全文获取类型
收费全文 | 478篇 |
免费 | 40篇 |
国内免费 | 72篇 |
专业分类
测绘学 | 27篇 |
大气科学 | 40篇 |
地球物理 | 176篇 |
地质学 | 113篇 |
海洋学 | 147篇 |
天文学 | 3篇 |
综合类 | 24篇 |
自然地理 | 60篇 |
出版年
2024年 | 2篇 |
2023年 | 5篇 |
2022年 | 11篇 |
2021年 | 8篇 |
2020年 | 9篇 |
2019年 | 20篇 |
2018年 | 6篇 |
2017年 | 17篇 |
2016年 | 15篇 |
2015年 | 17篇 |
2014年 | 27篇 |
2013年 | 33篇 |
2012年 | 12篇 |
2011年 | 43篇 |
2010年 | 24篇 |
2009年 | 26篇 |
2008年 | 43篇 |
2007年 | 42篇 |
2006年 | 32篇 |
2005年 | 30篇 |
2004年 | 22篇 |
2003年 | 19篇 |
2002年 | 19篇 |
2001年 | 17篇 |
2000年 | 16篇 |
1999年 | 16篇 |
1998年 | 7篇 |
1997年 | 6篇 |
1996年 | 4篇 |
1995年 | 4篇 |
1994年 | 11篇 |
1993年 | 9篇 |
1992年 | 4篇 |
1991年 | 3篇 |
1990年 | 1篇 |
1989年 | 1篇 |
1988年 | 2篇 |
1987年 | 2篇 |
1985年 | 1篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1980年 | 1篇 |
1978年 | 1篇 |
排序方式: 共有590条查询结果,搜索用时 15 毫秒
141.
Rutger de Wit Lucas J. Stal Bente Aa. Lomstein Rodney A. Herbert Hans van Gemerden Pierluigi Viaroli Victor-Ugo Cecherelli Francisco Rodríguez-Valera Marco Bartoli Gianmarco Giordani Roberta Azzoni Bart Schaub David T. Welsh Andrew Donnelly Ana Cifuentes Josefa Antn Kai Finster Lise B. Nielsen Anne-Grethe Underlien Pedersen Anne Turi Neubauer Marina A. Colangelo Sander K. Heijs 《Continental Shelf Research》2001,21(18-19)
“Buffer capacities” has been defined in ecology as a holistic concept (e.g., Integration of Ecosystem Theories: A Pattern, second ed. Kluwer, Dordrecht, 1997, 388pp), but we show that it can also be worked out in mechanistic studies. Our mechanistic approach highlights that “buffering capacities” can be depleted progressively, and, therefore, we make a distinction between current and potential “buffering capacities”. We have applied this concept to understand the limited “local stability” in seagrass ecosystems and their vulnerability towards structural changes into macro-algal dominated communities. We explored the following processes and studied how they confer buffering capacities to the seagrass ecosystem: (i) net autotrophy is persistent in Zostera noltii meadows where plant assimilation acts as a sink for nutrients, this contrasted with the Ulva system that shifted back and forth between net autotrophy and net heterotrophy; (ii) the Z. noltii ecosystem possesses a certain albeit rather limited capacity to modify the balance between nitrogen fixation and denitrification, i.e., it was found that in situ nitrogen fixation always exceeded denitrification; (iii) the nitrogen demand of organoheterotrophic bacteria in the sediment results in nitrogen retention of N in the sediment and hence a buffer against release of nitrogen compounds from sediments, (iv) habitat diversification in seagrass meadows provides shelter for meiofauna and hence buffering against adverse conditions, (v) sedimentary iron provides a buffer against noxious sulfide (note: bacterial sulfide production is enhanced in anoxic sediment niches by increased organic matter loading). On the other hand, in the coastal system we studied, sedimentary iron appears less important as a redox-coupled buffer system against phosphate loading. This is because most inorganic phosphate is bound to calcium rather than to iron. In addition, our studies have highlighted the importance of plant–microbe interactions in the seagrass meadows. 相似文献
142.
143.
Jorge Marcelo Gonnet 《Journal of Arid Environments》2001,48(4):569
Cattle grazing is an important disturbance in the Monte plain, reducing grass biomass and rates of fruit setting. Grass seeds are the most important food for granivorous birds (Emberizidae) during winter. The objective of this study was to test whether granivorous bird populations (Emberizidae), grass seed production, and vegetation structure differed at sites with different intensities of grazing. Emberizid density and species richness were higher in the ungrazed site than in the two grazed paddocks. Seed abundance was also higher in ungrazed vs. grazed sites. Woody vegetation, that serve as safe nest sites, did not differ among treatments. Granivorous bird populations seemed to be affected by cattle grazing; however, the main mechanisms of this process remain unknown. 相似文献
144.
145.
中国主要河流控制站氮、磷含量变化规律初探 总被引:33,自引:1,他引:32
根据黄河洛口站,长江武汉站和珠江高要站1980到1989年的水文,水化学资料,分别计算中国3条主要河流下游控制站80年代营养元素氮,磷平均含量,比较氮,磷含量的季节变化规律,并探讨影响它们含量变化的主要因素。 相似文献
146.
氮氧化物(NOX)在对流层中的收支与循环的研究 总被引:2,自引:0,他引:2
对氮氧化物(NOx)在对流层中的收支与循环问题,及其在对流层中的各种源汇过程进行了综述与讨论。其中包括NOx的人为排放与自然排放,干、湿沉降的清除及其相对重要性等。并对我国NOx的人为排放与自然排放进行了估算。 相似文献
147.
The contribution of nitrogen and phosphorus due to precipitation constitutes the second most important route after superficial runoff. The sampling carried out during a two-year period by means of a precipitation collector allows us to determine the contribution of this route both qualitatively and quantitatively. Nitrogen is mainly supplied in an inorganic form, while phosphorus is principally supplied as orthophosphate. During the period of this study (March 1986–February 1988) it was found that in the Santillana Reservoir Watershed the level of nitrogen supplied by precipitation constitutes an average of 4.87% and the level of phosphorus constitutes 8.01%. The contribution of nitrogen varies in inverse ratio to precipitation and the contribution of phosphorus varies in direct ratio. 相似文献
148.
H. D. Freyer 《Pure and Applied Geophysics》1978,116(2-3):393-404
Preliminary nitrogen isotope data for ammonia from animal urine, fuel combustion, fertilizer use and fertilizer factories have been measured or estimated. It turns out that direct nitrogen isotope measurements of atmospheric ammonia at Jülich are in the expected range calculated from the ranges of different sources. For deposition of atmospheric ammonium in Jülich-rain a depletion in15N with respect to atmospheric ammonia has been found which is explained by isotope fractionations during rainout and washout. In correspondence with this fractionation model are nitrogen isotope data of rain-ammonium from coastal areas, which are enriched in15N due to the fact that sea water acts as a sink for atmospheric ammonia.For Jülich rain-nitrate a pronounced seasonal trend has been detected with lower15N data in spring and summer than in autumn and winter. This trend is interpreted by different nitrogen isotope data of anthropogenic and natural nitric oxides which have been measured or estimated from isotope fractionation effects during nitrification and denitrification reactions in soils. It should be possible to get better global estimations for anthropogenic and natural nitric oxides from nitrogen isotope measurements. 相似文献
149.
Variation of phytoplankton biomass and primary production in Daya Bay during spring and summer 总被引:11,自引:0,他引:11
Song X Huang L Zhang J Huang X Zhang J Yin J Tan Y Liu S 《Marine pollution bulletin》2004,49(11-12):1036-1044
Environmental factors, phytoplankton biomass (Chl a) and primary production of two water areas in Daya Bay (Dapeng'ao Bay and Aotou Bay) were investigated during the transition period from spring to summer. Chl a ranged from 3.20 to 13.62 and 13.43 to 26.49 mg m(-3) in Dapeng'ao Bay and Aotou Bay respectively, if data obtained during red tides are excluded. Primary production varied between 239.7 and 1001.4 mg Cm(-2) d(-1) in Dapeng'ao Bay. The regional distribution of Chl a and primary production were mostly consistent from spring to summer in both bays. Seasonal transition characters have been found in Daya Bay from spring to summer, including high values of DO, nitrate and silicate. Size structures of phytoplankton and its primary production do not change very much from spring to summer, with micro-phytoplankton dominating and contributing about 50% of the whole. In Daya Bay, phytoplankton is limited by nitrogen in spring, and by phosphate in summer. Artificial impacts are evident from high temperature effluent from nuclear power stations, aquaculture and sewage. During the investigation, a red tide occurred in Aotou Bay, with a maximum Chl a of 103.23 mgm(-3) at surface and primary production of 2721.9 mg Cm(-2) d(-1) in the red tide center. Raised water temperature and nutrient supply from land-sources help to stimulate annual red tides. 相似文献
150.
The hydrogeochemistry of the Lake Waco drainage basin,Texas 总被引:1,自引:0,他引:1
The origin of surface water chemistry in highly impacted drainage basins must be investigated on a drainage-basin scale if the causes of the pollution are to be elucidated. This study characterizes and deciphers the surface water chemistry of a nutrient polluted river system in central Texas. Four tributaries of the Lake Waco reservoir were chemically characterized temporally and spatially in order to gain a complete understanding of the nature and origin of dissolved solids being transported into the lake. Temporal chemical variations measured at the base of each of the drainage basins are repetitive and seasonal. The most periodic and well-defined variation is exhibited by nitrate concentrations although many of the other solutes show seasonal changes as well. These temporal chemical changes are controlled by seasonal precipitation. During rainy seasons, the shallow aquifer is recharged resulting in stream discharge that is high in nitrate, calcium, and bicarbonate. When the shallow flow system is depleted in the summer, stream waters are dominated by deeper groundwater and become rich in sodium. Spatial variations in the chemistry of South Bosque surface waters were characterized using the snapshot technique. The spatial distribution of nitrate in surface waters is controlled by fertilizer application to row crops and the location of a munitions factory. The concentrations of naturally derived solutes such as Ca+, Na+, Cl–, and SO4–2are controlled by underlying lithologies. 相似文献