全文获取类型
收费全文 | 70篇 |
免费 | 61篇 |
国内免费 | 15篇 |
专业分类
大气科学 | 1篇 |
地球物理 | 8篇 |
地质学 | 47篇 |
海洋学 | 9篇 |
天文学 | 3篇 |
综合类 | 7篇 |
自然地理 | 71篇 |
出版年
2022年 | 6篇 |
2021年 | 3篇 |
2020年 | 2篇 |
2019年 | 3篇 |
2018年 | 5篇 |
2017年 | 2篇 |
2016年 | 1篇 |
2015年 | 4篇 |
2014年 | 5篇 |
2013年 | 2篇 |
2012年 | 5篇 |
2011年 | 3篇 |
2010年 | 8篇 |
2009年 | 6篇 |
2008年 | 6篇 |
2007年 | 7篇 |
2006年 | 8篇 |
2005年 | 7篇 |
2004年 | 11篇 |
2003年 | 5篇 |
2002年 | 7篇 |
2001年 | 4篇 |
2000年 | 3篇 |
1999年 | 4篇 |
1998年 | 6篇 |
1997年 | 3篇 |
1996年 | 1篇 |
1995年 | 3篇 |
1994年 | 4篇 |
1993年 | 3篇 |
1992年 | 2篇 |
1990年 | 3篇 |
1989年 | 1篇 |
1987年 | 3篇 |
排序方式: 共有146条查询结果,搜索用时 15 毫秒
81.
82.
83.
84.
Daniel Markewitz Ricardo de O. Figueiredo Eric A. Davidson 《Journal of Geochemical Exploration》2006,88(1-3):214
The objective of this study was to investigate the role of dissolved CO2 (H2CO3*) as a mechanism of cation removal from surface soils under secondary land uses in the tropics. Soil leachate columns were prepared with 0–10 cm soils from mature and secondary forest, and managed pastures, and extracted with H2CO3* from deionized water equilibrated with 0%, 0.5%, 1%, and 10% CO2 (g). Extraction of soil cations slowed over time following an exponential form for the cumulative data. The rate of cation concentration decline varied as a function of CO2 concentration with the 10% solution resulting in a greater percent decline with extraction volume. Potassium removal from the exchange sites of all soils and for all solutions was nearly complete ranging from 85% to 97% while removals of Mg (31% to 71%) and Ca (12% to 42%) were lower. The asymptotic patterns of cation loss observed in this study suggest that H2CO3* acid-driven losses of cations may become self-limiting over time. Other stronger acids from atmospheric deposition or organic sources may serve to perpetuate cation removal, and re-forestation on these cleared lands would certainly re-distribute cations from soils to vegetation. 相似文献
85.
土壤固钾与释钾机制研究进展 总被引:39,自引:0,他引:39
根据钾素在土壤中对作物的有效性,可将其分为速效钾、缓效钾和无效钾。各种形态钾素之间存在着渐变性,并处于一种动态平衡,其中,土壤钾素固定与释放对这一平衡产生重要影响,进而影响钾素的有效性。富含 2∶ 1型粘土矿物的土壤,不仅有丰富的缓效钾储量和较强的供钾能力,而且固钾能力也较强。在淋溶性较强的砂质土壤上,固钾作用可使钾素免受淋失,有利于保存肥力和提高肥效。但是,对于多数土壤来说,将使施入土壤的大部分钾素转化为非交换性钾,其中,只有小部分可被当季作物利用。在集约农业条件下,土壤的固定已成为提高钾素利用效率的主要障碍因素。系统阐述了钾素释放与固定的概念与过程,综述了影响钾素固定与释放的主要因素;系统分析了粘土矿物类型及其结构、土壤铵态氮肥和磷肥水平、土壤水分状况、有机物和羟基铝影响钾素固定、释放过程的作用机理;并指出了土壤钾素固定和释放研究领域的发展现状和重要动向。 相似文献
86.
Changes in the relationship between soil soluble ions and rainfall were studied on hillslopes at seven research sites that represent four climatic regions: Mediterranean, semi-arid, mildly arid and arid. At each site, soil samples were taken in several seasons and the ratio between the Na+ plus K+ content and the Ca2+ plus Mg2+ content (sodium and potassium adsorption RATIO=SPAR) was determined. In general, SPAR increased with increasing aridity except for the most arid site in which the soil contains a gypsic layer. The relationship between SPAR and rainfall was non-linear. An abiotic threshold, characterized by a sharp change in the SPAR, was found around the 200 mm isohyet: sites that receive less than 200 mm annual rainfall showed significantly higher SPAR than those that receive more than 200 mm, where SPAR was very low. 相似文献
87.
根据青藏高原16个气象观测台站有关气温和降水的多年数据,分析了近40年来青藏高原的气候变化特征。结果表明,近40年来青藏高原气候变化的总趋势是气温升高,降水量稍有增加。这种气候波动变化,对高原植被、冰雪水及土地沙化都有较大的影响。据预测到2100年青藏高原气温将上升2~3 6℃,降水量在高原中部和东部增加约0~300mm,而在西南部将减少0~500mm。气候的变化使植被群落发生明显的变化,分布界线向更高的海拔高度迁移。也使高原冰川退缩,但未来冰川不一定消失。 相似文献
88.
溶胶—凝胶工艺制备二氧化钛超微细粉过程中的物相和显微结构的变化 总被引:2,自引:0,他引:2
用溶胶—凝胶工艺制备了不同晶型的TiO_2超微细粉,锐钛矿型TiO_2超微细粉呈分散的球形颗粒,平均粒径约为0.03μm;金红石型TiO_2超微细粉呈分散的圆柱状颗粒,平均粒径约为0.07μm。用FT-IR.XRD.DTA/TGA和TEM等手段研究了TiO_2凝胶在煅烧过程中物相和显微结构的变化。发现TiO_2凝胶在200℃以下呈非晶态,在300℃时结晶成锐钛矿,在700℃时,锐钛矿开始转变为金红石,在1000℃时,锐钛矿消失,全部转变为金红石。 相似文献
89.
Displacement studies on leaching of potassium (K+) were conducted under unsaturated steady state flow conditions in nine undisturbed soil columns (15.5 cm in diameter and
25 cm long). Pulses of K+ applied to columns of undisturbed soil were leached with distilled water or calcium chloride (CaCl2) at a rate of 18 mm h−1. The movement of K+ in gypsum treated soil leached with distilled water was at a similar rate to that of the untreated soil leached with 15 mM
CaCl2. The Ca2+ concentrations in the leachates were about 15 mM, the expected values for the dissolution of the gypsum. When applied K+ was displaced with the distilled water, K+ was retained in the top 10–12.5 cm depth of soil. In the undisturbed soil cores there is possibility of preferential flow
and lack of K+ sorption. The application of gypsum and CaCl2 in the reclamation of sodic soils would be expected to leach K+ from soils. It can also be concluded that the use of sources of water for irrigation which have a high Ca2+ concentration can also lead to leaching of K+ from soil. Average effluent concentration of K+ during leaching period was 30.2 and 28.6 mg l−1 for the gypsum and CaCl2 treated soils, respectively. These concentrations are greater than the recommended guideline of the World Health Organisation
(12 mg K+ l−1). 相似文献
90.
Laura Schaefer 《Icarus》2005,173(2):454-468
We use chemical equilibrium calculations to model the speciation of alkalis and halogens in volcanic gases emitted on Io. The calculations cover wide temperature (500-2000 K) and pressure (10−6 to 10+1 bars) ranges, which overlap the nominal conditions at Pele (T=1760 K, P=0.01 bars). About 230 compounds of 11 elements (O, S, Li, Na, K, Rb, Cs, F, Cl, Br, I) are considered. The elemental abundances for O, S, Na, K, and Cl are based upon observations. CI chondritic elemental abundances relative to sulfur are used for the other alkalis and halogens (as yet unobserved on Io). We predict the major alkali species in Pele-like volcanic gases and the percentage distribution of each alkali are LiCl (73%), LiF (27%); NaCl (81%), Na (16%), NaF (3%); KCl (91%), K (5%), KF (4%); RbCl (93%), Rb (4%), RbF (3%); CsCl (92%), CsF (6%), Cs (2%). Likewise the major halogen species and the percentage distribution of each halogen are NaF (88%), KF (10%), LiF (2%); NaCl (89%), KCl (11%); NaBr (89%), KBr (10%), Br (1%); NaI (61%), I (30%), KI (9%). We predict the major halogen condensates and their condensation temperatures at P=0.01 bar are NaF (1115 K), LiF (970 K); NaCl (1050 K), KCl (950 K); KBr (750 K), RbBr (730 K), CsBr (645 K); and solid I2 (200 K). We also model disequilibrium chemistry of the alkalis and halogens in the volcanic plume. Based on this work and our prior modeling for Na, K, and Cl in a volcanic plume, we predict the major loss processes for the alkali halide gases are photolysis and/or condensation onto grains. Their estimated photochemical lifetimes range from a few minutes for alkali iodides to a few hours for alkali fluorides. Condensation is apparently the only loss process for elemental iodine. On the basis of elemental abundances and photochemical lifetimes, we recommend searching for gaseous KCl, NaF, LiF, LiCl, RbF, RbCl, CsF, and CsCl around volcanic vents during eruptions. Based on abundance considerations and observations of brown dwarfs we also recommend a search of Io's extended atmosphere and the Io plasma torus for neutral and ionized Li, Cs, Rb, and F. 相似文献