全文获取类型
收费全文 | 28697篇 |
免费 | 2773篇 |
国内免费 | 2565篇 |
出版年
2023年 | 717篇 |
2022年 | 709篇 |
2021年 | 886篇 |
2020年 | 974篇 |
2019年 | 727篇 |
2018年 | 1298篇 |
2017年 | 1533篇 |
2016年 | 1513篇 |
2015年 | 1599篇 |
2014年 | 1815篇 |
2013年 | 2007篇 |
2012年 | 1801篇 |
2011年 | 1655篇 |
2010年 | 1526篇 |
2009年 | 1597篇 |
2008年 | 1344篇 |
2007年 | 1280篇 |
2006年 | 1095篇 |
2005年 | 1043篇 |
2004年 | 915篇 |
2003年 | 767篇 |
2002年 | 690篇 |
2001年 | 660篇 |
2000年 | 580篇 |
1999年 | 562篇 |
1998年 | 461篇 |
1997年 | 340篇 |
1996年 | 288篇 |
1995年 | 237篇 |
1994年 | 259篇 |
1993年 | 188篇 |
1992年 | 233篇 |
1991年 | 229篇 |
1990年 | 219篇 |
1989年 | 176篇 |
1988年 | 144篇 |
1987年 | 146篇 |
1986年 | 104篇 |
1985年 | 110篇 |
1984年 | 111篇 |
1983年 | 105篇 |
1982年 | 120篇 |
1981年 | 109篇 |
1980年 | 95篇 |
1979年 | 82篇 |
1978年 | 71篇 |
1977年 | 75篇 |
1976年 | 60篇 |
1975年 | 61篇 |
1973年 | 59篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
Random Amplified Polymorphic DNA Variation among and within Selected Ixora (Rubiaceae) Populations and Mutants 总被引:1,自引:0,他引:1
RAJASEGER G.; TAN H. T. W.; TURNER I. M.; SAW L. G.; KUMAR P. P. 《Annals of botany》1999,84(2):253-257
Random amplified polymorphic DNA (RAPD) analysis was used tostudy variation among and within selectedIxora (Rubiaceae) populationsand mutants. Six populations of I. congesta yielded identicalbanding patterns suggesting genetic uniformity of this species.However, six populations of I. coccinea varieties (three red-flowered,two yellow-flowered and one red-flowered wild-type) exhibitedinfraspecific differences in RAPD profiles. Small and largeleaves of an atavistic mutant cultivar of I. coccinea were alsosubjected to RAPD analysis. An extra band was amplified in thelarge leaves that was absent in small leaves, suggesting thatthe phenotypic alteration in this taxon is due to genetic mutationrather than epigenetic changes. Similarly, an extra band wasdetected in the white sectors of I. Variegated compared to thegreen sectors, suggesting that the shoot apical meristems ofthis cultivar exist as a genetic chimera. DNA gel blot hybridizationwas performed to confirm the specificities of selected bands.Our study indicates that differences among individuals of variouspopulations and mutants may be detected using RAPD markers.Copyright 1999 Annals of Botany Company Ixora L., variegated variety, RAPD fingerprinting, DNA gel blot, intraspecific genetic similarity, atavistic mutant. 相似文献
72.
Microbiological and biotechnological characteristics of intensification of aerobic processing of organic waste have been reviewed, with a view for revealing two types of correlations: (1) between the quality of the composts obtained and the microorganisms involved in composting and (2) between physicochemical parameters and consumer properties of the composts. 相似文献
73.
Brett M. Bennett 《Journal of the history of biology》2011,44(1):125-145
Scholars studying the globalization of Australian trees have previously emphasized the rapid natural propagation of Australian
trees outside of their native habitats, believing their success to be a reversal of “ecological imperialism” from the “new
world” to the “old world.” This article argues that the expansion of Australian trees should not be viewed as a biological
phenomenon, but as the result of a long-term attempt by powerful states and state-sponsored scientists to select and breed
Australian species that could grow in a variety of climates and ecological conditions. Five non-biological factors largely
determined the success of these attempts to grow Australian trees: the abundance or paucity of natural forests, state power,
the amount of scientific research directed to planting Australian trees, the cost of labor, and the ability to utilize hardwood
timbers and bark. This paper compares the use of Australian trees in Australia, India, and South Africa to demonstrate that
biology was not the determining factor in the long-term success of many Australian genera and species. 相似文献
74.
Trace gas flux and the influence of short-term soil water and temperature dynamics in Australian sheep grazed pastures of differing productivity 总被引:1,自引:0,他引:1
S. J. Livesley R. Kiese J. Graham C. J. Weston K. Butterbach-Bahl S. K. Arndt 《Plant and Soil》2008,309(1-2):89-103
Temperate pastures are often managed with P fertilizers and N2-fixing legumes to maintain and increase pasture productivity which may lead to greater nitrous oxide (N2O) emissions and reduced methane (CH4) uptake. However, the diel and inter-daily variation in N2O and CH4 flux in pastures is poorly understood, especially in relation to key environmental drivers. We investigated the effect of pasture productivity, rainfall, and changing soil moisture and temperature upon short-term soil N2O and CH4 flux dynamics during spring in sheep grazed pasture systems in southeastern Australia. N2O and CH4 flux was measured continuously in a High P (23 kg P ha?1 yr?1) and No P pasture treatment and in a sheep camp area in a Low P (4 kg P ha?1 yr?1) pasture for a four week period in spring 2005 using an automated trace gas system. Although pasture productivity was three-fold greater in the High P than No P treatment, mean CH4 uptake was similar (?6.3?±?SE 0.3 to ?8.6?±?0.4 μg C m?2 hr?1) as were mean N2O emissions (6.5 to 7.9?±?0.8 μg N m?2 hr?1), although N2O flux in the No P pasture did not respond to changing soil water conditions. N2O emissions were greatest in the Low P sheep camp (12.4 μg?±?1.1 N m?2 hr?1) where there were also net CH4 emissions of 5.2?±?0.5 μg C m?2 hr?1. There were significant, but weak, relationships between soil water and N2O emissions, but not between soil water and CH4 flux. The diel temperature cycle strongly influenced CH4 and N2O emissions, but this was often masked by the confounding covariate effects of changing soil water content. There were no consistently significant differences in soil mineral N or gross N transformation rates, however, measurements of substrate induced respiration (SIR) indicated that soil microbial processes in the highly productive pasture are more N limited than P limited after >20 years of P fertilizer addition. Increased productivity, through P fertilizer and legume management, did not significantly increase N2O emissions, or reduce CH4 uptake, during this 4 week measurement period, but the lack of an N2O response to rainfall in the No P pasture suggests this may be evident over a longer measurement period. This study also suggests that small compacted and nutrient enriched areas of grazed pastures may contribute greatly to the overall N2O and CH4 trace gas balance. 相似文献
75.
Matthias Kinne Marzena Poraj-Kobielska Elisabet Aranda René Ullrich Kenneth E. Hammel Katrin Scheibner Martin Hofrichter 《Bioorganic & medicinal chemistry letters》2009,19(11):3085-3087
An extracellular peroxygenase of Agrocybe aegerita catalyzed the H2O2-dependent hydroxylation of the multi-function beta-adrenergic blocker propranolol (1-naphthalen-1-yloxy-3-(propan-2-ylamino)propan-2-ol) and the non-steroidal anti-inflammatory drug diclofenac (2-[2-[(2,6-dichlorophenyl)amino]phenyl]acetic acid) to give the human drug metabolites 5-hydroxypropranolol (5-OHP) and 4′-hydroxydiclofenac (4′-OHD). The reactions proceeded regioselectively with high isomeric purity and gave the desired 5-OHP and 4′-OHD in yields up to 20% and 65%, respectively. 18O-labeling experiments showed that the phenolic hydroxyl groups in 5-OHP and 4′-OHD originated from H2O2, which establishes that the reaction is mechanistically a peroxygenation. Our results raise the possibility that fungal peroxygenases may be useful for versatile, cost-effective, and scalable syntheses of drug metabolites. 相似文献
76.
Plant and Soil - To investigate the effects of polystyrene microplastics (PS-beads) on the soil properties, photosynthesis of Flowering Chinese cabbage, the rhizosphere microbial community and... 相似文献
77.
Alessio Bechini 《PloS one》2013,8(3)
Abstract models of proteins have been widely used as a practical means to computationally investigate general properties of the system. In lattice models any sterically feasible conformation is represented as a self-avoiding walk on a lattice, and residue types are limited in number. So far, only two- or three-dimensional lattices have been used. The inspection of the neighborhood of alpha carbons in the core of real proteins reveals that also lattices with higher coordination numbers, possibly in higher dimensional spaces, can be adopted. In this paper, a new general parametric lattice model for simplified protein conformations is proposed and investigated. It is shown how the supporting software can be consistently designed to let algorithms that operate on protein structures be implemented in a lattice-agnostic way. The necessary theoretical foundations are developed and organically presented, pinpointing the role of the concept of main directions in lattice-agnostic model handling. Subsequently, the model features across dimensions and lattice types are explored in tests performed on benchmark protein sequences, using a Python implementation. Simulations give insights on the use of square and triangular lattices in a range of dimensions. The trend of potential minimum for sequences of different lengths, varying the lattice dimension, is uncovered. Moreover, an extensive quantitative characterization of the usage of the so-called “move types” is reported for the first time. The proposed general framework for the development of lattice models is simple yet complete, and an object-oriented architecture can be proficiently employed for the supporting software, by designing ad-hoc classes. The proposed framework represents a new general viewpoint that potentially subsumes a number of solutions previously studied. The adoption of the described model pushes to look at protein structure issues from a more general and essential perspective, making computational investigations over simplified models more straightforward as well. 相似文献
78.
Afforestation of former croplands has been proposed as a promising way to mitigate rising atmospheric CO2 concentration in view of the commitment to the Kyoto Protocol. Central to this C sequestration is the dynamics of soil organic C (SOC) storage and stability with the development of afforested plantations. Our previous study showed that SOC storage was not changed after afforestation except for the 0–10 cm layer in a semi-arid region of Keerqin Sandy Lands, northeast China. In this study, soil organic C was further separated into light and heavy fractions using the density fractionation method, and their organic C concentration and 13C signature were analyzed to investigate the turnover of old vs. new SOC in the afforested soils. Surface layer (0–10 cm) soil samples were collected from 14 paired plots of poplar (Populus × xiaozhuanica W. Y. Hsu & Liang) plantations with different stand basal areas (the sum of the cross-sectional area of all live trees in a stand), ranging from 0.2 to 32.6 m2 ha−1, and reference maize (Zea mays L.) croplands at the same sites as our previous study. Soil ΔC stocks (ΔC refers to the difference in SOC content between a poplar plantation and the paired cropland) in bulk soil and light fraction were positively correlated with stand basal area (R
2 = 0.48, p<0.01 and R
2 = 0.40, p = 0.02, respectively), but not for the heavy fraction. SOCcrop (SOC derived from crops) contents in the light and heavy fractions in poplar plantations were significantly lower as compared with SOC contents in croplands, but tree-derived C in bulk soil, light and heavy fraction pools increased gradually with increasing stand basal area after afforestation. Our study indicated that cropland afforestation could sequester new C derived from trees into surface mineral soil, but did not enhance the stability of SOC due to a fast turnover of SOC in this semi-arid region. 相似文献
79.
80.
Robert S. Stelzer Damion R. Drover Susan L. Eggert Maureen A. Muldoon 《Biogeochemistry》2011,103(1-3):91-107
We measured net nitrate retention by mass balance in a 700-m upwelling reach of a third-order sand plains stream, Emmons Creek, from January 2007 to November 2008. Surface water and groundwater fluxes of nitrate were determined from continuous records of discharge and from nitrate concentrations based on weekly and biweekly sampling at three surface water stations and in 23 in-stream piezometers, respectively. Surface water nitrate concentration in Emmons Creek was relatively high (mean of 2.25 mg NO3?CN l?1) and exhibited strong seasonal variation. Net nitrate retention averaged 429 mg NO3?CN m?2 d?1 and about 2% of nitrate inputs to the reach. Net nitrate retention was highest during the spring and autumn when groundwater discharge was elevated. Groundwater discharge explained 57?C65% of the variation in areal net nitrate retention. Specific discharge and groundwater nitrate concentration varied spatially. Weighting groundwater solute concentrations by specific discharge improved the water balance and resulted in higher estimates of nitrate retention. Our results suggest that groundwater inputs of nitrate can drive nitrate retention in streams with high groundwater discharge. 相似文献