首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   81篇
  免费   2篇
  国内免费   1篇
测绘学   4篇
大气科学   6篇
地球物理   19篇
地质学   31篇
海洋学   8篇
天文学   8篇
综合类   1篇
自然地理   7篇
  2018年   1篇
  2017年   4篇
  2016年   2篇
  2015年   1篇
  2014年   2篇
  2013年   2篇
  2012年   5篇
  2011年   7篇
  2010年   2篇
  2009年   8篇
  2008年   4篇
  2007年   11篇
  2006年   4篇
  2005年   2篇
  2004年   3篇
  2003年   2篇
  2002年   9篇
  2001年   4篇
  2000年   2篇
  1999年   2篇
  1998年   1篇
  1994年   1篇
  1992年   1篇
  1991年   2篇
  1990年   1篇
  1987年   1篇
排序方式: 共有84条查询结果,搜索用时 11 毫秒
21.
United Nations climate change conferences have attracted an increasing number and range of observer participants, often outnumbering national delegates. The interactions between the formal and informal spaces of climate governance at the Conference of the Parties (COP) are explored by investigating why non-nation state actors (NNSAs) attend them and by measuring to what extent official UN Side Events provide relevant information for the formal negotiations. Based on primary empirical research at recent COPs, it is found that 60–75% of Side Events have related directly to items under negotiation in the post-2012 climate negotiations. In this regard, Side Events that facilitate informal exchange between stakeholders not only provide input into the negotiations but also allow issues beyond the realm of the negotiations to be discussed, reflecting the scope of climate change. Although Side Events are an effective forum to exchange ideas and network, their current format and purpose as being events ‘on the side’ does not offer a sufficient framework for coordination between the work of NNSAs and the United Nations Framework Convention on Climate Change (UNFCCC) process.  相似文献   
22.
23.
24.
Heike Koppitz   《Limnologica》2004,34(1-2):37
Changes in the contents and composition of dissolved amino acids and carbohydrates were monitored in different Phragmites australis stands in Germany. Four different reed clones planted in 1997 in a degraded fen area in NE-Germany were compared with respect to the seasonal development in the total amount of amino acids and sugars in the basal culm internodes. The individual Phragmites clones showed significant differences in the absolute content of both parameters indicating an influence of the genotype. Flooding affected the ratio of amino acids to carbohydrates within the reed clones. Plants grown in flooded parts of the fen area revealed a significantly higher total amino acid content together with a lower total content of carbohydrates.Furthermore, the effects of extreme flooding on the content and composition of amino acids and carbohydrates in the basal culm internodes of Phragmites australis were studied in nine reed stands with different degrees of damage on Lake Constance. Shoots from extremely damaged reed stands were characterised by a significant decrease in the total content of carbohydrates, an increase in the total content of amino acids and an increase in the share of γ-aminobutyric acid. These changes in the content and composition of carbohydrates and amino acids were discussed with respect to their value as stress indicators, such as for oxygen deprivation which causes a hypoxic metabolic state in Phragmites australis stands.  相似文献   
25.
In the absence of a sufficiently dense network of climate stations covering all topographic regions of the Indus River basin and delivering high quality data over the last 30 years or more, daily precipitation data were obtained from the National Centers for Environmental Prediction-Department of the Enviornment (NCEP-DOE) Reanalysis 2 dataset for the period 1979 to 2011. The daily precipitation data were transformed into time series of frequency of extreme precipitation events of 1-day and 10-day durations defined in terms of 90th and 99th percentile threshold exceedances. The non-parametric Mann-Kendall trend test was applied to determine whether statistically significant changes in precipitation extremes occurred over time, in due consideration of autocorrelation in the data.

Extreme precipitation showed a high spatial variability, with the highest daily and 10-day precipitation totals, and thus highest 90th and 99th percentiles, in the southeastern lowlands at the foot of the Himalayas and the lowest in the Karakorum. Significantly decreasing trends in extreme precipitation were observed in the western part of the Indus River basin; significantly increasing trends were mainly detected in the very high mountainous regions in the east (Transhimalaya and Himalayas) and in the north (Hindu Kush and Karakorum) of the Indus basin. High precipitation rates are not common in the arid climate of these high mountainous regions. Future flood management plans need to consider the increasing trends in extreme precipitation events in these areas.  相似文献   

26.
Fractional crystallization of fatty acid methyl esters (FAME) produced from waste cooking oil or animal fats was performed in a laboratory-scale facility. The raw esters and the “winterization” products were analyzed by determining the FAME pattern, iodine and peroxide numbers as well as oxidation stabilities and element concentrations. Fractionation experiments under air and nitrogen were conducted, but no differences concerning oxidative stability were observed. By improvement of the winterization and washing the filtration residue with methanol, the enrichment of saturated FAME in one step was raised up to a technically sufficient rate of 85%. Absorption of oxygen by FAME at different temperatures was measurable. Volatile decomposition products were registrated after heating the FAME at different temperatures. It was shown that antioxidants like butylhydroxytoluene or α-tocopherol were enriched in the phase of unsaturated FAME. This effect obviously affected the oxidative stability of the fraction of saturated FAME during winterization, although no oxidative damage of FAME was detectable at temperatures up to 20°C.  相似文献   
27.
Constructing quality: The multinational histories of chocolate   总被引:1,自引:0,他引:1  
Geographic research on food quality, while considering many of the ways in which quality is socially constructed, has largely focused on the place-based aspects of the raw materials of food production. Here, we use French convention theory to look at a highly processed food in order to show how place associations in the social construction of food quality extend to manufacturing. For chocolate, quality is based on material characteristics whose relative importance in determining quality depends on the country in which different stages of economic innovation took place. Struggles over the definition of quality chocolate, as exemplified by the “European Chocolate War,” show how quality issues are connected to geographies of manufacturing and innovation.  相似文献   
28.
Refractory compounds are responsible for the long-term sequestration of organic matter in soil. The aim of this study is to assess the storage of refractory compounds, i.e. compounds with long turnover times, across size separates in arable soils. The contents and distribution of organic carbon (OC) and nitrogen (N) in size fractions were examined for two contrasting treatment types from long-term agroecosystem experiments, i.e. C-depleted and fertilized plots. The soil organic carbon (SOC) pool of the C-depleted plots is considered to be relatively enriched in refractory compounds compared with the SOC in the fertilized counterparts. In two of the three long-term experiments, the relative retention of OC in separates <20 μm was considerably higher than in separates 2000–20 μm (OC contents in depleted plots compared with fertilized plots). Highest residual contents of OC were found in fractions <6 μm. In the third experiment, additionally to the very fine fractions, separates 250–20 μm retained a high proportion of OC. The behavior of N was analogous to that of OC: the highest relative residues in the depleted plots were found in fine separates. These results indicate that in the investigated arable soils, C and N compounds associated with fine separates are most stable. Refractory OC in arable soils may be largely stored in fine particle-size fractions.  相似文献   
29.
To test whether the silicate reference glasses BAM‐S005‐A and BAM‐S005‐B from BAM (The Federal Institute for Materials Research and Testing, Germany) are suitable materials for microanalysis, we investigated the homogeneity of these reference glasses using the microanalytical techniques EPMA, LA‐ICP‐MS and SIMS. Our study indicated that all major and most trace elements are homogeneously distributed at micrometre sampling scale in both types of glass. However, some trace elements (e.g., Cs, Cl, Cr, Mo and Ni) seem to be inhomogeneously distributed. We also determined the composition of BAM‐S005‐A and BAM‐S005‐B. The EPMA data of major elements confirmed the information values specified by the certificate. With the exception of Sr, Ba, Ce and Pb, our trace element data by LA‐ICP‐MS were also in agreement with the certified values within the stated uncertainty limits. The reasons for the discrepancy in these four elements are still unclear. In addition, we report new data for twenty‐two further trace elements, for which the concentrations were not certified. Based on our investigation, we suggest that both of these materials are suitable for many microanalytical applications.  相似文献   
30.
After 1990, external loads of Central European rivers with inorganic nutrients (nitrogen and phosphorus) and organic material were reduced because of changed environmental laws. However, in the eutrophic lowland River Warnow, North-Eastern Germany, nitrate concentrations remained high with 35–185 μmol l−1 without a significant decrease since 1992. In contrast, phosphate concentrations, varying between 0.3 and 5.2 μmol l−1 during the growth season 2002, decreased significantly over the years. However, its concentrations still exceeded 1 μmol l−1 regularly in the growth seasons. This load led to a substantial accumulation of organic matter additional to high terrestrial inputs. Despite the high organic load, the remineralising bacteria were mainly inactive in River Warnow. Therefore, the composition of the dissolved organic material, especially its bioavailability, were investigated during the growth season 2002 and discussed with other potential controlling factors. River Warnow carried a high load of dissolved organic carbon (14 mg l−1), especially of humic substances (5.5 mg C l−1). Bacterial abundance (12×106 ml−1) as well as production (1.7 μg C l−1 h−1) depended on temperature. During late spring and summer at constantly higher temperatures, bacterial production correlated positively to readily utilisable substrates and to humic compounds. Thus, the bacterial community in River Warnow may be well adapted or contain enough species using the available amino acids and carbohydrates as well as humic matter compounds. However, calculated from protozoan biomass, grazing may control bacterial biomass and perhaps community composition profoundly, what lead to the low percentages of active bacteria.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号