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   41606篇
  免费   453篇
  国内免费   565篇
安全科学   1286篇
废物处理   1876篇
环保管理   5119篇
综合类   6902篇
基础理论   11057篇
环境理论   21篇
污染及防治   10812篇
评价与监测   2846篇
社会与环境   2438篇
灾害及防治   267篇
  2022年   406篇
  2021年   392篇
  2020年   319篇
  2019年   362篇
  2018年   617篇
  2017年   650篇
  2016年   958篇
  2015年   747篇
  2014年   1163篇
  2013年   3319篇
  2012年   1433篇
  2011年   1928篇
  2010年   1572篇
  2009年   1596篇
  2008年   1879篇
  2007年   1965篇
  2006年   1683篇
  2005年   1442篇
  2004年   1333篇
  2003年   1417篇
  2002年   1282篇
  2001年   1663篇
  2000年   1202篇
  1999年   708篇
  1998年   474篇
  1997年   517篇
  1996年   521篇
  1995年   601篇
  1994年   623篇
  1993年   490篇
  1992年   515篇
  1991年   489篇
  1990年   541篇
  1989年   477篇
  1988年   409篇
  1987年   398篇
  1986年   340篇
  1985年   370篇
  1984年   380篇
  1983年   365篇
  1982年   342篇
  1981年   317篇
  1980年   272篇
  1979年   292篇
  1978年   262篇
  1977年   213篇
  1976年   181篇
  1975年   209篇
  1974年   193篇
  1972年   201篇
排序方式: 共有10000条查询结果,搜索用时 265 毫秒
491.
492.
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   
493.
494.
IntroductionTheuseofchemicals,includingpesticides,hasbecomeanintegralandeconomicallyessentialpartofmodernagriculture.Pesticide?..  相似文献   
495.
IntroductionInnatureCooccursintwooxidationstates ,Co2 + andCo3+ ,andformationofthecomplexanionCo(OH) - 3 isalsopossible .DuringweatheringCoisrelativelymobileinoxidizingacidenvironment,butduetoahighsorptionbyFeandMnoxides,aswellasbyclayminerals,thismetaldoesnotm…  相似文献   
496.
本文主要研究了肯尼亚姆瓦彻湾(属于浅潮滩红树林湿地)泥沙交换的动力机制.该港湾属于半日潮,大、小潮的潮差分别为.2m和1.4m,大潮高水位的水面面积为17km2.  相似文献   
497.
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   
498.
499.
500.
The present paper describes heavy metal contamination of soils and crops resulting from auto traffic, sewage sludge, pig manure and chemical fertilizers, in three separate experiments.The first experiment revealed that although the ADDT (average annual daily traffic) value of the Agricultural Area was lower, higher concentrations of Cu, Cd, Zn, Pb and Mn were obtained in soil when compared with the Commercial-Residential Area. Contaminations of trace metals from various source, e.g., chemical fertilizer, animal manure, night soil and pesticides of the nearby agricultural land were suspected.The second experiment compared the effects of adding a commercial fertilizer and digested sludge which contain substantial amounts of various trace metals to a sandy loam for growing Chinese radish. The results indicated that repeated applications of the fertilizer elevated the contents of various trace metals in the roots of Chinese radish. The phenomenon was further aggravated when digested sludge was also incorporated.The third experiment compared the contents of trace metals in the root and leaf tissues of tree seedlings (Acacia confusa) added with a single dose of the trace metalenriched fertilizer, and different portions of pig manure compost. It was revealed that the fertilizer-treated tree seedlings had a higher content of trace metals in the tissues than those amended with manure compost. Although the metal contents of soil increased as more manure compost was added, the contents in plant tissues did not increase correspondingly.  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