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   43044篇
  免费   472篇
  国内免费   595篇
安全科学   1365篇
废物处理   1957篇
环保管理   5589篇
综合类   6721篇
基础理论   11400篇
环境理论   21篇
污染及防治   11585篇
评价与监测   2871篇
社会与环境   2319篇
灾害及防治   283篇
  2023年   188篇
  2022年   437篇
  2021年   440篇
  2020年   321篇
  2019年   394篇
  2018年   669篇
  2017年   659篇
  2016年   1036篇
  2015年   812篇
  2014年   1235篇
  2013年   3553篇
  2012年   1484篇
  2011年   1955篇
  2010年   1592篇
  2009年   1674篇
  2008年   1987篇
  2007年   2043篇
  2006年   1772篇
  2005年   1480篇
  2004年   1414篇
  2003年   1498篇
  2002年   1357篇
  2001年   1711篇
  2000年   1214篇
  1999年   731篇
  1998年   522篇
  1997年   513篇
  1996年   554篇
  1995年   616篇
  1994年   610篇
  1993年   520篇
  1992年   517篇
  1991年   481篇
  1990年   532篇
  1989年   462篇
  1988年   418篇
  1987年   394篇
  1986年   335篇
  1985年   356篇
  1984年   395篇
  1983年   383篇
  1982年   370篇
  1981年   352篇
  1980年   284篇
  1979年   321篇
  1978年   269篇
  1977年   201篇
  1975年   215篇
  1972年   212篇
  1971年   186篇
排序方式: 共有10000条查询结果,搜索用时 281 毫秒
501.
502.
503.
504.
505.
506.
507.
508.
The rate of carbon accumulation in the atmosphere can be reduced by decreasing emissions from the burning of fossil fuels and by increasing the net uptake (or reducing the net loss) of carbon in terrestrial (and aquatic) ecosystems. The Kyoto Protocol addresses both the release and uptake of carbon. Canada is developing a National Forest Carbon Monitoring, Accounting and Reporting System in support of its international obligations to report greenhouse gas sources and sinks. This system employs forest-inventory data, growth and yield information, and statistics on natural disturbances, management actions and land-use change to estimate forest carbon stocks, changes in carbon stocks, and emissions of non-CO2 greenhouse gases. A key component of the system is the Carbon Budget Model of the Canadian Forest Sector (CBM-CFS). The model is undergoing extensive revisions to enable analyses at four spatial scales (national, provincial, forest management unit and stand) and in annual time steps. The model and the supporting databases can be used to assess carbon-stock changes between 1990 and the present, and to predict future carbon-stock changes based on scenarios of future disturbance rates and management actions.  相似文献   
509.
Large scale dairy operations are common. In many cases the manure is deposited on a paved surface and then removed with a flushing system, after which the solids are separated, the liquid stored in ponds, and eventually the liquid applied on adjacent crop land. Management of liquid manure to maximize the fertilizer value and minimize water quality degradation requires knowledge of the interactive effects of mineralization of organic N (ON) to NH4+, crop uptake of mineral N, and leaching of NO3 on a temporal basis. The purpose of the research was to use the ENVIRO-GRO model to simulate how the amount of applied N, timing of N application, ON mineralization rates, chemical form of N applied, and irrigation uniformity affected (1) yields of corn (Zea mays) in summer and a forage grass in winter in a Mediterranean climate and (2) the amount of NO3 leached below the root zone. This management practice is typical for dairies in the San Joaquin Valley of California. The simulations were conducted for a 10-year period. Steady state conditions, whereby an equivalent amount of N applied in the organic form will be mineralized in a given year, are achieved more rapidly for materials with high mineralization rates. Both timing and total quantity of N application are important in affecting crop yield and potential N leaching. Major conclusions from the simulations are as follows. Frequent low applications are preferred to less frequent higher applications. Increasing the amount of N application increased both the crop yield and the amount of NO3 leached. Increasing irrigation uniformity increased crop yields but had variable effects on the amount of NO3 leached. A winter forage crop following a summer corn crop effectively reduced the leaching of residual soil N following the corn crop.  相似文献   
510.
We studied the Noel KempffMercado Climate Action Project (NKMCAP),Bolivia, to assess whether forestprotection carbon (C) projects cansignificantly benefit local people. Wehypothesized that forest protection canonly securely deliver C if significantstakeholders are meaningfully andtransparently involved, traditional orcustomary rights are recognized and theirloss compensated for, and there are directlinkages between conservation anddevelopment objective. Our researchfocused on 53 members of the communities ofFlorida, Porvenir and Piso Firme and 36secondary stakeholders. In each of thevillages we held half-day meetings withcommunity leaders, complemented bysemi-structured one-hour interviews with 5,10, and 7 families, representing 20%, 10%and 8% of each community. The long-termimpact of the NKMCAP on the localcommunities may well be positive. However,in the short run, certain sections of thelocal communities are financially poorer. Forest protection projects clearly have thepotential to sequester C, protectbiodiversity and simultaneously contributeto sustainable rural development, but ifthey really are to improve rurallivelihoods, they must be designed andimplemented carefully and participatively.  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