全文获取类型
收费全文 | 1286篇 |
免费 | 68篇 |
国内免费 | 57篇 |
专业分类
测绘学 | 47篇 |
大气科学 | 127篇 |
地球物理 | 271篇 |
地质学 | 512篇 |
海洋学 | 139篇 |
天文学 | 157篇 |
综合类 | 15篇 |
自然地理 | 143篇 |
出版年
2023年 | 9篇 |
2022年 | 10篇 |
2021年 | 30篇 |
2020年 | 28篇 |
2019年 | 27篇 |
2018年 | 42篇 |
2017年 | 52篇 |
2016年 | 37篇 |
2015年 | 44篇 |
2014年 | 60篇 |
2013年 | 96篇 |
2012年 | 51篇 |
2011年 | 79篇 |
2010年 | 59篇 |
2009年 | 65篇 |
2008年 | 56篇 |
2007年 | 43篇 |
2006年 | 51篇 |
2005年 | 42篇 |
2004年 | 42篇 |
2003年 | 46篇 |
2002年 | 45篇 |
2001年 | 30篇 |
2000年 | 35篇 |
1999年 | 17篇 |
1998年 | 21篇 |
1997年 | 16篇 |
1996年 | 15篇 |
1995年 | 12篇 |
1994年 | 10篇 |
1993年 | 9篇 |
1991年 | 10篇 |
1990年 | 15篇 |
1988年 | 4篇 |
1987年 | 11篇 |
1986年 | 17篇 |
1985年 | 15篇 |
1984年 | 10篇 |
1983年 | 10篇 |
1982年 | 9篇 |
1981年 | 14篇 |
1980年 | 17篇 |
1979年 | 11篇 |
1978年 | 8篇 |
1976年 | 7篇 |
1974年 | 10篇 |
1973年 | 9篇 |
1972年 | 4篇 |
1971年 | 4篇 |
1968年 | 5篇 |
排序方式: 共有1411条查询结果,搜索用时 46 毫秒
51.
WANG Fang HE Yongtao FU Gang NIU Ben ZHANG Haorui LI Meng WANG Zhipeng WANG Xiangtao ZHANG Xianzhou 《资源与生态学报(英文版)》2020,11(3):290-297
Enclosure is one of the most widely used management tools for degraded alpine grassland on the northern Tibetan Plateau, but the responses of different types of grassland to enclosure may vary, and research on these responses can provide a scientific basis for improving ecological conservation. This study took one site for each of three grassland types (alpine meadow, alpine steppe and alpine desert) on the northern Tibetan Plateau as examples, and explored the effects of enclosure on plant and soil nutrients by comparing differences in plant community biomass, leaf-soil nutrient content and their stoichiometry between samples from inside and outside the fence. The results showed that enclosure can significantly increase all aboveground biomass in these three grassland types, but it only increased the 10-20 cm underground biomass in the alpine desert. Enclosure also significantly increased the leaf nutrient content of the dominant plants and contents of total nitrogen (N), total potassium (K), and organic carbon (C) in 10-20 cm soil in alpine desert, thus changing the stoichiometry between C, N and P (phosphorus). However, enclosure significantly increased only the N content of dominant plant leaves in alpine steppe, while other nutrients and stoichiometries of both plant leaves and soil did not show significant differences in alpine meadow and alpine steppe. These results suggested that enclosure has differential effects on these three types of alpine grasslands on the northern Tibetan Plateau, and the alpine desert showed the most active ecological conservation in the responses of its soil and plant nutrients. 相似文献
52.
Intensification of Southern Hemisphere westerly winds 2000–1000 years ago: evidence from the subantarctic Campbell and Auckland Islands (52–50°S)
下载免费PDF全文
![点击此处可从《第四纪科学杂志》网站下载免费的PDF全文](/ch/ext_images/free.gif)
53.
54.
55.
Rachel E. Adams Ayumi Hyodo Toby SantaMaria Cynthia L. Wright Thomas W. Boutton Jason B. West 《水文研究》2020,34(4):991-1003
Questions persist about interpreting isotope ratios of bound and mobile soil water pools, particularly relative to clay content and extraction conditions. Interactions between pools and resulting extracted water isotope composition are presumably related to soil texture, yet few studies have manipulated the bound pool to understand its influence on soil water processes. Using a series of drying and spiking experiments, we effectively labelled bound and mobile water pools in soils with varying clay content. Soils were first vacuum dried to remove residual water, which was then replaced with heavy isotope-enriched water prior to oven drying and spiking with heavy isotope-depleted water. Water was extracted via centrifugation or cryogenic vacuum distillation (at four temperatures) and analysed for oxygen and hydrogen isotope ratios via isotope ratio mass spectrometry. Water from centrifuged samples fell along a mixing line between the two added waters but was more enriched in heavy isotopes than the depleted label, demonstrating that despite oven drying, a residual pool remains and mixes with the mobile water. Soils with higher clay + silt content appeared to have a larger bound pool. Water from vacuum distillation samples have a significant temperature effect, with high temperature extractions yielding progressively more heavy isotope-enriched values, suggesting that Rayleigh fractionation occurred at low temperatures in the vacuum line. By distinctly labelling bound and mobile soil water pools, we detected interactions between the two that were dependent on soil texture. Although neither extraction method appeared to completely extract the combined bound and mobile (total water) pool, centrifugation and high temperature cryogenic vacuum distillations were comparable for both δ2H and δ18O of soil water isotope ratios. 相似文献
56.
Erodibility of low‐compaction steep‐sloped reclaimed surface mine lands in the southern Appalachian region,USA
下载免费PDF全文
![点击此处可从《水文研究》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The use of loose spoils on steep slopes for surface coal mining reclamation sites has been promoted by the US Department of Interior, Office of Surface Mining for the establishment of native forest, as prescribed by the Forest Reclamation Approach (FRA). Although low‐compaction spoils improve tree survival and growth, erodibility on steep slopes was suspected to increase. This study quantified a combined KC factor (combining the effects of the soil erodibility K factor and cover management C) for low compaction, steep‐sloped (>20°) reclaimed mine lands in the Appalachian region, USA. The combined KC factor was used because standard Unit Plot conditions required to separate these factors, per Revised Universal Soil Loss Equation (RUSLE) experimental protocols, were not followed explicitly. Three active coal mining sites in the Appalachian region of East Tennessee, each containing four replicate field plots, were monitored for rainfall and sediment yields during a 14‐month period beginning June 2009. Average cumulative erosivity for the study sites during the monitoring period was measured as 5248.9 MJ·mm·ha?1·h?1. The KC ranged between 0.001 and 0.05 t·ha·h·ha?1·MJ?1·mm?1, with the highest values occurring immediately following reclamation site construction as rills developed (June – August 2009). The KC for two study sites with about an 18–20 mm spoil D84 were above 0.01 t·ha·h·ha?1·MJ?1·mm?1 during rill development, and below 0.003 t·ha·h·ha?1·MJ?1·mm?1 after August 2009 for the post‐rill development period. The KC values for one site with a 40 mm spoil D84 were never above 0.008 t·ha·h·ha?1·MJ?1·mm?1 and also on average were lower, being more similar to the other two sites after the rill development period. Based on an initial KC factor (Ke) measured during the first few storm events, the average C factor (Ce) was estimated as 0.58 for the rill development period and 0.13 for the post‐rill development period. It appears that larger size fractions of spoils influence KC and Ce factors on low‐compaction steep slopes reclamation sites. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
57.
58.
59.
60.
J. Rogan T.M. Wright J. Cardille H. Pearsall Y. Ogneva-Himmelberger R. Riemann 《地理信息系统科学与遥感》2016,53(4):506-519
Forest fragmentation has been studied extensively with respect to biodiversity loss, disruption of ecosystem services, and edge effects although the relationship between forest fragmentation and human activities is still not well understood. We classified the pattern of forests in Massachusetts using fragmentation indicators to address these objectives: 1) characterize the spatial pattern of forest fragmentation in Massachusetts towns using Morphological Spatial Pattern Analysis (MSPA); and (2) identify regional trends using archetypal towns in relation to town history, geography and socioeconomic characteristics. Six fragmentation indicators were calculated using MSPA for each town to represent patterns and processes of fragmentation. We then used these indicators and the proportion of forested land to group towns across Massachusetts with similar patterns of fragmentation. Six representative towns typify different types of forest fragmentation, and illustrate the commonalities and differences between different fragmentation types. The objective selection of representative towns suggests that they might be used as the target of future studies, both in retrospective studies that seek to explain current patterns and in analyses that predict future fragmentation trends. 相似文献