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Vogt RD Seip HM Larssen T Zhao D Xiang R Xiao J Luo J Zhao Y 《The Science of the total environment》2006,367(1):394-404
Acid rain may cause soil acidification possibly leading to indirect forest damage. Assessment of acidification potential of atmospheric deposition is problematic where dry and occult deposition is significant. Furthermore, uncertainty is enhanced where a substantial part of the potential acidity is represented by deposition of ammonium (NH(4)(+)) since the degree of assimilation and nitrification is not readily available. Estimates of dry deposition based on deposition velocity are highly uncertain and the models need to be verified or calibrated by field measurements of total deposition. Total deposition may be monitored under the forest canopy. The main problem with this approach is the unknown influence of internal bio-cycling. Moreover, bio-cycling may neutralize much of the acidity by leaching of mainly K(+). When the water percolates down into the rooting zone this K(+) is assimilated again and acidity is regenerated. Most monitoring stations only measure deposition. Lacking measurements of output flux of both NH(4)(+) and NO(3)(-) from the soil one cannot assess current net N transformation rates. Assumptions regarding the fate of ammonium in the soil have strong influence on the estimated acid load. Assuming that all the NH(4)(+) is nitrified may lead to an overestimation of the acidifying potential. In parts of the world where dry deposition and ammonium are important special consideration of these factors must be made when assessing the acidification potential of total atmospheric loading. In China dry and occult deposition is considerable and often greater than wet deposition. Furthermore, the main part of the deposited N is in its reduced state (NH(4)(+)). The IMPACTS project has monitored the water chemistry as it moves through watersheds at 5 sites in China. This paper dwells at two important findings in this study. 1) Potassium leached from the canopy by acid rain is assimilated again upon entering the mineral soil. 2) Nitrification apparently mainly takes place in forest floor (H- and O-) horizon as NH(4)(+) that escapes this horizon is efficiently assimilated in the A-horizon. This suggests that the potential acidification capacity of the deposition may be found in the throughfall and forest floor solution by treating K(+) and NH(4)(+), respectively, as acid cations in a base neutralization capacity (BNC) calculation. 相似文献
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The influence of sediment redox conditions on solubility of selected metals and nutrients in sediment from a coastal Louisiana freshwater lake (Lake Cataouatche) receiving diverted Mississippi River water was quantified. Sediment redox was cycled step wise in 50 mV increments between oxidized (-200 to +500 mV) and reduced (+500 to -200 mV) conditions. Changes in sediment oxidation/reduction status and pH influenced solubility of both metals and nutrients. When redox potential (Eh) was increased from -200 to +500 mV, sediment pH decreased from 7.1 to 5.7. When the sediment Eh decreased from +500 to -200 mV, pH increased from 5.7 to 7.1. The increase in sediment acidity upon oxidation resulted in the release of the Pb, Ca, Mg, Al, and Zn into solution. The solution concentration of these elements was inversely proportional to Eh (P=0.05). The concentration of Fe, Mn, and P in sediment suspension was strongly governed by changes in oxidation-reduction status of sediment. The oxidation of reduced sediment resulted in a decrease in amount of Fe and Mn in solution, a result of the conversion of soluble ferrous and manganous form to less soluble ferric and manganic form. Following reduction of oxidized sediment, Fe and Mn became more soluble because the ferric iron and manganic manganese form changed to ferrous and manganous form. Phosphorus behavior as influenced by oxidation/reduction status was closely related to Fe and Mn chemistry with an increase in phosphate following iron reduction. This study demonstrated that sediment redox potential is an important parameter affecting metal and nutrient solubility and mobility in Louisiana coastal freshwater lake sediment. Reduction or aeration status of sediment should be considered in predicting the release of these elements into the aquatic environment. 相似文献
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Tindall CI Moore RV Bosley JD Swetnam RD Bowie R De Rudder A 《The Science of the total environment》2006,360(1-3):223-232
The Urban Regeneration and the Environment Research Programme (URGENT) required a system for cataloguing its datasets and enabling its scientific community to discover what data were available to it. This community was multidisciplinary in nature and therefore needed a range of facilities for searching. Of particular importance were facilities to help those unfamiliar with specialist terminology. To meet these needs, four applications were designed and developed: a Metadata Capture Tool for describing datasets in compliance with the National Geospatial Data Framework (NGDF) standard, a Term Entry Tool for creating an ISO compliant thesaurus, a Thesaurus Builder for merging thesauri and a Search Tool. To encourage users to help in cataloguing data, the capture tools were written as stand alone applications, which users could keep and use to build their own metadatabases. The tools contained export and import facilities that allowed the URGENT Data Centre to build a central database and publish it upon the web. During the development work, it was found necessary to extend the NGDF standard as it could not adequately describe time variant or 3-D atmospheric datasets. The four applications met their design objectives. However, a number of ergonomic issues will need to be addressed if the system is to meet the needs of the much larger up coming programmes. The main challenges will be moving from the NGDF standard to the ISO standard, hence bringing the work into line with the recommendations of the INSPIRE Project, and merging the metadatabase with the scientific database, which enable metadata maintenance to be semi-automated. 相似文献
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《影像视觉》一直在孜孜不倦地建议摄影师和爱好者使用RAW格式拍摄。为什么应该这么做呢?我们在使用RAW格式拍摄时应该注意些什么?且听我们娓娓道来。 相似文献
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我们将告诉大家,如何使用RAW文件中包含的丰富色彩信息得到优秀的黑白摄影作品。软件准备:Photoshop CS5学习目标:如何使用ACR中HSL/灰度面板的转换为灰度命令对黑白转换过程中的不同颜色明暗进行选择性控制。操作耗时:15分钟控制黑白转换时的画面明暗变化并不是一件容易的事情,我们期待画面元素反差分明、轮廓突出,可结果往往事与愿违,稍不留神整张照片就变得灰蒙蒙一片。今天我 相似文献
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我们将用实例告诉大家如何发掘画面的纹理与色彩潜力,增强作品的视觉冲击力。在无风的日子里进行风光摄影创作,水面就像一面镜子,能给我们带来完美的对称构图。但景物在经过水面反射以后,其通透程度和色彩表现力往往逊色于画面中的实体,破坏了画面的亮度平衡。想改善这类风光摄影作品的画面表现力,首先要做的就是对反射部分进行分区调整,增强该区域的色彩、反差与细节表现。幸好Pnotoshop为我们提供了功能强大的调整图层与图层蒙版。在它们的帮助下,这项 相似文献
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