全文获取类型
收费全文 | 765篇 |
免费 | 64篇 |
国内免费 | 96篇 |
出版年
2024年 | 3篇 |
2023年 | 8篇 |
2022年 | 10篇 |
2021年 | 11篇 |
2020年 | 20篇 |
2019年 | 26篇 |
2018年 | 31篇 |
2017年 | 23篇 |
2016年 | 31篇 |
2015年 | 16篇 |
2014年 | 30篇 |
2013年 | 41篇 |
2012年 | 28篇 |
2011年 | 38篇 |
2010年 | 28篇 |
2009年 | 38篇 |
2008年 | 32篇 |
2007年 | 39篇 |
2006年 | 21篇 |
2005年 | 37篇 |
2004年 | 29篇 |
2003年 | 24篇 |
2002年 | 32篇 |
2001年 | 24篇 |
2000年 | 10篇 |
1999年 | 17篇 |
1998年 | 15篇 |
1997年 | 19篇 |
1996年 | 11篇 |
1995年 | 16篇 |
1994年 | 15篇 |
1993年 | 18篇 |
1992年 | 18篇 |
1991年 | 3篇 |
1990年 | 13篇 |
1989年 | 16篇 |
1988年 | 11篇 |
1987年 | 7篇 |
1986年 | 8篇 |
1985年 | 15篇 |
1984年 | 19篇 |
1983年 | 14篇 |
1982年 | 14篇 |
1981年 | 10篇 |
1980年 | 3篇 |
1979年 | 6篇 |
1978年 | 9篇 |
1977年 | 8篇 |
1974年 | 2篇 |
1973年 | 3篇 |
排序方式: 共有925条查询结果,搜索用时 859 毫秒
81.
A model study of the microfiltration of fatty acids (C2 to C8) dissolved in a synthetic ultrafiltrate was performed. Ceramic membranes of 0.1 and 0.2 m were used. Results showed that minerals from the ultrafiltrate were involved in membrane fouling, while fatty acids were poorly rejected.On 0.1 m membranes, modelisation of fouling exhibited two different steps of fouling, a first one due to adsorption of solutes, and a second one involving particles deposit onto the membrane. On 0.2 m membranes, only fouling due to particles deposit was observed in modelisation. Results of fouling and rejection pointed out the great difference between 0.1 and0.2 m membranes.This revised version was published online in October 2005 with corrections to the Cover Date. 相似文献
82.
Soil inorganic carbon storage pattern in China 总被引:1,自引:0,他引:1
NA MI SHAOQIANG WANG JIYUAN LIU GUIRUI YU WENJUAN ZHANG ESTEBAN JOBBÁGY 《Global Change Biology》2008,14(10):2380-2387
Soils with pedogenic carbonate cover about 30% (3.44 × 106 km2) of China, mainly across its arid and semiarid regions in the Northwest. Based on the second national soil survey (1979–1992), total soil inorganic carbon (SIC) storage in China was estimated to be 53.3±6.3 PgC (1 Pg=1015 g) to the depth investigated to 2 m. Soil inorganic carbon storages were 4.6, 10.6, 11.1, and 20.8 Pg for the depth ranges of 0–0.1, 0.1–0.3, 0.3–0.5, and 0.5–1 m, respectively. Stocks for 0.1, 0.3, 0.5, and 1 m of depth accounted for 8.7%, 28.7%, 49.6%, and 88.9% of total SIC, respectively. In contrast with soil organic carbon (SOC) storage, which is highest under 500–800 mm yr−1 of mean precipitation, SIC storage peaks where mean precipitation is <400 mm yr−1. The amount and vertical distribution of SIC was related to climate and land cover type. Content of SIC in each incremental horizon was positively related with mean annual temperature and negatively related with mean annual precipitation, with the magnitude of SIC content across land cover types showing the following order: desert, grassland >shrubland, cropland >marsh, forest, meadow. Densities of SIC increased generally with depth in all ecosystem types with the exception of deserts and marshes where it peaked in intermediate layers (0.1–0.3 m for first and 0.3–0.5 m for latter). Being an abundant component of soil carbon stocks in China, SIC dynamics and the process involved in its accumulation or loss from soils require a better understanding. 相似文献
83.
An alkaline inorganic pyrophosphatase (IP) from leaves of an orchid, Aranda Christine 130 (Arachnis hookerana var. luteola × Vanda Hilo Blue) was purified by acetone precipitation and chromatography on Sephadex G-75 and DEAE-cellulose. The IP gave a single band on non-denaturing gel electrophoresis at pH 8.3 and its M, determined by gel filtration, was 28 000. The pH optimum was 9 and the IP required Mg2+ for its activity and stability. The IP exhibited high specificity for PPi and attained a maximum activity at a Mg2+: PPi ratio of 10:1. Other cations tested could not replace Mg2+ and they were also found to be inhibitory. The IP was also inhibited by EDTA and F? but not by iodoacetamide. 相似文献
84.
Effect of Vegetation Rehabilitation on Soil Carbon and Its Fractions in Mu Us Desert,Northwest China 总被引:1,自引:0,他引:1
Jia-Bin Liu Bin Wu Shu-Gao Qin Xin Jia Ke-Yu Fa 《International journal of phytoremediation》2015,17(6):529-537
Although vegetation rehabilitation on semi-arid and arid regions may enhance soil carbon sequestration, its effects on soil carbon fractions remain uncertain. We carried out a study after planting Artemisia ordosica (AO, 17 years), Astragalus mongolicum (AM, 5 years), and Salix psammophila (SP, 16 years) on shifting sand land (SL) in the Mu Us Desert, northwest China. We measured total soil carbon (TSC) and its components, soil inorganic carbon (SIC) and soil organic carbon (SOC), as well as the light and heavy fractions within soil organic carbon (LF-SOC and HF-SOC), under the SL and shrublands at depths of 100 cm. TSC stock under SL was 27.6 Mg ha?1, and vegetation rehabilitation remarkably elevated it by 40.6 Mgha?1, 4.5 Mgha?1, and 14.1 Mgha?1 under AO, AM and SP land, respectively. Among the newly formed TSC under the three shrublands, SIC, LF-SOC and HF-SOC accounted for 75.0%, 10.7% and 13.1% for AO, respectively; they made up 37.0%, 50.7% and 10.6% for AM, respectively; they occupied 68.6%, 18.8% and 10.0% for SP, respectively. The accumulation rates of TSC within 0–100 cm reached 238.6 g m?2y?1, 89.9 g m?2y?1 and 87.9 g m?2y?1 under AO, AM and SP land, respectively. The present study proved that the accumulation of SIC considerably contributed to soil carbon sequestration, and vegetation rehabilitation on shifting sand land has a great potential for soil carbon sequestration. 相似文献
85.
A new Na3Ca2(SO4)3F: Ce3+ phosphor synthesized by a solid state diffusion method is reported. The photoluminescence study showed a single high intensity emission peak at 307 nm wavelength when excited by UV light of wavelength 278 nm. An unresolved peak of comparatively less intensity was also observed at 357 nm along with the main peak. The characteristic emission of dopant Ce in Na3Ca2(SO4)3F phosphor clearly indicated that it resides in the host lattice in trivalent form. The emission peak can be attributed to 5d → 4f transition of rare earth Ce3+. The prepared sample is also characterized for its thermoluminescence properties. The TL glow curve of prepared sample showed a single broad peak at 147°C. The trapping parameters are also evaluated by Chen's method. The values of trap depth (E) and frequency factor (s) were found to be 0.64 ± 0.002 eV and 1.43 × 107 s–1 respectively. The study of PL and TL along with evaluation of trapping parameters has been undertaken and discussed for the first time. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
86.
Ana Rey 《Global Change Biology》2015,21(5):1752-1761
Widespread recognition of the importance of soil CO2 efflux as a major source of CO2 to the atmosphere has led to active research. A large soil respiration database and recent reviews have compiled data, methods, and current challenges. This study highlights some deficiencies for a proper understanding of soil CO2 efflux focusing on processes of soil CO2 production and transport that have not received enough attention in the current soil respiration literature. It has mostly been assumed that soil CO2 efflux is the result of biological processes (i.e. soil respiration), but recent studies demonstrate that pedochemical and geological processes, such as geothermal and volcanic CO2 degassing, are potentially important in some areas. Besides the microbial decomposition of litter, solar radiation is responsible for photodegradation or photochemical degradation of litter. Diffusion is considered to be the main mechanism of CO2 transport in the soil, but changes in atmospheric pressure and thermal convection may also be important mechanisms driving soil CO2 efflux greater than diffusion under certain conditions. Lateral fluxes of carbon as dissolved organic and inorganic carbon occur and may cause an underestimation of soil CO2 efflux. Traditionally soil CO2 efflux has been measured with accumulation chambers assuming that the main transport mechanism is diffusion. New techniques are available such as improved automated chambers, CO2 concentration profiles and isotopic techniques that may help to elucidate the sources of carbon from soils. We need to develop specific and standardized methods for different CO2 sources to quantify this flux on a global scale. Biogeochemical models should include biological and non‐biological CO2 production processes before we can predict the response of soil CO2 efflux to climate change. Improving our understanding of the processes involved in soil CO2 efflux should be a research priority given the importance of this flux in the global carbon budget. 相似文献
87.
A metal‐ion chelate immobilized enzyme reactor (IMER) supported on organic–inorganic hybrid silica monolith was developed for rapid digestion of proteins. The monolithic support was in situ prepared in a fused silica capillary via the polycondensation between tetraethoxysilane hydrolytic sol and iminodiacetic acid conjugated glycidoxypropyltrimethoxysilane. After activated by Cu2+, trypsin was immobilized onto the monolithic support via metal chelation. Proteolytic capability of such an IMER was evaluated by the digestion of myoglobin and BSA, and the digests were further analyzed by microflow reversed‐phase liquid chromatography with ESI‐MS/MS. Similar sequence coverages of myoglobin and BSA were obtained by IMER, in comparison to those obtained by in‐solution digestion (91 versus 92% for 200 ng myoglobin, and 26 versus 26% for 200 ng BSA). However, the digestion time was shortened from 12 h to 50 s. When the enzymatic activity was decreased after seven runs, the IMER could be easily regenerated by removing Cu2+ via EDTA followed by trypsin immobilization with fresh Cu2+ introduced, yielding the equal sequence coverage (26% for 200 ng BSA). For ~5 μg rat liver extract, even more proteins were identified with the immobilized trypsin digestion within 150 s in comparison to the in‐solution digestion for 24 h (541 versus 483), demonstrating that the IMER could be a promising tool for efficient and high‐throughput proteome profiling. 相似文献
88.
Tamiko Ok-hama Friederike Siebelt Kazuo Furihata Haruo Seto Shigetoh Miyachi Masayuki Ohmori 《Journal of phycology》1986,22(4):485-490
Using “P nuclear magnetic resonance analysis, total inorganic polyphosphate in algae could be quantitatively estimated, For this purpose the algal suspension, which had been kept in cold trichloroacetic acid, was further treated with 6 mM EDTA, or the cells were kept in 2 N KOH containing 100 mM EDTA for 18 h at 37°C. These simple methods avoid hydrolysis of cellular inorganic polyphosphate and, therefore, are useful for the study of phosphorus metabolism in algae. The effects of these treatments on visualization of the signal for inorganic polyphosphate in nuclear magnetic resonance spectra were discussed in comparison with in vivo, ‘P nuclear magnetic resonance spectra of algae. 相似文献
89.
We investigated the uptake of inorganic elements (Be, Na, Mg, K, Ca, Sc, Mn, Co, Zn, Se, Rb, Sr, Y, Zr, Ce, Pm, Gd, and Hf)
and the effect of Ca on their uptake in carrots (Daucus carota cv. U.S. harumakigosun) by the radioactive multitracer technique. The experimental results suggested that Na, Mg, K, and
Rb competed for the functional groups outside the cells in roots with Ca but not for the transporter-binding sites on the
plasma membrrane of the root cortex cells. In contrast, Y, Ce, Pm, and Gd competed with Ca for the transporters on the plasma
membrane. The selectivity, which was defined as the value obtained by dividing the concentration ratio of an elemental pair,
K/Na, Rb/Na, Be/Sr, and Mg/Sr, in the presence of 0.2 and 2 ppm Ca by that of the corresponding elemental pair in the absence
of Ca in the solution was estimated. The selectivity of K and Rb in roots was increased in the presence of Ca. The selectivity
of Be in roots was not affected, whereas the selectivity of Mg was increased by Ca. These observations suggest that the presence
of Ca in the uptake solution enhances the selectivity in the uptake of metabolically important elements against unwanted elements. 相似文献
90.
Mineral contents of Aloe vera leaf gel and their role on streptozotocin-induced diabetic rats 总被引:1,自引:0,他引:1
The role of some inorganic elements like vanadium, zinc, sodium, potassium, calcium, copper, manganese, and traces of chromium
in the improvement of impaired glucose tolerance and their indirect role in the management of diabetes mellitus are being
increasingly recognized. In traditional methods, medicinal plants are being used, which contain both organic and inorganic
constituents. In the present study, an attempt has been made to analyze the inorganic elements present in Aloe vera leat gel and their role on diabetes-related biochemical alterations in experimental rats. Special emphasis was given to the
inorganic parts by carefully preparing ash of the leaf gel. The results clearly indicate the presence of several hypoglycemic-activity-possessing
elements in the gel. The ash treatment also resulted in hypoglycemic action. In conclusion, the presence of various inorganic
trace elements in the gel might account for the hypoglycemic nature of the plant. 相似文献