全文获取类型
收费全文 | 3039篇 |
免费 | 176篇 |
国内免费 | 1篇 |
专业分类
林业 | 118篇 |
农学 | 80篇 |
基础科学 | 5篇 |
505篇 | |
综合类 | 449篇 |
农作物 | 291篇 |
水产渔业 | 244篇 |
畜牧兽医 | 1287篇 |
园艺 | 50篇 |
植物保护 | 187篇 |
出版年
2023年 | 18篇 |
2022年 | 30篇 |
2021年 | 63篇 |
2020年 | 54篇 |
2019年 | 57篇 |
2018年 | 82篇 |
2017年 | 69篇 |
2016年 | 77篇 |
2015年 | 63篇 |
2014年 | 79篇 |
2013年 | 132篇 |
2012年 | 170篇 |
2011年 | 215篇 |
2010年 | 100篇 |
2009年 | 89篇 |
2008年 | 170篇 |
2007年 | 192篇 |
2006年 | 112篇 |
2005年 | 146篇 |
2004年 | 127篇 |
2003年 | 124篇 |
2002年 | 126篇 |
2001年 | 54篇 |
2000年 | 52篇 |
1999年 | 39篇 |
1998年 | 24篇 |
1997年 | 25篇 |
1996年 | 26篇 |
1995年 | 16篇 |
1994年 | 19篇 |
1992年 | 29篇 |
1991年 | 44篇 |
1990年 | 34篇 |
1989年 | 33篇 |
1988年 | 26篇 |
1987年 | 32篇 |
1986年 | 31篇 |
1985年 | 37篇 |
1984年 | 22篇 |
1983年 | 22篇 |
1979年 | 27篇 |
1978年 | 15篇 |
1977年 | 21篇 |
1974年 | 21篇 |
1973年 | 24篇 |
1972年 | 19篇 |
1971年 | 16篇 |
1970年 | 20篇 |
1969年 | 19篇 |
1966年 | 13篇 |
排序方式: 共有3216条查询结果,搜索用时 343 毫秒
991.
Michael Axelsson William R. Driedzic Anthony P. Farrell Stefan Nilsson 《Fish physiology and biochemistry》1989,6(5):315-326
Coeliac artery blood flow (Fca) before and after feeding was recorded in the sea raven. To obtain basic information about the scope of cardiovascular adjustment in the sea raven, a separate series of experiments was performed, in which ventral (Pva), and dorsal (Pda) aortic blood pressure, heart rate (HR) and cardiac output (jaz) were monitored during rest and encouraged exercise.Measurements of coeliac artery flow showed that visceral blood flow is substantial, particularly after feeding, and variations in the visceral vascular conductance affect Pda directly. Simultaneous recordings of intestinal and dorsal aortic blood pressures showed no measurable difference in the two arterial pressures, refuting the idea of a vascular control at the level of the main coeliac artery. Thus, in the sea raven, the adrenergic tonus affecting the visceral vasculature presumably acts at the arteriolar level.Sea ravens encouraged to exercise increased theirjaz by 64%; 32% through HR and 25% through stroke volume. The increase injaz during encouraged exercise was sufficient to produce an elevation of both Pva and Pda, despite an increase of systemic vascular conductance, -adrenoceptor blockade with sotalol, however, severely impaired the increase injaz during exercise, and the change in Pda was reversed.During rest there were both an adrenergic and a cholinergic tonus affecting the HR, as revealed by the effects of injected pharmacological antagonists. Swimming activity decreased the cholinergic tonus, while the adrenergic tonus increased. 相似文献
992.
K D Gallicano H Park J Yee L M Young P W Saschenbrecker 《Journal of the Association of Official Analytical Chemists》1988,71(1):48-50
A reverse-phase liquid chromatographic (LC) method is described for simultaneously determining 5 coccidiostats--aklomide, dinsed, ethopabate, nitromide, and zoalene in chicken liver. The method entails blender extraction of 10 g liver with ethyl acetate, column chromatography through Sephadex LH-20 and neutral alumina, and LC analysis on a C18 column with UV detection at 260 nm. The drugs were eluted from Sephadex with methanol-benzene (10 + 90), from alumina with methanol-dichloromethane (10 + 90), and from C18 with acetonitrile-water (linear gradient: 25% acetonitrile for 10 min, increasing to 55% over 15 min; flow rate 1 mL/min). Liquid chromatography was completed in 40 min and calculations were based on peak height measurements. Average recoveries of the coccidiostats from fortified liver ranged from 72 to 97%, except for dinsed, which showed a relatively constant average recovery of 57%. The detection limit for the standards was 2.5 ng on column. Levels as low as 50 ng/g were detected in fortified liver samples. 相似文献
993.
Mineralization of carbon and nitrogen from cowpea leaves decomposing in soils with different levels of microbial biomass 总被引:2,自引:0,他引:2
Kathrin Franzluebbers Richard W. Weaver Anthony S. R. Juo Alan J. Franzluebbers 《Biology and Fertility of Soils》1995,19(2-3):100-102
Soils with greater levels of microbial biomass may be able to release nutrients more rapidly from applied plant material. We tested the hypothesis that the indigenous soil microbial biomass affects the rate of decomposition of added green manure. Cowpea (Vigna unguiculata L.) Walp.] leaves were added to four soils with widely differing microbial biomass C levels. C and N mineralization of the added plant material was followed during incubation at 30°C for 60 days. Low levels of soil microbial biomass resulted in an initially slower rate of decomposition of soil-incorporated green manure. The microbial biomass appeared to adjust rapidly to the new substrate, so that at 60 days of incubation the cumulative C loss and net N mineralization from decomposing cowpea leaves were not significantly affected by the level of the indigenous soil microbial biomass. 相似文献
994.
Soil and microbiological properties of a tropical rain forest soil were evaluated 17 years after oil spillage to access the chronic effects of, and interrelationship between population of petroleum hydrocarbon utilizing and nitrifying microorganisms. The spatial distribution of petroleum hydrocarbons (oil), the nutrient status and the abundance of heterotrophic microbes along soil transect lines in the contaminated zones served as the index for corroborating the results. The pH status of soil in the contaminated (Heavy impact — HI, and moderate inpact — MI) zones varied from acidic, that is 4.0 to near neutral PH, that is 6.0. The C content of soils decreased from 3.6% at the HI zones to 2.84% at the MI zones. Total N in the HI and MI zones differed by a factor of 0.10%. Available P was higher at the MI than HI zone, while CEC decreased from a combined mean of 6.48 at the HI zones to 4.46 at the MI zones. Although residual oil content was higher in the HI zones than MI zone, the soil nutrient status within these two zones did not vary significantly (P=0.05). However, soil microbes responded differently. For instance, petroleum hydrocarbon utilizers correlated positively with the distribution of oil in the environment. But, not the nitrifying microorganisms. Aerobic nitrifiers were abundant at the HI than MI zones, while anaerobic nitrifiers were higher at the MI than HI zones. With the presence of petroleum hydrocarbon utilizers anf nitrifying microbes. It is possible to enhance the degradation of oil in the 17 yr old spillage by adopting bioremediation. 相似文献
995.
The relative effectiveness of average-well-color-development-normalized single-point absorbance readings (AWCD) vs the kinetic parameters mu(m), lambda, A, and integral (AREA) of the modified Gompertz equation fit to the color development curve resulting from reduction of a redox sensitive dye from microbial respiration of 95 separate sole carbon sources in microplate wells was compared for a dilution series of rhizosphere samples from hydroponically grown wheat and potato ranging in inoculum densities of 1 x 10(4)-4 x 10(6) cells ml-1. Patterns generated with each parameter were analyzed using principal component analysis (PCA) and discriminant function analysis (DFA) to test relative resolving power. Samples of equivalent cell density (undiluted samples) were correctly classified by rhizosphere type for all parameters based on DFA analysis of the first five PC scores. Analysis of undiluted and 1:4 diluted samples resulted in misclassification of at least two of the wheat samples for all parameters except the AWCD normalized (0.50 abs. units) data, and analysis of undiluted, 1:4, and 1:16 diluted samples resulted in misclassification for all parameter types. Ordination of samples along the first principal component (PC) was correlated to inoculum density in analyses performed on all of the kinetic parameters, but no such influence was seen for AWCD-derived results. The carbon sources responsible for classification differed among the variable types with the exception of AREA and A, which were strongly correlated. These results indicate that the use of kinetic parameters for pattern analysis in CLPP may provide some additional information, but only if the influence of inoculum density is carefully considered. 相似文献
996.
997.
The purpose of this work was to determine the degree of oil retention from an oil spill area in view of natural physical weathering, and to evaluate the compositional changes of the natural oil retained in the soil material. The percentage of oil retained over the 1 yr period of weathering was determined at various intervals, and the compositional changes were studied by G.L.C. and I.R. The results of G.L.C. indicated the loss of low molecular weight n-paraffins, and that of I.R. indicated the formation of sulfones and ether groups as a result of oxidation reaction due to natural exposure. It was established that up to 4% oil was retained in the soil material after 6 mo of exposure, and that about 3.5% remained after a year. 相似文献
998.
999.
Summary Restriction fragment length polymorphism (RFLP) analysis has several advantages over traditional methods of genetic linkage mapping, one of these being the starting point for map-based cloning. The recent development of an RFLP map of cowpea (Vigna unguiculata L. Walp) has allowed the investigation of associations between genes of interest and RFLP markers. A cross between an aphid (Aphis craccivora Koch) resistant cultivated cowpea, TT84S-2246-4, and an aphid susceptible wild cowpea, NI 963, was screened for both aphid phenotype and RFLP marker segregation. One RFLP marker, bg4D9b, was found to be tightly linked to the aphid resistance gene (Rac
1) and several flanking markers in the same linkage group (linkage group 1) were also identified. The close association of Rac
1 and RFLP bg4D9b presents a real potential for cloning this insect resistance gene. 相似文献
1000.
Summary Field experiments were carried out to determine the effects of single and mixed inoculations with Rhizobium and vesicular-arbuscular mycorrhiza (VAM) on nodulation, symbiotic N2 fixation and yield of soybeans in six Taiwan subtropical-tropical sites. Inoculation with Rhizobium alone significantly increased nodulation, nodule weight and nitrogenase activity of nodules in three out of six experimental fields, and affected soybean yields in the range –13% to + 134%. Inoculation with VAM fungi alone did not have a significant effect on nodulation and nitrogenase activity. Mycorrhiza inoculation affected soybean yields in the range –13% to + 65%, but only the yield increases at one out of six sites with N application were statistically significant. Mixed inoculation with Rhizobium and mycorrhiza affected yields in the range –8% to + 145% A synergistic effect from mixed inoculation of Rhizobium-mycorrhiza on soybean yields was found in one out of six experimental fields. The yield response to N application (40 kg N ha–1) in these six paddy-field trials was not significant. These results suggest that single or mixed inoculation of rhizobia can greatly assist soybean grain yields and can replace N fertilizers. 相似文献