全文获取类型
收费全文 | 907篇 |
免费 | 47篇 |
国内免费 | 2篇 |
专业分类
林业 | 37篇 |
农学 | 40篇 |
基础科学 | 6篇 |
153篇 | |
综合类 | 112篇 |
农作物 | 52篇 |
水产渔业 | 34篇 |
畜牧兽医 | 444篇 |
园艺 | 16篇 |
植物保护 | 62篇 |
出版年
2023年 | 5篇 |
2021年 | 18篇 |
2020年 | 23篇 |
2019年 | 21篇 |
2018年 | 16篇 |
2017年 | 19篇 |
2016年 | 15篇 |
2015年 | 11篇 |
2014年 | 13篇 |
2013年 | 38篇 |
2012年 | 37篇 |
2011年 | 49篇 |
2010年 | 43篇 |
2009年 | 22篇 |
2008年 | 38篇 |
2007年 | 44篇 |
2006年 | 42篇 |
2005年 | 46篇 |
2004年 | 35篇 |
2003年 | 41篇 |
2002年 | 54篇 |
2001年 | 19篇 |
2000年 | 19篇 |
1999年 | 21篇 |
1998年 | 11篇 |
1997年 | 12篇 |
1996年 | 5篇 |
1995年 | 11篇 |
1994年 | 7篇 |
1992年 | 12篇 |
1991年 | 16篇 |
1990年 | 13篇 |
1989年 | 7篇 |
1988年 | 12篇 |
1987年 | 7篇 |
1986年 | 11篇 |
1985年 | 11篇 |
1984年 | 7篇 |
1980年 | 5篇 |
1979年 | 8篇 |
1978年 | 8篇 |
1977年 | 7篇 |
1976年 | 9篇 |
1975年 | 9篇 |
1974年 | 8篇 |
1973年 | 4篇 |
1972年 | 5篇 |
1969年 | 4篇 |
1967年 | 4篇 |
1943年 | 5篇 |
排序方式: 共有956条查询结果,搜索用时 31 毫秒
51.
Henry C Vitrac X Decendit A Ennamany R Krisa S Mérillon JM 《Journal of agricultural and food chemistry》2005,53(3):798-803
Two stilbenes (trans-piceid and its aglycone trans-resveratrol) were investigated in the uptake across the apical membrane of the human intestinal cell line Caco-2 in order to determine their mechanisms of transport. The uptake was quantified using a reverse phase high-performance liquid chromatography method with fluorescence detection. The rate of cellular accumulation in the cells was found to be higher for trans-resveratrol than for trans-piceid. In addition, trans-resveratrol uses passive transport to cross the apical membrane of the cells, whereas the transport of trans-piceid is likely active. With regard to the mechanisms of transport, the involvement of the active transporter SGLT1 in the absorption of trans-piceid was deduced using various inhibitors directly or indirectly exploiting the activity of this transporter (glucose, phlorizin, and ouabain). Moreover, we investigated the involvement of the multidrug-related protein 2 (MRP2), an efflux pump present on the apical membrane, in stilbene efflux by Caco-2 cells. The effect of MK-571 (an MRP inhibitor) seems to implicate MRP2 as responsible for apical efflux of trans-piceid and trans-resveratrol. 相似文献
52.
Horvatovich P Miesch M Hasselmann C Delincée H Marchioni E 《Journal of agricultural and food chemistry》2005,53(15):5836-5841
The 2-alkylcyclobutanones (2-ACBs) are formed from triglycerides by irradiation treatment and may be used as markers for this type of food processing. This paper describes a detection method for the analysis of monounsaturated alkyl side chain 2-ACBs, which is formed upon irradiation from monounsaturated fatty acids which frequently are the most abundant fatty acids in foods. The estimated radioproduction yields of the cis-2-(dodec-5'-enyl)-cyclobutanones (cis-2-dDeCB) and the cis-2-(tetradec-5'-enyl)-cyclobutanones (cis-2-tDeCB) were 1.0 +/- 0.5 and 0.9 +/- 0.2 nmol.mmol(-1) precursor fatty acid.kGy(-1), respectively, being similar to that of saturated 2-ACBs. The stability study of the s- and mu-2-ACBs in poultry meat samples irradiated at 10 kGy and stored for 3-4 weeks at 4 degrees C and 25 degrees C showed that these compounds undergo some transformation, their amounts being reduced by about 50%. These storage losses did not depend on the saturation state of the alkyl side chain. The EI-MS detection limit of 2-tDeCB is 3 times higher (0.6 pmol) than that of 2-dodecylcyclobutanone (0.2 pmol). Consequently, when the oleic acid content of the analyzed food exceeds the content of palmitic acid by a factor of 3, it would be of an advantage to apply 2-tDeCB as a marker for detection of the irradiation treatment. 相似文献
53.
Plant species-level research that comprises inventories, impact studies and monitoring is necessary if plant resources are to be harvested sustainably by human populations living adjacent to protected areas in sub-Saharan Africa. This research assessed the extraction of plant products from two montane forest ecosystems, Uzungwa Scarp Forest Reserve (USFR) and Bwindi Impenetrable National Park (BINP), East Africa. In USFR, data from vegetation sampling and interviews with local people were used to understand the ecological and socio-economic aspects of non-timber forest product (NTFP) harvesting. The densities of eight commonly harvested tree species, most of which were used as building poles, were approximately 2.4-4.5 times lower in disturbed versus undisturbed habitats across all four sites in USFR. Interviews with 91 women and 54 men suggested that most species were harvested for medicinal uses (57 species) and building purposes (50 species). In BINP, the liana Loeseneriella apocynoides (Apocynaceae), is harvested for basketry weaving. Evidence suggested that the liana was harvested in both protected and unprotected areas of BINP. Data collected suggested a negative impact on this species in an unprotected versus a protected area, with stem diameters larger than 1 cm significantly more abundant in the protected area. This study reveals that harvesting of NTFPs occurs even in these two protected forest areas, and that over-exploitation not only threatens species of high-demand, but could also alter forest structure and composition. Management practices that encourage the monitoring of sustainable harvesting levels of species and promote alternative plants for the same uses should be considered as part of conservation strategies. 相似文献
54.
Soil and plant samples were collected from chlorotic plantations of Grand fir (Abies grandis) and Douglas-fir (Pseudotsuga menziesii) in Winter, 1989. The soils had been amended in 1981 with an average of 300 dry Mg ha?1 of municipal sewage sludge. The sludge amendment resulted in an N application rate of approximately 8000 kg ha?1. Foliage analysis indicated a severe Mg deficiency (0.25 g kg?1 in sludge-treated vs. 0.93 g kg?1 in untreated area) might be the cause of chlorosis. No other nutrient showed concentrations in the deficient or toxic ranges. Trace metal levels in foliage were elevated significantly for Ni, Cd and Cr in sludge-treated sites, but not toxic levels. Soil samples taken to a depth of 1.4 m indicated the potential for soil acidification (up to 0.9 pH unit) in soil surface horizons. In addition, exchangeable Ca, Mg and K may have been depleted in surface horizons. Exchangeable Al and Fe were greater in the surface of sludge-treated sites. These observations and the loss of much of the nitrogen added during the sludge amendment indicated that nitrification and cation leaching was likely the mechanism for acidification and depletion of exchangeable cations. Fertilization of the plantation with MgSO4 or dolomitic limestone was carried out in Spring 1990. New foliage collected in June, 1990 was non-chlorotic and significantly higher in Mg concentration than unfertilized foliage (1.1. vs. 0.7 g kg?1, respectively). Results of this study indicate that it is important to assess the potential for initiating a nutrient deficiency due to secondary effects of sludge application in forest systems. 相似文献
55.
Claudio E. Rafanelli Gianna L. Petriconi Henry M. Papee 《Water, air, and soil pollution》1978,9(4):403-412
The dissipation of chemical elements from ash basin effluent was determined by use of neutron activation analysis of sediment, water, plant, invertebrate, and vertebrate samples taken from successive sites through the drainage system. Samples were tested for concentrations of 40 elements for a 12 mo period. Three different modes of dissipation were found. The major mechanism for removal of chemical elements from effluent was sedimentation into the benthic sediment. All biota had greater concentrations of chemicals than did water, with invertebrates having slightly higher levels than did plants and vertebrates. Ten elements were more concentrated in at least one biotic group than in the sediment. Concentrations of six elements in the drainage system exceeded the EPA recommended limits for surface waters. The observed modes of dissipation in this aquatic system emphasize the necessity for stable biotic communities present to permit cycling of chemical elements. 相似文献
56.
Measurement of coupled nitrification-denitrification in paddy fields affected by Terrazole,a nitrification inhibitor 总被引:2,自引:0,他引:2
David?CarrascoEmail author Eduardo?Fernández-Valiente Yoanna?Ariosa Antonio?Quesada 《Biology and Fertility of Soils》2004,39(3):186-192
Coupled nitrification-denitrification and potential denitrification were measured as 15N2O and 15N2 evolution rates in ammonium sulphate-treated rice soils with or without Terrazole [5-ethoxy-3 (trichloromethyl) 1,2,3 Thiadizole] under laboratory and field conditions. The greatest coupled nitrification-denitrification activity was found after drying and rewetting the soil, with maximum values of 322 ng N cm–2 h–1 in the laboratory and 90.8 ng N cm–2 h–1 in the field. These 15N2O + 15N2 evolution rates were about 10 times lower than potential denitrification in these soils. These results and the observed decrease in 15N2O + 15N2 evolution rate in soils treated with Terrazole (60% under laboratory conditions and 52% under field conditions) indicate that denitrification was limited by coupled nitrification-denitrification activity. Oxygen and previous addition of ammonium sulphate appear to control the rate of 15N2O + 15N2 evolution in ammonium sulphate-fertilised soils. 相似文献
57.
Bartels RA Paul A Green H Kapteyn HC Murnane MM Backus S Christov IP Liu Y Attwood D Jacobsen C 《Science (New York, N.Y.)》2002,297(5580):376-378
We present spatial coherence measurements of extreme ultraviolet (EUV) light generated through the process of high-harmonic up-conversion of a femtosecond laser. With a phase-matched hollow-fiber geometry, the generated beam was found to exhibit essentially full spatial coherence. The coherence of this laser-like EUV source was shown by recording Gabor holograms of small objects. This work demonstrates the capability to perform EUV holography with a tabletop experimental setup. Such an EUV source, with low divergence and high spatial coherence, can be used for experiments involving high-precision metrology, inspection of optical components for EUV lithography, and microscopy and holography with nanometer resolution. Furthermore, the short time duration of the EUV radiation (a few femtoseconds) will enable EUV microscopy and holography to be performed with ultrahigh time resolution. 相似文献
58.
Bockelée-Morvan D Biver N Moreno R Colom P Crovisier J Gérard E Henry F Lis DC Matthews H Weaver HA Womack M Festou MC 《Science (New York, N.Y.)》2001,292(5520):1339-1343
The gas activity of comet C/1999 S4 (LINEAR) was monitored at radio wavelengths during its disruption. A runaway fragmentation of the nucleus may have begun around 18 July 2000 and proceeded until 23 July. The mass in small icy debris (=30-centimeter radius) was comparable to the mass in the large fragments seen in optical images. The mass budget after breakup suggests a small nucleus ( approximately 100- to 300-meter radius) that had been losing debris for weeks. The HNC, H2CO, H2S, and CS abundances relative to H2O measured during breakup are consistent with those obtained in other comets. However, a deficiency in CH3OH and CO is observed. 相似文献
59.
Wahlen M Tanaka N Henry R Deck B Zeglen J Vogel JS Southon J Shemesh A Fairbanks R Broecker W 《Science (New York, N.Y.)》1989,245(4915):286-290
Measurements of carbon-14 in small samples of methane from major biogenic sources, from biomass burning, and in "clean air" samples from both the Northern and Southern hemispheres reveal that methane from ruminants contains contemporary carbon, whereas that from wetlands, pat bogs, rice fields, and tundra is somewhat, depleted in carbon-14. Atmospheric (14)GH(4) seems to have increased from 1986 to 1987, and levels at the end of 1987 were 123.3 +/- 0.8 percent modern carbon (pMC) in the Northern Hemisphere and 120.0 +/- 0.7 pMC in the Southern Hemisphere. Model calculations of source partitioning based on the carbon-14 data, CH(4) concentrations, and delta(13)C in CH(4) indicate that 21 +/- 3% of atmospheric CH(4) was derived from fossil carbon at the end of 1987. The data also indicate that pressurized water reactors are an increasingly important source of (14)CH(4). 相似文献
60.