全文获取类型
收费全文 | 1318篇 |
免费 | 96篇 |
国内免费 | 144篇 |
出版年
2024年 | 4篇 |
2023年 | 25篇 |
2022年 | 33篇 |
2021年 | 52篇 |
2020年 | 47篇 |
2019年 | 56篇 |
2018年 | 45篇 |
2017年 | 41篇 |
2016年 | 26篇 |
2015年 | 41篇 |
2014年 | 64篇 |
2013年 | 106篇 |
2012年 | 57篇 |
2011年 | 48篇 |
2010年 | 30篇 |
2009年 | 46篇 |
2008年 | 60篇 |
2007年 | 48篇 |
2006年 | 55篇 |
2005年 | 60篇 |
2004年 | 47篇 |
2003年 | 64篇 |
2002年 | 51篇 |
2001年 | 44篇 |
2000年 | 41篇 |
1999年 | 25篇 |
1998年 | 22篇 |
1997年 | 43篇 |
1996年 | 24篇 |
1995年 | 21篇 |
1994年 | 21篇 |
1993年 | 32篇 |
1992年 | 17篇 |
1991年 | 18篇 |
1990年 | 19篇 |
1989年 | 15篇 |
1988年 | 27篇 |
1987年 | 10篇 |
1986年 | 6篇 |
1985年 | 6篇 |
1984年 | 10篇 |
1983年 | 7篇 |
1982年 | 11篇 |
1981年 | 13篇 |
1980年 | 6篇 |
1979年 | 4篇 |
1978年 | 4篇 |
1976年 | 3篇 |
1975年 | 1篇 |
1974年 | 1篇 |
排序方式: 共有1558条查询结果,搜索用时 252 毫秒
1.
2.
A Comparison of Bone Mineral Density and Its Predictors in HIV-Infected and HIV-Uninfected Older Men
《Endocrine practice》2021,27(12):1225-1231
ObjectiveBone health in older individuals with HIV infection has not been well studied. This study aimed to compare bone mineral density (BMD), trabecular bone score (TBS), and bone markers between HIV-infected men and age- and body mass index (BMI)-matched HIV-uninfected men aged ≥60 years. We investigated the associations of risk factors related to fracture with BMD, TBS, and bone markers in HIV-infected men.MethodsThis cross-sectional study included 45 HIV-infected men receiving antiretroviral therapy and 42 HIV-uninfected men. Medical history, BMD and TBS measurements, and laboratory tests related to bone health were assessed in all the participants. HIV-related factors known to be associated with bone loss were assessed in the HIV-infected men.ResultsThe mean BMD, TBS, and osteopenia or osteoporosis prevalence were similar among the cases and controls. The HIV-infected men had significantly higher mean N-terminal propeptide of type 1 procollagen and C-terminal cross-linking telopeptide of type I collagen levels. Stepwise multiple linear regression analysis demonstrated that low BMI (lumbar spine, P = .015; femoral neck, P = .018; and total hip, P = .005), high C-terminal cross-linking telopeptide of type I collagen concentration (total hip, P = .042; and TBS, P = .010), and low vitamin D supplementation (TBS, P = .035) were independently associated with low BMD and TBS.ConclusionIn older HIV-infected men with a low fracture risk, the mean BMD and TBS were similar to those of the age- and BMI-matched controls. The mean bone marker levels were higher in the HIV group. Traditional risk factors for fracture, including low BMI, high C-terminal cross-linking telopeptide of type I collagen level, and low vitamin D supplementation, were significant predictors of low BMD and TBS. 相似文献
3.
Xi Chen 《Geomicrobiology journal》2016,33(9):822-831
Previous studies revealed the thermodynamic properties of DNA adsorption on pure minerals or biomasses; however, there has been little attempt to develop such studies on bacteria–mineral composites. Equilibrium adsorption experiments, attenuated total reflectance Fourier transform infrared spectroscopy, and isothermal titration calorimetry were employed to investigate the adsorption of DNA by Bacillus subtilis, Pseudomonas putida, and their composites with minerals. Similar capacity and affinity were observed for DNA adsorption on two bacterial cells. However, different patterns were found in the adsorption of DNA by bacteria–mineral composites. The Gram-positive bacterium B. subtilis enhanced the adsorption of DNA on its mineral composites compared with their individual components, while the composites of Gram-negative bacterial cells with kaolinite and goethite bound lower amounts of DNA than the predicted values. The thermodynamic parameters and the Fourier transform infrared spectra showed that van der Waals force and hydrogen bonding are responsible for the DNA adsorption on B. subtilis–minerals and P. putida–kaolinite. By contrast, the entropy increases of excluded water rearrangement and dehydration effect play key roles in the interaction between DNA and P. putida–montmorillonite/goethite composites. 相似文献
4.
Summary The influence of total nitrification to nitrate or partial nitrification to nitrite on the soil organic nitrogen status was examined. NH
4
+
–15N was added to the soil in the absence and the presence of NaClO3, respectively nitrapyrin. The first chemical inhibits only nitrate formation, the second inhibits total nitrification. The accumulation of nitrite nitrogen in the soil at levels up to 5 mg kg–1 increased the loss of nitrogen. Yet, it did not increase the binding of mineral nitrogen into soil organic matter, relative to the control soil. The data suggest that the biochemistry of the nitrite formation process, rather than the levels of nitrite ions formed, are of primary importance in the role of nitrification mediated nitrosation of soil organic matter. 相似文献
5.
V. Carré‐Missio F.A. Rodrigues D.A. Schurt R.S. Resende N.F.A. Souza D.C. Rezende W.R. Moreira L. Zambolim 《The Annals of applied biology》2014,164(3):396-403
Coffee leaf rust, caused by Hemileia vastatrix, is the most devastating disease of coffee. Since limited information is available in the literature on silicon (Si) affecting plant diseases in coffee, this study was designed to investigate foliar application of potassium silicate (PS), a source of soluble (Si), on infection process of coffee leaf rust at the microscopic level. The foliar Si concentration for plants sprayed with water and PS has no significant difference (0.24 and 0.30 dag kg?1, respectively). X‐ray microanalysis indicated that the deposition of Si on the leaves of the plants that were sprayed with PS was greater in comparison to the leaf samples from the plants sprayed with water. Rust severity on leaves of plants sprayed with water or sprayed with PS reached 44% and 32%, respectively, at 36 days after inoculation (dai). Plates of polymerised PS were observed on the leaf surfaces of the plants sprayed with the product, in contrast to its absence on the leaf surfaces of plants sprayed with water. At 36 dai, a greater number of uredia were observed on the leaf surfaces of plants sprayed with water in comparison to the leaf surfaces of plants sprayed with PS. On fractured leaf tissues that were sprayed with PS, less fungal colonisation was observed in comparison to the leaves of plants sprayed with water. In conclusion, the results of this study suggest that the effect of foliar‐applied Si on the control of the coffee leaf rust development may be attributed to the physical role of the polymerised PS, its osmotic effect against urediniospores germination, or both. 相似文献
6.
Electron probe microanalysis of transverse sections was used to provide semiquantitative estimates of Al and other cations
in the leaf tissues of some Al-accumulating plants of the cerrado vegetation of central Brazil. Concentrations of Al were
generally higher than those of other cations in the phloem elements of these plants growing on dystrophic as well as fertile
acid soils. When one of the Al-accumulating species,Vochysia thyrsoidea, was grown in a calcareous soil, the concentration of Al in the phloem elements of leaves was lower than that of Ca and K
though the cotyledons showed higher concentrations of Al than those of other cations. 相似文献
7.
Synopsis The caloric density of stomach contents in juvenile chinook salmon,Oncorhynchus tshawytscha, was not affected by gastric evacuation, suggesting a constant caloric density of stomach contents during evacuation. Differences in the caloric density of prey consumed did affect caloric density of stomach contents over a 24-h period. Consumption of the amphipodCorophium sp. was associated with reduced caloric densities of stomach contents. During periods whenCorophium contributed more than 4% of the stomach contents, average caloric density declined from 5.56 to 5.33 kcal g–1. Despite this difference, estimates of daily energy intake of juvenile chinook salmon were only 3%, greater when developed from the mean caloric density of stomach contents excludingCorophium. 相似文献
8.
Plants ofDatura stramonium var.tatula L. Torr. were grown for 24 weeks on vermiculite and provided with mineral solutions where the relative proportions of the major anions (N, S, P) and cations (K, Ca, Mg) were varied within a constant total.With all treatments a cyclic transport of alkaloids (hyoscyamine) from the roots to the upper plant parts appeared. The fluctuations in alkaloid content in the leaves followed those in the roots with a delay of 4 weeks.The interionic ratios seemed to influence growth and alkaloid content rather than to affect the ontogenic variations. In general, the response to the different treatments could be divided into two types. The nitrogen treatments which gave at the same time a higher crop yield and an increased hyoscyamine content, and the other treatments where a much lower yield and content was obtained.The scopolamine content on the other hand seemed to be more affected by the developmental stage. Also, the time of maximal yield was not influenced by the ion-balance but rather the climatic conditions. 相似文献
9.
Curtiss D. Hunt Edward S. Halas Marilou J. Eberhardt 《Biological trace element research》1988,16(2):97-113
The purpose of this study was threefold: 1. to determine the long-term effects of interactions between lactational zinc deficiency
and gender on bone mineral composition in repleted rat offspring, 2. to determine the nutritional efficacy of the second of
two commercially designed, modified Luecke diets (ML2) during the gestational and lactational stress, and 3. determine the
ultratrace element contents of Ralston Rodent Laboratory Chow #5001. The ML2 basal diet, based on dextrose, sprayed egg white,
and corn oil contained 0.420 μg Zn/g, was supplemented with Zn (as zinc acetate) at 0 (diet 0ML2) or 30 (diet 30ML2) μg/g,
and was mixed and pelleted commercially. all rat dams were fed the 30ML2 diet ad libitum during gestation. Beginning at parturition,
the dams were fed either the 1. 0ML2, 2. 30ML2 (food restricted), or 3. 30ML2 (ad libitum) diets. All pups were fed the 30ML2
diet ad libitum from 23 to 40 d of age. From d 40 to 150, all pups were fed Ralston Rodent Laboratory Chow. The 30ML2 diet
was found to be nutritionally efficacious; litter size and pup growth were normal and pup mortality was only 1.2%. Pups (ZD)
with access to the 0ML2 diet until 23 d of age and nursed by dams fed the 0ML2 diet, when compared to pups (PF) fed restricted
amounts of the 30ML2 diet, exhibited increased mortality and decreased concentrations of tibial zinc but no change in growth.
Inadequate zinc nutriture during infancy, despite postlactational zinc repletion, induced imbalances in adult bone mineral
metabolism. Thus, at 150 d of age, the ZD pups exhibited increased levels of bone P and Mg and decreased concentrations of
K as compared to the PF pups. 相似文献
10.
Laboratory incubation and field experiments were conducted to evaluate thiourea, ATC (4-amino-1, 2, 4 triazole hydrochloride)
and N-Serve 24 E (2-chloro-6-trichloromethyl-pyridine) as inhibitors of nitrification of fertilizer N. In the incubation experiment,
most of the added aqueous NH3 or urea was nitrified at 14 days on both soils, but addition of the inhibitors to fertilizer N decreased the conversion of
NH4−N to NO3−N markedly. There was less nitrification for ATC and thiourea but not for N-Serve 24 E when the fertilizers and the inhibitors
were placed at a point as opposed to when mixed into soil. After 28 days, ATC and N-Serve 24 E were more effective in inhibiting
nitrification than thiourea. ATC and N-Serve 24 E also inhibited release of mineral N (NH4−N+NO3−N) from native soil N. In the uncropped field experiment, which received N fertilizers in the fall, nitrification of fall-applied
N placed in the 15-cm bands was almost complete by early May in the Malmo soil, but not in the Breton soil. When ATC or thiourea
had been applied with urea, nitrification of fall-applied N was depressed by May and the recovery of applied N as NH4−N was greater with increasing band spacing to 60 cm or placing N fertilizer in nests (a method of application where urea
prills were placed at a point in the soil in the center of 60×60 cm area). In late June, the percentage recovery of fall-applied
N in soil as NH4−N or mineral N increased with wide band spacing, or nest placement, or by adding ATC to fertilizer N on both soils. These
results indicate that placing ammonium-based N fertilizers in widely-spaced bands or in nests with low rates of inhibitors
slows nitrification enough to prevent much of the losses from fall-applied N.
Scientific Paper No. 552, Lacombe Research Station, Research Branch, Agric, Can. 相似文献