is published with his kind permission 相似文献