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   17008篇
  免费   48篇
  国内免费   59篇
林业   3684篇
农学   1337篇
基础科学   149篇
  2816篇
综合类   795篇
农作物   2144篇
水产渔业   1890篇
畜牧兽医   1250篇
园艺   1128篇
植物保护   1922篇
  2024年   1篇
  2023年   11篇
  2022年   16篇
  2021年   29篇
  2020年   18篇
  2019年   16篇
  2018年   2759篇
  2017年   2724篇
  2016年   1200篇
  2015年   76篇
  2014年   40篇
  2013年   56篇
  2012年   829篇
  2011年   2161篇
  2010年   2130篇
  2009年   1278篇
  2008年   1349篇
  2007年   1606篇
  2006年   49篇
  2005年   116篇
  2004年   117篇
  2003年   192篇
  2002年   121篇
  2001年   57篇
  2000年   75篇
  1999年   14篇
  1998年   7篇
  1997年   4篇
  1996年   3篇
  1995年   3篇
  1993年   12篇
  1992年   8篇
  1990年   1篇
  1989年   8篇
  1988年   13篇
  1987年   1篇
  1982年   1篇
  1978年   1篇
  1977年   4篇
  1975年   1篇
  1972年   1篇
  1969年   1篇
  1968年   4篇
  1967年   1篇
  1965年   1篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
42.
Decision support systems (DSSs) are indispensable tools in preparing a forest management plan for a better combination of multiple forest values. This study attempted to develop and explain a stand-based forest management DSS (Ecosystem-based multiple-use forest planning [ETÇAP]) comprising a traditional simulation, linear programming (LP), metaheuristics and geographic information system. The model consists of five submodels; traditional management approach to handle inventory data, an empirical growth and yield model, a simulation to conceptualize management actions, a LP technique to optimize resource allocation and a simulated annealing approach to directly create a spatially feasible harvest schedule. The ETÇAP model has been implemented in a comparative two case study areas; Denizli–Honaz and Akseki–Ibrad?. Both simulation and optimization models outperformed to the traditional management plan. The periodical change of growing stock, allowable cuts, carbon sequestration and water production are used as performance indicators. The results showed that more amount of wood could be harvested over time compared to traditional level of harvesting. It could be concluded that various management strategies allowed managers to stimulate more decision options for better outputs through intertemporal trade-offs of management interventions as the model provided tools to quantify forest dynamics over time and space. Challenges exist to establish the functional relationships between forest structure and values for better quantification and integration into the management plans.  相似文献   
43.
Symbiotic N2 fixation by lucerne (Medicago sativa) has capacity to provide significant inputs of N to agro-ecosystems, and the species has also been shown to scavenge soil mineral N and thus act as a sink for excess reactive N. The balance between these two N cycle processes was investigated in an extensive irrigated lucerne growing region where nitrate contamination of groundwater has been reported. We sampled 18 permanent pure lucerne stands under irrigation for standing dry matter, total shoot N, and N2 fixation using 15N natural abundance along with activity of the inducible enzyme nitrate reductase as indicators of use of soil NO3 by lucerne. On average 65% of lucerne N was obtained from symbiotic N2 fixation. Converting standing dry matter estimates to annual N2 fixation amounts we calculated average N2 fixation of 311 kg N/ha, including N in roots and nodules. Uptake of N from soil by lucerne was calculated to be 181 kg N/ha/year. We were not able to identify the source of this soil mineral N, although nitrate reductase activity of lucerne was higher than that of non-N2 fixing species examined.  相似文献   
44.
45.
46.
Umma Tamima 《林业研究》2016,27(4):853-861
The objective was to evaluate the performance of the co-management of Nishorgo Support Project at Chunati Wildlife Sanctuary in Bangladesh. I adopted the Focus Groups Discussion method for opinion survey and applied the SWOTAHP technique for data analysis. Local people did not participate in the decision-making process of forest management and they perceived co-management as a threat to their livelihoods. Moreover, mistrust and misunderstanding among different stakeholders, political and ethical conflicts, lack of property rights of tribal communities, and lack of accountability and transparency were identified as the drawbacks of co-management.  相似文献   
47.
48.
49.
50.
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