全文获取类型
收费全文 | 76370篇 |
免费 | 3795篇 |
国内免费 | 52篇 |
专业分类
林业 | 3596篇 |
农学 | 2341篇 |
基础科学 | 392篇 |
7912篇 | |
综合类 | 15522篇 |
农作物 | 3080篇 |
水产渔业 | 3394篇 |
畜牧兽医 | 38507篇 |
园艺 | 904篇 |
植物保护 | 4569篇 |
出版年
2018年 | 856篇 |
2017年 | 925篇 |
2016年 | 856篇 |
2015年 | 779篇 |
2014年 | 952篇 |
2013年 | 2600篇 |
2012年 | 1676篇 |
2011年 | 2069篇 |
2010年 | 1359篇 |
2009年 | 1285篇 |
2008年 | 2035篇 |
2007年 | 2076篇 |
2006年 | 1894篇 |
2005年 | 1831篇 |
2004年 | 1751篇 |
2003年 | 1780篇 |
2002年 | 1690篇 |
2001年 | 2215篇 |
2000年 | 2219篇 |
1999年 | 1846篇 |
1998年 | 700篇 |
1997年 | 698篇 |
1995年 | 812篇 |
1993年 | 696篇 |
1992年 | 1489篇 |
1991年 | 1551篇 |
1990年 | 1666篇 |
1989年 | 1644篇 |
1988年 | 1527篇 |
1987年 | 1477篇 |
1986年 | 1527篇 |
1985年 | 1515篇 |
1984年 | 1247篇 |
1983年 | 1144篇 |
1982年 | 771篇 |
1981年 | 717篇 |
1979年 | 1248篇 |
1978年 | 979篇 |
1977年 | 848篇 |
1976年 | 803篇 |
1975年 | 913篇 |
1974年 | 1201篇 |
1973年 | 1240篇 |
1972年 | 1255篇 |
1971年 | 1182篇 |
1970年 | 1102篇 |
1969年 | 1003篇 |
1968年 | 863篇 |
1967年 | 906篇 |
1966年 | 837篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
941.
Oxidation of methane in the rhizosphere of rice plants 总被引:14,自引:0,他引:14
Oxidation of CH4 in the rhizosphere of rice plants was quantified using (1) methyl fluoride, a specific inhibitor of CH4 oxidation, and (2) measuring changes in plant-mediated CH4 emission after incubation under air, N2, or 40% O2. No significant rhizospheric CH4 oxidation was observed from rice plants in the ripening stage. CH4 emission from rice plants 1 week before panicle initiation increased by 40% if CH4 oxidation in the rhizosphere was blocked. The growth stage of the rice plant is an important factor determining the rhizospheric CH4 oxidation. Fluctuation of rhizospheric CH4 oxidation during the growing season may help to explain the observed seasonal CH4 emission patterns in field studies. Measurements from four rice varieties showed that one variety, Pokkali, had higher rhizospheric CH4 oxidation. This was probably because Pokkali was in an earlier growth stage than the other three varieties. Both in the early and in the late growth stages, incubation under N2 caused a much stronger CH4 flux than inhibition of CH4 oxidation alone. Apparently, N2 incubation not only blocked CH4 oxidation but also stimulated methanogenesis in the rhizosphere. Incubation under a higher O2 atmosphere (40% O2) than ambient air decreased the CH4 flux, suggesting that increasing the oxidation of the rice rhizosphere may help in reducing CH4 fluxes from rice agriculture. The O2 pressure in the rhizosphere is an important factor that reduces the plant-mediated CH4 flux. However, inhibition of methanogenesis in the rhizosphere may contribute more to CH4 flux reduction than rhizospheric CH4 oxidation. 相似文献
942.
Vermicomposting of coffee pulp using the earthworm Eisenia fetida: Effects on C and N contents and the availability of nutrients 总被引:1,自引:0,他引:1
In Colombia, more than 1 million tons of coffee pulp are produced every year. Its transformation into compost by means of turned piles has led to a final product with poor physical and chemical characteristics and vermicomposting has been suggested as an alternative method of transforming these wastes into a useful organic fertilizer. The ability of the earthworm Eisenia fetida to transform coffee pulp into a valuable compost was evaluated. The influence of bed depth and time on different C fractions, N content and availability of nutrients was studied. The results showed that the C and N contents were not affected by the depth of the bed, whereas time affected both. An increase in the fractionation ratio, determined by calculating the C in the fraction smaller than 100 m as a percentage of C in the samples as a whole, and low values of humic-like substances were recorded during vermicomposting. After ingestion of the pulp by the earthworms, an increase in available P, Ca, and Mg but a decrease in K were detected. 相似文献
943.
Glomus-wetland rice mycorrhizas influenced by nursery inoculation techniques under high fertility soil conditions 总被引:2,自引:0,他引:2
We investigated the effect of nursery inoculation techniques on mycorrhizal colonization and sporulation, growth responses,
and nutrient (N and P) uptake to determine the suitable nursey inoculation method of wetland rice (Oryza sativa L.) under high-fertility soil conditions. Seedlings were produced in dry-nursery (DN, watered to 60% of –0.03 MPa) and wet-nursery
(WN, 3–5 cm water from the soil surface) conditions with or without arbuscular mycorrhizal fungal (Glomus spp.) inoculation. Soil was γ-ray sterilized before use in this experiment. Mycorrhizal fungal colonization was 56% in DN
and 23% in WN plants at 6 weeks of growth. The arbuscular mycorrhizal fungal colonization was significantly higher in plants
of DN origin than in WN plants after transplantation to the pots, irrespective of growing stages. Mycorrhizal colonization
was significantly decreased to 28% in DN plants and to 25% in WN plants at harvest. The grain yield was significantly influenced
by nursery conditions. N and P acquisition of wetland rice plants inoculated with Glomus spp. was significantly greater than that of non-inoculated plants at maturity, especially in those originating from DN conditions.
P translocation from shoots to grain was accelerated by mycorrhizas.
Received: 6 April 1997 相似文献
944.
Phenolic antioxidants from the leaves of Corchorus olitorius L. 总被引:3,自引:0,他引:3
K Azuma M Nakayama M Koshioka K Ippoushi Y Yamaguchi K Kohata Y Yamauchi H Ito H Higashio 《Journal of agricultural and food chemistry》1999,47(10):3963-3966
Six phenolic antioxidative compounds [5-caffeoylquinic acid (chlorogenic acid), 3,5-dicaffeoylquinic acid, quercetin 3-galactoside, quercetin 3-glucoside, quercetin 3-(6-malonylglucoside), and quercetin 3-(6-malonylgalactoside) (tentative)] were identified from the leaves of Corchorus olitorius L. (moroheiya) by NMR and FAB-MS. The contents of these phenolic compounds, ascorbic acid, and alpha-tocopherol in C. olitorius leaves were determined, and their antioxidative activities were measured using the radical generator-initiated peroxidation of linoleic acid. The results obtained showed that 5-caffeoylquinic acid was a predominant phenolic antioxidant in C. olitorius leaves. 相似文献
945.
N H Low M McLaughlin H J Hofsommer D A Hammond 《Journal of agricultural and food chemistry》1999,47(10):4261-4266
The equilibrium method is introduced for the detection of invert sugar addition to apple juice. The method consists of a pre-equilibration of the sample with dry pyridine at 50 degrees C for 20 min followed by the addition of trimethylsilylimidazole and heating at 75 degrees C for 40 min. The resulting derivatized carbohydrates are then analyzed by capillary gas chromatography. This method was successfully used by independent laboratories to distinguish heated pure, intentionally adulterated (with invert sugar), and intentionally adulterated and then heated apple juice concentrates. The equilibrium method was shown to give significantly lower coefficients of variation for this sample set when compared to the original capillary gas chromatographic method. In addition, these results indicate that it may also be an effective method for the detection of medium invert sugar, depending on the level of the fingerprint oligosaccharides in this sweetener. 相似文献
946.
Flavonoid constituents of Chorizanthe diffusa with potential cancer chemopreventive activity 总被引:11,自引:0,他引:11
Chung HS Chang LC Lee SK Shamon LA van Breemen RB Mehta RG Farnsworth NR Pezzuto JM Kinghorn AD 《Journal of agricultural and food chemistry》1999,47(1):36-41
An ethyl acetate-soluble extract of Chorizanthe diffusa was found to exhibit significant antioxidant activity, as judged by scavenging stable 1,1-diphenyl-2-picrylhydrazyl (DPPH) free radicals and inhibition of 12-O-tetradecanoylphorbol 13-acetate (TPA)-induced free radical formation with cultured HL-60 cells. Bioassay-directed fractionation of this extract using the DPPH antioxidant assay as a monitor led to the isolation of five structurally related flavonoids (1-5), including the novel compound 5,8,3',4',5'-pentahydroxy-3, 7-dimethoxyflavone (1). Isolates 1-5 demonstrated varying degrees of antioxidant or antimutagenic activity. Two of the compounds, 5,7,3', 4'-tetrahydroxy-3-methoxyflavone (2) and quercetin (4), were subsequently found to inhibit carcinogen-induced preneoplastic lesions in a mouse mammary organ culture model. Inhibitory activity of this type is known to correlate with cancer chemopreventive effects in full-term models of tumorigenesis. 相似文献
947.
van het Hof KH Gärtner C Wiersma A Tijburg LB Weststrate JA 《Journal of agricultural and food chemistry》1999,47(4):1582-1586
Palm oil carotenoids are a mixture of alpha- and beta-carotenes, which are used as food colorants. They may also be applied as a functional food ingredient because of the provitamin A activity of alpha- and beta-carotenes and their proposed beneficial roles in the prevention of chronic diseases. This paper discusses the results of an incomplete balanced crossover study with 69 healthy adult volunteers to compare palm oil carotenoids with synthetic beta-carotene in their efficacies to increase plasma levels of carotenoids. Four days of supplementation with natural palm oil carotenoids (7.6 mg/day of alpha-carotene, 11.9 mg/day of all-trans-beta-carotene, 7.5 mg/day of cis-beta-carotene) or synthetic beta-carotene (23.8 mg/day of all-trans-beta-carotene, 4.4 mg/day of cis-beta-carotene), added to a mixed meal, resulted in significant increases in plasma levels of the supplied carotenoids as compared to consumption of a low-carotenoid meal (i.e., 7.2-fold increase in alpha-carotene and 3.5-fold increase in all-trans-beta-carotene following palm oil carotenoids; 6.9-fold increase in all-trans beta-carotene following synthetic beta-carotene). As the carotenoid content differed between the treatments, the relative plasma responses were calculated per milligram of beta-carotene intake. These were similar for the two supplements, suggesting that the presence of alpha-carotene does not affect the bioavailability of beta-carotene from palm oil. It was concluded that 4 days of supplementation with palm oil carotenoids or synthetic beta-carotene improves the plasma beta-carotene status substantially, whereas alpha-carotene is additionally delivered by the palm oil supplement. 相似文献
948.
Fractional extraction of compounds from grape seeds by supercritical fluid extraction and analysis for antimicrobial and agrochemical activities 总被引:3,自引:0,他引:3
Palma M Taylor LT Varela RM Cutler SJ Cutler HG 《Journal of agricultural and food chemistry》1999,47(12):5044-5048
White grape seeds were subjected to sequential supercritical fluid extraction. By increasing the polarity of the supercritical fluid using methanol as a modifier of CO(2), it was possible to fractionate the extracted compounds. Two fractions were obtained; the first, which was obtained with pure CO(2), contained mainly fatty acids, aliphatic aldehydes, and sterols. The second fraction, obtained with methanol-modified CO(2), had phenolic compounds, mainly catechin, epicatechin, and gallic acid. The fractions were bioassayed. Antimicrobial activities were checked on human pathogens, and a high degree of activity was obtained with the lipophilic fraction. Agrochemical activities on phytopathogenic fungi and activities on the etiolated wheat coleoptile bioassay were also checked. The more polar fraction was active in the latter bioassay. 相似文献
949.
Shehata Salwa A. Lasheen Mohamed R. Ali Gamila H. Kobbia Imam A. 《Water, air, and soil pollution》1999,110(1-2):119-135
The toxic effect of multi metals mixture which exist simultaneously in aquatic ecosystem on natural phytoplankton assemblages (green algae, blue-green algae and diatoms) was studied. For this purpose a laboratory scale unit was designed to evaluate the effect of continuous flow metals mixture in forms if triple and penta metals in Nile water algae. Clear changes in algal biomass in terms of chlorophyll a (chl a) took place when subjected to metals combination. The rise or decline in chl a was in relation with other algal pigments (chl b, chl c, carotenoides and phenophytin), protein and carbohydrate content of algal cells. Substantial changes in phytoplankton community structure was detected and the most tolerant group was blue-green algae followed by green algae while diatoms was the most sensitive group. The most dominant species in all cases were blue-green alga Oscillatoria mougeotii and green alga Scenedesmus quadricauda. In addition clear changes in morphological shape was observed for tolerant species belonging to the three algal groups. Nile water algae has ability to remove and accumulate metals in the following order therefore Zn > Cd > Ni > Cu > Cr. In addition, phytoplankton has ability to recovered from the stress of metals when eliminated from the media and the recovered biomass was nearly equivalent to that before exposing to metals stress. The overall effect of metals mixture depending on the type and number of metals, the algal community structure and ratio between different morphological forms of algae (unicellular, colonial and filamentous). 相似文献
950.
M Yamamoto M Ishikawa T Masui H Nakazawa Y Kabasawa 《Journal of the Association of Official Analytical Chemists》1985,68(3):493-494
A simple and rapid liquid chromatographic method is described for the determination of barbaloin (aloin, 10-D-glucopyranosyl-1,8-dihydroxy-3-(hydroxymethyl)-9(10H)-anthraceno ne) in foods. Barbaloin is extracted with water from foods containing aloe and the extract is cleaned up on a disposable cartridge by using methanol-water (55 + 45) as eluant. The eluted barbaloin is separated by liquid chromatography on a YMC A-302 column with methanol-water (50 + 50) mobile phase, and detected at 293 nm. Recoveries of barbaloin added to foods at the levels of 0.05 and 0.50 mg/g were 94.4-100%. Assay results for commercial food samples indicated that the present method is applicable to a variety of foods supplemented with aloe. 相似文献