全文获取类型
收费全文 | 9198篇 |
免费 | 768篇 |
国内免费 | 1117篇 |
出版年
2023年 | 100篇 |
2022年 | 98篇 |
2021年 | 184篇 |
2020年 | 198篇 |
2019年 | 314篇 |
2018年 | 292篇 |
2017年 | 261篇 |
2016年 | 219篇 |
2015年 | 253篇 |
2014年 | 686篇 |
2013年 | 620篇 |
2012年 | 524篇 |
2011年 | 602篇 |
2010年 | 507篇 |
2009年 | 634篇 |
2008年 | 500篇 |
2007年 | 477篇 |
2006年 | 425篇 |
2005年 | 390篇 |
2004年 | 282篇 |
2003年 | 299篇 |
2002年 | 256篇 |
2001年 | 187篇 |
2000年 | 171篇 |
1999年 | 161篇 |
1998年 | 165篇 |
1997年 | 163篇 |
1996年 | 136篇 |
1995年 | 117篇 |
1994年 | 117篇 |
1993年 | 149篇 |
1992年 | 112篇 |
1991年 | 86篇 |
1990年 | 83篇 |
1989年 | 70篇 |
1988年 | 69篇 |
1987年 | 54篇 |
1986年 | 68篇 |
1985年 | 113篇 |
1984年 | 188篇 |
1983年 | 106篇 |
1982年 | 122篇 |
1981年 | 95篇 |
1980年 | 102篇 |
1979年 | 99篇 |
1978年 | 50篇 |
1977年 | 42篇 |
1976年 | 44篇 |
1975年 | 40篇 |
1974年 | 28篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
Elena I. Katzy Alexander D. Iosipenko Dmitry A. Egorenkov Elena A. Zhuravleva Valery I. Panasenko Vldimir V. Ignatov 《FEMS microbiology letters》1990,72(1-2):1-4
Indole acetic acid (IAA) production in Azospirillum brasilense strain Sp245 is controlled by a 85 MDa plasmid naturally present in this bacterium. In the presence of L-tryptophan, anthranilic acid production and almost no IAA production occurs in a derivative strain harbouring a Tn5-Mob insertion in the 85 MDa plasmid. Agrobacterium tumefaciens strain GM19023, upon transfer of Tn5-Mob labelled 85 MDa plasmid of A. brasilense Sp245, gains the ability to produce anthranilic acid. 相似文献
2.
3.
E. S. Jensen 《Plant and Soil》1991,133(1):83-92
Simultaneous determination of 15N and total N using an automated nitrogen analyser interfaced to a continuous-flow isotope ratio mass spectrometer (ANA-MS method) was evaluated. The coefficient of variation (CV) of repeated analyses of homogeneous standards and samples at natural abundance was lower than 0.1%. The CV of repeated analyses of 15N-labelled plant material and soil samples varied between 0.3% and 1.1%. The reproductibility of repeated total N analyses using the automated method was comparable to results obtained with a semi-micro Kjeldahl procedure. However, the automated method gave results which were 3% to 5% higher than those obtained with the Kjeldahl procedure. Since only small samples can be analysed, careful sample homogenization and fine grinding are very important. Evaluation of a diffusion method for preparing nitrate and ammonium in solution for automated 15N analysis showed that the recovery of inorganic N in the NH3 trap was lower when the N was diffused from water than from 2 M KCl. The results also indicated that different proportions of the NO3
- and the NH4
+ in aqueous solution were recovered in the trap after combined diffusion. The method is most suited for diffusing either NO3
- or NH4
+ alone, but can be used for combined diffusion of the two ions. 相似文献
4.
5.
This study was conducted to investigate the influence of soil water potential, depth of N placement, timing, and cultivar on uptake of a small dose of labeled N applied after anthesis by wheat (Triticum aestivum L.) Understanding postanthesis N accumulation should allow better control of grain protein concentration through proper manipulation of inputs. Two hard, red spring-wheat cultivars were planted in early and late fall each yr of a 2-yr field experiment. Less than 1 kg N ha–1 as K 15NO3 was injected into the soil at two depths: shallow (0.05 to 0.08 m) and deep (0.15 to 0.18 m). In both years an irrigation was applied at anthesis, and injections of labeled N were timed 4, 12, and 20 days after anthesis (DAA). Soil water potential was estimated at the time of injection. Mean recovery of 15N in grain and straw was 57% of the 15N applied. Recovery did not differ between the high-protein (Yecora Rojo) and the low-protein (Anza or Yolo) cultivars. Mean recovery from deep placement was 60% versus only 54% from shallow placement (p < 0.01). Delaying the time of injection decreased mean recovery significantly from 58% at 4 DAA to 54% at 20 DAA. This decrease was most pronounced in the shallow placement, where soil drying was most severe. Regressions of recovery on soil water potential of individual cultivar x yr x planting x depth treatments were significant only under the driest conditions. Stepwise regression of 15N recovery on soil water potential and yield parameters using data from all treatments of both years resulted in an equation including soil water potential and N yield, with a multiple correlation coefficient of 0.64. The translocation of 15N to grain was higher (0.89) than the nitrogen harvest index (0.69), and showed a highly significant increase with increase in DAA. This experiment indicates that the N uptake capacity of wheat remains reasonably constant between 4 and 20 DAA unless soil drying is severe. 相似文献
6.
A.C. Cockcroft H.H. Du Preez 《Journal of experimental marine biology and ecology》1990,140(3):159-171
Nitrogen excretion and assimilation efficiencies in Lithognathus mormyrus (Linnaeus), a marine teleost present in high-energy surf zones in Algoa Bay, South Africa, were determined under controlled laboratory conditions at 15, 20 and 25 °C. Ammonia was the major form of nonfaecal nitrogen excreted by starved and fed L. mormyrus. Urea and amino acids were secondary excretory products. Ammonia excretion rates were temperature independent and the excretion rates of fed fish were significantly higher than starved fish at 20 and 25 °C but not at 15 °C. The mass component (b) of the mass/ammonia excretion equation was temperature independent and ranged from 0.590 to 0.669 and from 0.670 to 0.767 for starved and fed fish, respectively. The mean percentage of food energy lost via the dissolved nonfaecal excretory products (exogenous plus endogenous) was 4.12%. Assimilation efficiencies ranged from 71.89 to 98.78% for dry matter and from 96.89 to 99.88% on an energy basis. The combined nonfaecal and faecal energy loss was calculated at 10.11% of the ingested energy. The omnivorous ichthyofauna present in the surf zone ecosystem recycle 33 g N · m strip · yr−1. This constitutes < 1 % of total phytoplankton nitrogen requirements. 相似文献
7.
8.
9.
Fire can cause severe nitrogen (N) losses from grassland, chaparral, and temperate and boreal forest ecosystems. Paradoxically,
soil ammonium levels are markedly increased by fire, resulting in high rates of primary production in re-establishing plant
communities. In a manipulative experiment, we examined the influence of wild-fire ash residues on soil, microbial and plant
N pools in a recently burned Californian bishop pine (Pinus muricata D. Don) forest. Ash stimulated post-fire primary production and ecosystem N retention through direct N inputs from ash to
soils, as well as indirect ash effects on soil N availability to plants. These results suggest that redistribution of surface
ash after fire by wind or water may cause substantial heterogeneity in soil N availability to plants, and could be an important
mechanism contributing to vegetation patchiness in fire-prone ecosystems. In addition, we investigated the impact of fire
on ecosystem N cycling by comparing 15N natural abundance values from recently burned and nearby unburned P. muricata forest communities. At the burned site, 15N natural abundance in recolonising species was similar to that in bulk soil organic matter. By contrast, there was a marked
15N depletion in the same species relative to the total soil N pool at the unburned site. These results suggest that plant uptake
of nitrate (which tends to be strongly depleted in 15N because of fractionation during nitrification) is low in recently burned forest communities but could be an important component
of eco- system N cycling in mature conifer stands.
Received: 29 June 1999 / Accepted: 24 October 1999 相似文献
10.
P Robertson K A Koehler R G Hiskey 《Biochemical and biophysical research communications》1979,86(2):265-270
The binding of magnesium ions to two tripeptides, -Arg--Gla--Gla-OMe and Z--Arg(NO2)--Gla--Gla-OMe, and to bovine prothrombin fragment 1 as a function of pH has been monitored by 25Mg NMR spectroscopy. Binding to the tripeptide was dependent on peptide ionizations occurring at pH 4.6 – 4.8. The pH dependence of magnesium ion binding to fragment 1 reveals two inflection points 4.2 may be attributed to the deprotonation of the third side chain carboxylic acid group of the double γ-carboxyglutamic acid sequence. The origin of the increased binding of magnesium ions to fragment 1 at pH values above 7 is unknown. 相似文献