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   33010篇
  免费   356篇
  国内免费   235篇
安全科学   832篇
废物处理   1426篇
环保管理   4259篇
综合类   5431篇
基础理论   9151篇
环境理论   15篇
污染及防治   8494篇
评价与监测   2126篇
社会与环境   1652篇
灾害及防治   215篇
  2022年   267篇
  2021年   238篇
  2019年   213篇
  2018年   405篇
  2017年   457篇
  2016年   670篇
  2015年   516篇
  2014年   833篇
  2013年   2584篇
  2012年   978篇
  2011年   1377篇
  2010年   1154篇
  2009年   1131篇
  2008年   1369篇
  2007年   1436篇
  2006年   1243篇
  2005年   1065篇
  2004年   1062篇
  2003年   1019篇
  2002年   996篇
  2001年   1259篇
  2000年   939篇
  1999年   526篇
  1998年   387篇
  1997年   440篇
  1996年   442篇
  1995年   509篇
  1994年   479篇
  1993年   412篇
  1992年   429篇
  1991年   409篇
  1990年   444篇
  1989年   416篇
  1988年   359篇
  1987年   371篇
  1986年   337篇
  1985年   319篇
  1984年   370篇
  1983年   329篇
  1982年   364篇
  1981年   314篇
  1980年   279篇
  1979年   294篇
  1978年   257篇
  1977年   241篇
  1976年   198篇
  1975年   205篇
  1974年   226篇
  1973年   234篇
  1972年   215篇
排序方式: 共有10000条查询结果,搜索用时 140 毫秒
391.
392.
Several management practices are available to conserve and sequester C in the agricultural sector of the former Soviet Union (FSU). The highest rate of C accumulation would result from the implementation of a no-till management option which will only continue during the first ten years until new C equilibrium is reached. Agroforestry management options provide a longer period for C accumulation, but at a lower rate. It is possible that the longest period of C conservation may be achieved by increasing the area under perennial grasses in the crop rotation. During the first decade of implementation of the management practices, the amount of C conserved or sequestered would be approximately equal to the current rate of net C sequestration in FSU forest sector. At present, agricultural soils and vegetation of the FSU store approximately 120 Pg C; the accumulation of soil organic matter is 0.032 Pg C yr-1. The annual C loss in the FSU agricultural sector was estimated at 0.21 Pg C yr-1.  相似文献   
393.
Jørgensen  C. K.  Reisfeld  R.  Berg  W. F.  Jaenicke  L. 《Die Naturwissenschaften》1977,64(9):490-492
The Science of Nature -  相似文献   
394.
395.
396.
Focus on eco-friendly processing techniques makes vegetable tanning a viable option in leather processing and establishes the subsequent need for the more efficient methods of extraction in tannin manufacture. Application of ultrasound has been tried in the extraction of tannins from myrobalan nuts in order to improve the extraction efficiency, to perform the extraction under milder process conditions and to reduce the process time. The influence of process parameters such as ultrasonic output power, time and temperature has been studied. Scale-up trials and the use of ultrasound in pulse mode have also been attempted. The results show that a three- to fivefold improvement is possible with ultrasonic output from 20 to 100 W. Extraction efficiency has been calculated from the maximum extractable materials from myrobalan nuts. Extraction efficiency is found to be 90% for ultrasound, 100 W without external heating as compared to 77% for control process at 70 °C for 4 h. Therefore, ultrasound could be employed even dispensing with provision for temperature controls. The use of ultrasound in pulse mode offered 70% extraction efficiency of continuous mode. Scale-up trials indicate that there exists an optimum ultrasonic output power depending on the amount of nuts used, to achieve better extraction efficiency. The effectiveness of ultrasonically extracted tannin solution has also been tested in the tanning process for its applicability. The degree of tanning efficacy has been assessed by shrinkage temperature measurement. The results indicate that ultrasonically extracted tannin solution is suitable for tanning process. Therefore, application of ultrasound in tannin extract manufacture is a viable option with added advantages.  相似文献   
397.
398.
399.
400.
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