全文获取类型
收费全文 | 2275篇 |
免费 | 166篇 |
国内免费 | 77篇 |
出版年
2023年 | 15篇 |
2022年 | 25篇 |
2021年 | 26篇 |
2020年 | 27篇 |
2019年 | 48篇 |
2018年 | 68篇 |
2017年 | 59篇 |
2016年 | 50篇 |
2015年 | 64篇 |
2014年 | 90篇 |
2013年 | 149篇 |
2012年 | 77篇 |
2011年 | 126篇 |
2010年 | 81篇 |
2009年 | 102篇 |
2008年 | 95篇 |
2007年 | 131篇 |
2006年 | 107篇 |
2005年 | 115篇 |
2004年 | 110篇 |
2003年 | 73篇 |
2002年 | 69篇 |
2001年 | 60篇 |
2000年 | 46篇 |
1999年 | 55篇 |
1998年 | 54篇 |
1997年 | 58篇 |
1996年 | 44篇 |
1995年 | 45篇 |
1994年 | 35篇 |
1993年 | 38篇 |
1992年 | 26篇 |
1991年 | 29篇 |
1990年 | 29篇 |
1989年 | 21篇 |
1988年 | 19篇 |
1987年 | 14篇 |
1986年 | 20篇 |
1985年 | 23篇 |
1984年 | 23篇 |
1983年 | 20篇 |
1982年 | 35篇 |
1981年 | 25篇 |
1980年 | 19篇 |
1979年 | 22篇 |
1978年 | 12篇 |
1977年 | 7篇 |
1976年 | 7篇 |
1974年 | 8篇 |
1973年 | 9篇 |
排序方式: 共有2518条查询结果,搜索用时 557 毫秒
181.
Hagedorn Frank Bucher Jürg B. Tarjan David Rusert Peter Bucher-Wallin Inga 《Plant and Soil》2000,224(2):273-286
The objectives of this study were to estimate how soil type, elevated N deposition (0.7 vs. 7 g N m–2y–1) and tree species influence the potential effects of elevated CO2 (370 vs. 570 mol CO2 mol–1) on N pools and fluxes in forest soils. Model spruce-beech forest ecosystems were established on a nutrient-rich calcareous sand and on a nutrient-poor acidic loam in large open-top chambers. In the fourth year of treatment, we measured N concentrations in the soil solution at different depths, estimated N accumulation by ion exchange resin (IER) bags, and quantified N export in drainage water, denitrification, and net N uptake by trees. Under elevated CO2, concentrations of N in the soil solution were significantly reduced. In the nutrient-rich calcareous sand, CO2 enrichment decreased N concentrations in the soil solution at all depths (–45 to –100%). In the nutrient-poor acidic loam, the negative CO2 effect was restricted to the uppermost 5 cm of the soil. Increasing the N deposition stimulated the negative impact of CO2 enrichment on soil solution N in the acidic loam at 5 cm depth from –20% at low N inputs to –70% at high N inputs. In the nutrient-rich calcareous sand, N additions did not influence the CO2 effect on soil solution N. Accumulation of N by IER bags, which were installed under individual trees, was decreased at high CO2 levels under spruce in both soil types. Under beech, this decrease occurred only in the calcareous sand. N accumulation by IER bags was negatively correlated with current-years foliage biomass, suggesting that the reduction of soil N availability indices was related to a CO2-induced growth enhancement. However, the net N uptake by trees was not significantly increased by elevated CO2. Thus, we suppose that the reduced N concentrations in the soil solution at elevated CO2 concentrations were rather caused by an increased N immobilisation in the soil. Denitrification was not influenced by atmospheric CO2 concentrations. CO2 enrichment decreased nitrate leaching in drainage by 65%, which suggests that rising atmospheric CO2 potentially increases the N retention capacity of forest ecosystems. 相似文献
182.
Different field experiments were performed to discover the main factor(s) responsible for the poor leaf growth, moderate propagation and leaf chlorosis of Oxalis acetosella in an old Norway spruce stand with acid soil (Höglwald Forest). A previous study had suggested, that Ca (or Mg) deficiency or Mn toxicity could be involved.In a Main Diagnostic Field Experiment with an intact population, Ca and Mg were either applied as sulphate or carbonate to distinguish between nutritional and pH effects. Mn and Si were also applied to aggravate or overcome possible effects of Mn toxicity. Enhancement experiments with different amounts of CaSO4 were conducted to investigate the Ca dose-effect relationship under field conditions. Additional trials with SrCO3, BaCO3 and NaHCO3 had the goal to raise the soil pH without supply of nutrients.Greenhouse experiments with Oxalis acetosella supplemented the field studies by investigating the Ca and Mn dose-effect relationships under controlled conditions. Growth, vitality and nutrition of Oxalis were studied in a nutrient solution culture at pH 4.0 over a range of concentrations of Ca (20 to 5000 mol L-1) and Mn (5 to 1000 mol L-1) respectively. Furthermore, the effects of two contrasting ammonium/nitrate ratios were tested. The nutritional composition of the basal nutrient solution and the microclimate in the greenhouse were as far as possible adjusted to the environmental conditions of the plant in the Höglwald Forest.All these studies led to the conclusion, that the moderate growth and vitality of Oxalis in the Höglwald Forest was mainly due to an insufficient Ca supply, rather than an effect of Mg deficiency, low soil pH or Mn toxicity. The application of CaSO4 caused a similar stimulation of the growth as CaCO3. A clearly positive, close CaSO4 dose-effect relationship was detected in field experiments as well as in the nutrient solution study. The same type of leaf chloroses as in the field was reproduced through low Ca nutrient solutions. Predominant ammonium nutrition may significantly impair Ca uptake.Oxalis acetosella displayed a relatively high leaf tissue tolerance of excessive Mn. There was no indication for a Mn-induced Ca deficiency in the Höglwald Forest. Enhanced Si uptake led to a partial vitalization of Oxalis; the reason for that remained unclear. 相似文献
183.
Effects of sulfate on cadmium uptake by Swiss chard: II. Effects due to sulfate addition to soil 总被引:3,自引:0,他引:3
Sulfate complexation of Cd in nutrient solution has been shown to have little impact on Cd uptake by plants. This study examined the effect of sulfate added to soil on Cd concentrations in soil solution and Cd uptake by Swiss chard (Beta vulgaris L. cv. Fordhook Giant). Swiss chard was grown in soil which was wetted with complete nutrient solution containing equivalent salt concentrations of NaNO3 or Na2SO4. Plant growth was reduced by increasing both NO3 and SO4 concentrations in soil solution, with growth reductions similar for both salts. The Cd concentration in soil solution increased P< 0.05) more consistently with increasing concentrations of SO4 compared to NO3 in soil solution. Solution speciation, calculated with GEOCHEM-PC, showed significant increases of Cd2+ activities with increasing salt rates. Shoot Cd content in 19-day-old Swiss chard plants was marginally but significantly P <0.05) increased with increasing SO4 concentration but no effect was observed with increasing NO3 concentration. These results are compared with earlier work on the marked effect of Cl- salinity on Cd availability in Swiss chard. Possible mechanisms explaining the smaller effect of SO4 compared to Cl on Cd availability are proposed. 相似文献
184.
White clover (Trifolium repens L.) is widely distributed in the Appalachian region, except on highly acid soils. We used a procedure where a thin layer of soil is placed on top of solidified water agar to characterize effects of acid soil on seedling root growth. Our objectives were to evaluate the soil-on-agar technique by using four soils (non-limed and limed) with diverse chemical characteristics and to relate root emergence to the chemical properties of the soils. We used three white clover cultivars, Grasslands Huia, Grasslands Tahora and Sacramento. Daily counts of root emergence from soil into agar were made for 12 d. Liming hastened white clover root emergence in three of the four soils. Days to 40% emergence were closely related (P < 0.01) to soil pH
and to species of soil solution Al that are associated with Al toxicity in dicotyledonous plants. The r2 values for the regression of days to 40% root emergence on
were 0.95, 0.96, 0.94 and 0.96, respectively. Apparently, the primary factor responsible for delayed root emergence in the soil-on-agar procedure was Al toxicity. Because of the close relationship between root emergence and activity of toxic species of soil solution Al, we propose that the soil-on-agar technique should be useful for characterizing the response of many small-seeded species to Al. 相似文献
185.
Phytotoxicity of hydrogen fluoride and fluoroborate and their uptake from solution culture by Lycopersicon esculentum and Avena sativa 总被引:6,自引:0,他引:6
The aims of this paper were to determine the phytoavailability and phytotoxicity of hydrogen fluoride (HF) and fluoroborate (BF
4
-
) in solution when exposed to the root of the plant. As fluoroborate undergoes a slow hydrolysis to F and borate ions, the stability of BF
4
-
under solution culture conditions was determined. Fluoroborate was found to have a zero order rate constant of 0.0136 and took approximately 72 days to hydrolyse completely.Tomato (Lycopersicon esculentum) and oat (Avena sativa) plants were grown in dilute nutrient solutions which contained a range of activities of HF and BF
4
-
. Dry matter production of both tomato and oat plants grown in nutrient solutions were found to be restricted by increased activity of HF and BF
4
-
in solution. Tomatoes were more sensitive to HF and BF
4
-
than oats. Limitations to dry matter production coincided with increased uptake of F for F concentrations in tissue of both tomatoes and oats. Fluoride uptake of both HF and BF by tomatoes and oats was orders of magnitude higher compared to similar activities of other ionic species of F reported in previous studies. Possible mechanisms of uptake are discussed. 相似文献
186.
Toshiyuki Tanaka James B. Ames Masatsune Kainosho Lubert Stryer Mitsuhiko Ikura 《Journal of biomolecular NMR》1998,11(2):135-152
The three-dimensional solution structure of recombinant bovine myristoylated recoverin in the Ca2+-free state has been refined using an array of isotope-assisted multidimensional heteronuclear NMR techniques. In some experiments, the myristoyl group covalently attached to the protein N-terminus was labeled with 13C and the protein was unlabeled or vice versa; in others, both were 13C-labeled. This differential labeling strategy was essential for structural refinement and can be applied to other acylated proteins. Stereospecific assignments of 41 pairs of -methylene protons and 48 methyl groups of valine and leucine were included in the structure refinement. The refined structure was constructed using a total of 3679 experimental NMR restraints, comprising 3242 approximate interproton distance restraints (including 153 between the myristoyl group and the polypeptide), 140 distance restraints for 70 backbone hydrogen bonds, and 297 torsion angle restraints. The atomic rms deviations about the averaged minimized coordinate positions for the secondary structure region of the N-terminal and C-terminal domains are 0.44 ± 0.07 and 0.55 ± 0.18 Å for backbone atoms, and 1.09 ± 0.07 and 1.10 ± 0.15 Å for all heavy atoms, respectively. The refined structure allows for a detailed analysis of the myristoyl binding pocket. The myristoyl group is in a slightly bent conformation: the average distance between C1 and C14 atoms of the myristoyl group is 14.6 Å. Hydrophobic residues Leu28, Trp31, and Tyr32 form a cluster that interacts with the front end of the myristoyl group (C1-C8), whereas residues Phe49, Phe56, Tyr86, Val87, and Leu90 interact with the tail end (C9-C14). The relatively deep hydrophobic pocket that binds the myristoyl group (C14:0) could also accommodate other naturally occurring acyl groups such as C12:0, C14:1, and C14:2 chains. 相似文献
187.
Solution structure of barley lipid transfer protein complexed with palmitate. Two different binding modes of palmitate in the homologous maize and barley nonspecific lipid transfer proteins. 总被引:2,自引:0,他引:2
下载免费PDF全文
![点击此处可从《Protein science : a publication of the Protein Society》网站下载免费的PDF全文](/ch/ext_images/free.gif)
M. H. Lerche F. M. Poulsen 《Protein science : a publication of the Protein Society》1998,7(12):2490-2498
The structure of a nonspecific lipid transfer protein from barley (ns-LTPbarley) in complex with palmitate has been determined by NMR spectroscopy. The structure has been compared to the structure of ns-LTPbarley in the absence of palmitate, to the structure of ns-LTPbarley in complex with palmitoyl coenzyme A, to the structure of ns-LTPmaize in its free form, and to the maize protein complexed with palmitate. Binding of palmitate only affects the structure of ns-LTPbarley moderately in contrast to the binding of palmitoyl coenzyme A, which leads to a considerable expansion of the protein. The modes of binding palmitate to the maize and barley protein are different. Although in neither case there are major conformational changes in the protein, the orientation of the palmitate in the two proteins is exactly opposite. 相似文献
188.
189.
190.
Synergistic Impact of Solvent and Polymer Additives on the Film Formation of Small Molecule Blend Films for Bulk Heterojunction Solar Cells
下载免费PDF全文
![点击此处可从《Liver Transplantation》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Caitlin McDowell Maged Abdelsamie Kui Zhao Detlef‐M. Smilgies Guillermo C. Bazan Aram Amassian 《Liver Transplantation》2015,5(18)
The addition of polystyrene (PS), a typical insulator, is empirically shown to increase the power conversion efficiencies (PCEs) of a solution‐deposited bulk heterojunction (BHJ) molecular blend film used in solar cell fabrication: p‐DTS(FBTTh2)2/PC71BM. The performance is further improved by small quantities of diiodooctane (DIO), an established solvent additive. In this study, how the addition of PS and DIO affects the film formation of this bulk heterojunction blend film are probed via in situ monitoring of absorbance, thickness, and crystallinity. PS and DIO additives are shown to promote donor crystallite formation on different time scales and through different mechanisms. PS‐containing films retain chlorobenzene solvent, extending evaporation time and promoting phase separation earlier in the casting process. This extended time is insufficient to attain the morphology for optimal PCE results before the film sets. Here is where the presence of DIO comes into play: its low vapor pressure further extends the time scale of film evolution and allows for crystalline rearrangement of the donor phase long after casting, ultimately leading to the best BHJ organization. 相似文献