全文获取类型
收费全文 | 4011篇 |
免费 | 642篇 |
国内免费 | 997篇 |
专业分类
测绘学 | 53篇 |
大气科学 | 717篇 |
地球物理 | 768篇 |
地质学 | 2106篇 |
海洋学 | 886篇 |
天文学 | 102篇 |
综合类 | 173篇 |
自然地理 | 845篇 |
出版年
2024年 | 16篇 |
2023年 | 81篇 |
2022年 | 209篇 |
2021年 | 180篇 |
2020年 | 173篇 |
2019年 | 197篇 |
2018年 | 158篇 |
2017年 | 166篇 |
2016年 | 197篇 |
2015年 | 213篇 |
2014年 | 289篇 |
2013年 | 328篇 |
2012年 | 285篇 |
2011年 | 316篇 |
2010年 | 200篇 |
2009年 | 299篇 |
2008年 | 303篇 |
2007年 | 245篇 |
2006年 | 271篇 |
2005年 | 210篇 |
2004年 | 191篇 |
2003年 | 153篇 |
2002年 | 133篇 |
2001年 | 128篇 |
2000年 | 128篇 |
1999年 | 107篇 |
1998年 | 96篇 |
1997年 | 80篇 |
1996年 | 49篇 |
1995年 | 45篇 |
1994年 | 31篇 |
1993年 | 33篇 |
1992年 | 14篇 |
1991年 | 14篇 |
1990年 | 12篇 |
1989年 | 9篇 |
1988年 | 20篇 |
1987年 | 8篇 |
1986年 | 17篇 |
1985年 | 15篇 |
1984年 | 9篇 |
1983年 | 4篇 |
1982年 | 8篇 |
1981年 | 8篇 |
1980年 | 1篇 |
1954年 | 1篇 |
排序方式: 共有5650条查询结果,搜索用时 15 毫秒
71.
随着分析技术的进步,δ13C观测在全球碳循环研究中越来越受到重视。在讨论关于碳循环中δ13C的Suess效应、分布特征和同位素分馏等基本概念的基础上,结合现有的观测事实,介绍应用δ13C辨别碳的源汇问题的方法,通过比较不同KeelingPlot方法分析该方法在研究大气CO2背景特征中的意义。着重讨论δ13C在研究大气C... 相似文献
72.
73.
74.
鄂尔多斯盆地苏里格地区奥陶系马家沟组马五段白云岩的地球化学特征 总被引:1,自引:0,他引:1
苏里格地区奥陶系马家沟组马五段白云岩是重要的油气储集层.多年来,马家沟组白云岩的成因一直是地质研究者讨论的热点话题.对苏里格地区马家沟组马五段白云岩的微量元素及C、O、Sr同位素地球化学资料的分析表明,微量元素表现为高铁、低锰、低锶、低钠;白云石、白云质角砾和孔洞充填白云岩的δ13C和δ18O值较为近似,且白云岩的δ1... 相似文献
75.
76.
77.
Employing the Unit Soil Carbon Amount (USCA) approach, soil carbon storage was calculated across the Northeast Plain of China based on the Multi-purpose Regional Geochemical Survey conducted in 2004 – 2006 (MRGS). The results indicated that the soil organic carbon (SOC) storage in topsoil (0 – 0.2 m), subsoil (0 – 1 m) and deep soil (0 – 1.8 m) was 768.1 Mt, 2978.4 Mt and 3729.2 Mt with densities of 3327.8 t/km2, 12,904.7 t/km2 and 16,157.5 t/km2, respectively. These values were consistent with national averages, whereas the soil carbon densities showed a clear increasing trend from the southern area of the Northeast Plain (Liaoning), to the middle (Jilin) and the northern Plain (Heilongjiang) — particularly in terms of topsoil carbon density, which increased from 2284.2, to 3436.7 and 3861.5 t/km2, respectively. In comparison to carbon data obtained from the Second National Soil Survey in 1984 – 1986 (SNSS), the topsoil SOC storage values from the MRGS were found to have decreased by 320.59 Mt (29.4%), with an average annual decline of 16.0 Mt (l.73%) over the 20 years. In the southern, middle and northern areas of the plain, soil carbon densities decreased by 1060.6 t/km2, 1646.4 t/km2 and 1300.2 t/km2, respectively, with an average value of 1389.0 t/km2 for the whole plain. These findings indicate that the decrease in soil carbon density varied according to the different ecosystems and land use types. Therefore, ratios of soil carbon density were calculated in order to study the carbon dynamic balance between ecosystems, and to further explore distribution characteristics, as well as the sequestration potential of SOC. 相似文献
78.
Dark respiration (non-photorespiratory mitochondrial respiration),which occurs both in the light and in darkness,is vital for growth and survival of algae and plays a critical role in modulating the carbon balance of them.In the present study,we have investigated dark respiration in the light (R L) and in darkness (R D) in three marine macroalgal species,Hizikia fusiformis (phaeophyta),Gracilaria lemaneiformis (Rhodophyta) and Ulva lactuca (Chlorophyta),cultured at 20℃ using aeration with two CO 2 conditions:current ambient (CO 2 concentration about 380 μl/L) and elevated CO 2 (approximately 720 μl/L) air.R L was estimated by using the Kok method,whereas R D was determined as the rate of O 2 influx at zero light.The results showed that both R L and R D were unchanged for the elevated CO 2-grown algae relative to ambient CO 2 concentration for all the algal species tested.However,R L was significantly lower than R D across all the algal species and growth CO 2 treatments,demonstrating that daytime respiration was partly depressed by the light.The percentage of inhibition of respiration by light was similar between ambient and elevated CO 2 grown algae.The ratio of respiration to photosynthesis,which tended to decrease when estimated using R L instead of R D,was not altered for the elevated relative to ambient CO 2 concentration.The results suggest that R L,rather than R D,is a more accurate estimate of nonphotorespiratory carbon loss in marine macroalgae during the daytime.It would not be anticipated that elevated atmospheric CO 2 would exert a substantial influence on respiratory flux either in the light or in darkness in these particular marine macroalgal species. 相似文献
79.
80.
Growth and grazing rate dynamics of major phytoplankton groups in an oligotrophic coastal site 总被引:1,自引:0,他引:1
Andrés Gutiérrez-Rodríguez Mikel Latasa Renate Scharek Ramon Massana Gemma Vila Josep M. Gasol 《Estuarine, Coastal and Shelf Science》2011
There has been more attention to phytoplankton dynamics in nutrient-rich waters than in oligotrophic ones thus requiring the need to study the dynamics and responses in oligotrophic waters. Accordingly, phytoplankton community in Blanes Bay was overall dominated by Prymnesiophyceae, remarkably constant throughout the year (31 ± 13% Total chlorophyll a, Tchl a) and Bacillariophyta with a more episodic appearance (20 ± 23% Tchl a). Prasinophyceae and Synechococcus contribution became substantial in winter (Prasinophyceae = 30% Tchl a) and summer (Synechococcus = 35% Tchl a). Phytoplankton growth and grazing mortality rates for major groups were estimated by dilution experiments in combination with high pressure liquid chromatography and flow cytometry carried out monthly over two years. Growth rates of total phytoplankton (range = 0.30–1.91 d−1) were significantly higher in spring and summer (μ > 1.3 d−1) than in autumn and winter (μ ∼ 0.65 d−1) and showed a weak dependence on temperature but a significant positive correlation with day length. Microzooplankton grazing (range = 0.03–1.4 d−1) was closely coupled to phytoplankton growth. Grazing represented the main process for loss of phytoplankton, removing 60 ± 34% (±SD) of daily primary production and 70 ± 48% of Tchl a stock. Chla synthesis was highest during the Bacillarophyceae-dominated spring bloom (Chl asynt = 2.3 ± 1.6 μg Chl a L−1 d−1) and lowest during the following post-bloom conditions dominated by Prymnesiophyceae (Chl asynt = 0.23 ± 0.08 μg Chl a L−1 d−1). This variability was smoothed when expressed in carbon equivalents mainly due to the opposite dynamics of C:chl a (range = 11–135) and chl a concentration (range = 0.07–2.0 μg chl a L−1). Bacillariophyta and Synechococcus contribution to C fluxes was higher than to biomass because of their fast-growth rate. The opposite was true for Prymnesiophyceae. 相似文献