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排序方式: 共有498条查询结果,搜索用时 78 毫秒
491.
This paper reviews methods for carbon-14 dating using accelerators. 14C is accelerated to high energies. Subsequently 14C is separated from 14N by passing it through a xenon gas cell or electrostatic separator. Advantages, limits and applications of the techniques are discussed.  相似文献   
492.
Nitrogen is often provided to impoverished overburden dumps through the establishment of legumes. Low indigenous soil nutrient levels, summer drought conditions and an acidic mining overburden represent major obstacles to successful rehabilitation of open-cut coal mining at Collie in southwest Western Australia. In this study,Acacia pulchella, a native Western Australian species often used in rehabilitation of mined lands, was shown to nodulate and grow in coal mining overburden with pH values less than 4.0 under glasshouse conditions. Plant growth (both top and root dry weight), nodule fresh weight, and nodulation success was best at a pH near 5.0, a value only slightly lower than the typical soil pH of the native jarrah (Eucalyptus marginata) forest. Acetylene reduction rates were reduced by acidity and ranged from 8.2 m C2H4g–1hr–1 at pH 6.77 to 3.0 m C2H4g–1hr–1 at a pH of 3.98. Four additional plant species were found to occur and to nodulate on acid overburden material at Collie.  相似文献   
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The radiocarbon content in creosote bush in the greater Los Angeles area has been found to reflect the distribution of smog. Indications are that smog in the even larger air basin of the San Joaquin Valley of California may be caused in part by recent biospheric emissions rather than by fossil fuel combustion alone.  相似文献   
496.
The assessment of environmental impacts along the life cycle of biofuels is a complex and resource-demanding task that cannot be afforded by small producers in developing countries. Therefore, certification schemes bear the risk that small and independent producers will be locked out and the market for sustainable biofuels will be dominated by international investors and large-scale plantations. However, many environmental impacts of various production chains of biofuels and feedstocks are already known. This knowledge has been used to create a web-based questionnaire for a “Sustainability Quick Check for Biofuels” (SQCB, http://www.sqcb.org). SQCB reduces the need for user entries to the most relevant and best-known parameters of the biofuel production chain. Based on this user input, a specific inventory is automatically modelled and linked to background data. SQCB then calculates the environmental impact assessment and checks the results against sustainability criteria. Since the results are calculated immediately, key environmental factors can be interactively analyzed. One major goal of the SQCB is to support the market entrance for local biofuel producers, given that strengthening local stakeholders is a key driver for empowering rural communities in development countries.  相似文献   
497.
In order to come up with a sound assessment of the effects of material efficiency innovations on growth, employment and competitiveness, different economic mechanisms have to be considered. From a methodological point of view, the analysis must be able to account for technology specific aspects. The employment impacts of five innovations are analyzed: plastic and paper recycling, increased life span of automobiles, car sharing, and bio-based products. Time horizons for all strategies are scenarios until 2020.The economic impacts are context specific with regard to innovation, technology and country. The economic impacts highlight substantial structural adjustments. The overall economic net impact on employment tends to be neutral to positive. Stronger positive economic impacts result if first mover advantages can be established.  相似文献   
498.
Malpaisomys insularis is a mouse-like rodent endemic to the eastern Canary Islands. It became extinct during the fourteenth century. It was a remarkable species living under hyperarid conditions. A dental microwear analysis was performed in order to determine its former diet. The elevated number of fine scratches found in Malpaisomys molars suggests that it consumed a significant part of Poaceae, grass consumption leaving the most distinctive features on dental wear facets. A graminivorous diet with a high amount of abrasive items is in agreement with the broad teeth of Malpaisomys, considered as adapted to grass consumption. However, in the absence of potential competitors over its native range, it is likely that Malpaisomys also foraged on dicots to meet higher nutrient and energetic requirements. The ecology of Malpaisomys is discussed from these results in the context of the desertic climatic conditions of the eastern Canary Islands and with a special concern on its small body size in contrast to other large-sized island murine species such as the giant rats of the central Canary Islands.  相似文献   
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