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   4026篇
  免费   115篇
  国内免费   194篇
林业   296篇
农学   134篇
基础科学   106篇
  889篇
综合类   2482篇
农作物   40篇
水产渔业   9篇
畜牧兽医   98篇
园艺   119篇
植物保护   162篇
  2024年   8篇
  2023年   35篇
  2022年   46篇
  2021年   56篇
  2020年   53篇
  2019年   65篇
  2018年   48篇
  2017年   173篇
  2016年   177篇
  2015年   191篇
  2014年   206篇
  2013年   270篇
  2012年   315篇
  2011年   416篇
  2010年   378篇
  2009年   336篇
  2008年   307篇
  2007年   324篇
  2006年   260篇
  2005年   144篇
  2004年   88篇
  2003年   93篇
  2002年   59篇
  2001年   41篇
  2000年   26篇
  1999年   30篇
  1998年   29篇
  1997年   20篇
  1996年   20篇
  1995年   10篇
  1994年   23篇
  1993年   39篇
  1992年   21篇
  1991年   11篇
  1990年   7篇
  1989年   5篇
  1988年   3篇
  1987年   1篇
  1985年   1篇
排序方式: 共有4335条查询结果,搜索用时 31 毫秒
51.
刘敏 《安徽农业科学》2012,40(16):9093-9095
分析了国家土地督察制度实施5年来存在的主要问题,主要是:土地督察制度建设的法制化不足;土地督察业务建设不规范;工作机制不完善;土地督察队伍仍需加强。针对这些问题,提出了推进土地督察制度建设的建议:加强土地督察制度的法制化和制度化建设;完善土地督察核心业务建设;完善土地督察工作机制;加强土地督察队伍建设。  相似文献   
52.
在城市建设中,土地的收购、储备、出让行为,对城市建设和发展的影响非常直接,极其关键。因此,要规范土地收储管理,优化土地资源配置。  相似文献   
53.
阿勒泰地区位于北纬44°59′-49°11′和东经85°32′-91°01′,区内分布有高山、丘陵、河谷、平原、沙漠、戈壁和湖泊,土地面积为17750万亩。对该区土地类型进行研究,有助于认识影响土地分异的因素和探讨农业自然分区,为合理利用土地资源,发展农、林、牧业生产提供科学依据。  相似文献   
54.
本文分析了鄂尔多斯西部地区荒漠化的现状及发展过程,划分了轻度、中度、重度和严重荒漠化阶段,并分出极易、较易、较难三种潜在荒漠化等级,分别测算了面积,做出了图表。说明了风蚀与水蚀过程的特征。阐述了自然因素与人为因素在土地荒漠化过程中的交互作用。提出了退耕还牧封沙育草、实行防风蚀耕作、发展灌溉农业、合理轮牧、保护植被等防治荒漠化的有效措施。  相似文献   
55.
Recent research shows that land use history is an important determinant of current ecosystem function. In the United States, characterization of land use change following European settlement requires reconstruction of the original property mosaic. However, this task is difficult in unsystematically surveyed areas east of the Appalachian Mountains. The Gwynns Falls watershed (Baltimore, MD) was originally surveyed in the 1600-1700s under a system of warrants and patents (commonly known as ‘metes and bounds’). A method for the reconstruction and mapping of warrant and patent properties is presented and used to map the original property mosaic in the Gwynns Falls watershed. Using the mapped mosaic, the persistence of properties and property lines in the current Gwynns Falls landscape is considered. The results of this research indicate that as in agricultural areas, the original property lines in the Gwynns Falls watershed are persistent. At the same time, the results suggest that the property mosaic in heavily urbanized/suburbanized areas is generally ‘reset.’ Further, trends in surveying technique, parcel size, and settlement patterns cause property line density and property shape complexity to increase in the less urbanized upper watershed. The persistence of original patterns may be damping expression of heterogeneity gradients in this urban landscape. This spatial pattern of complexity in the original mosaic is directly opposite of hypothesized patterns of landscape heterogeneity arising from urbanization. The technique reported here and the resulting observations are important for landscape pattern studies in areas settled under unsystematic survey systems, especially the heavily urbanized areas of the eastern United States. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   
56.
<正> 全球变暖问题已经是全世界关注的焦点,不仅各国科学家们进行了大量的研究,各国政府也耗费巨资组成了政府间专家组(IPCC,Intergovernmental panel on ClimateChange研究气候变化的影响。IPCC包括3个工作组,Ⅰ.科学分析工作组(Working  相似文献   
57.
<正> The Ordos Plateau is a unified geographical region demarcated by the Yellow River's elbow section on the north and the Great Wall on the south, administratively comprisingthe whole Ih-Ju League of Inner Mongolia, part of Yulin Prefecture of Shaanxi Province,and part of Yanchi County of Ningxia. The unique sandy and denuded landscape of the pla-  相似文献   
58.
针对巢湖市"五早"作物生产的实施成效和面临的形势与问题,从优质高产、市场流通、配套服务等多方面提出了可持续发展的"三大"转变措施.  相似文献   
59.
Improved understandings of the agricultural and range ecologies ofsemi-arid Africa require better information on the spatiotemporal distributionof domestic livestock across agropastoral landscapes. An empirical GIS-basedapproach was developed for estimating distributions of herded livestock acrossthree agropastoral territories (around 100 km2 each)over a two-year period. Algorithms developed from regression analyses of herdtracking data (with R2s 0.67) are used to transform a morecomprehensive but incomplete set of data generated from herders accounts oftheir herds grazing itineraries (400 herds following 6500 itineraries). Theresulting characterization registers 40 000 days of livestock activitiesacross694 land units (averaging 70 ha) over the study period. This studydemonstrates that rural producers knowledge of their daily extractionpracticescan be translated to fine-grained characterizations of extraction densitiesacross mixed landscapes. The spatiotemporal distribution of livestock that isrevealed by this approach diverges strongly from that predicted bycommonly-usedpoint-diffusion estimation procedures. Instead, the distribution reflects localpatterns of land use, topography, vegetation, settlements, and water points.Grazing and nongrazing times spent in land units are not spatially correlatedand the seasonality of grazing pressure is spatially variable. Therefore, theecological impacts of livestock grazing are spatially variable at fine scalesand there is a significant potential for livestock-mediated nutrient transfersacross agropastoral landscapes. The georeferenced data produced by thisapproachnot only will help evaluate the impact and sustainability of differentmanagement practices but also provides a strong empirical base for improvedspatial modeling of herded livestock.This revised version was published online in May 2005 with corrections to the Cover Date.  相似文献   
60.
Introduction   总被引:1,自引:0,他引:1  
Pedroli  Bas  Harms  Bert 《Landscape Ecology》2002,17(1):1-12
The realization of strategies for sustainable land use assumes specificresearch concepts from the local to the global scale (micro-, meso- andmacroscale). Therefore, landscape ecological science has to provideinvestigation methods for all these different scales. By combiningtop-down and bottom-up approaches in addition tocoupled GIS-model applications and traditional methods, the investigation oflandscape ecological structures and processes seems to be possible. Thepresented studies show this approach on examples of two study areas in EasternGermany: A watershed of 400 km2 and an administrativedistrict of about 4000 km2. The scale-specificapplicability of several models and methods were tested for theseinvestigations, and the validation of the calculated results are presented. Animportant outcome of the project should be the prevention of conflicts betweenagriculture, water management and soil, and water and nature conservation;based onrecommendations for land use variants with decreased pollutant loading withinagricultural areas. The scale specific investigations can be considered as abase for establishing sustainable land use.This revised version was published online in May 2005 with corrections to the Cover Date.  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