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   7315篇
  免费   445篇
  国内免费   585篇
林业   537篇
农学   360篇
基础科学   327篇
  609篇
综合类   3710篇
农作物   535篇
水产渔业   296篇
畜牧兽医   1099篇
园艺   553篇
植物保护   319篇
  2024年   29篇
  2023年   98篇
  2022年   298篇
  2021年   292篇
  2020年   242篇
  2019年   275篇
  2018年   186篇
  2017年   360篇
  2016年   209篇
  2015年   333篇
  2014年   359篇
  2013年   468篇
  2012年   642篇
  2011年   646篇
  2010年   622篇
  2009年   592篇
  2008年   570篇
  2007年   530篇
  2006年   449篇
  2005年   327篇
  2004年   220篇
  2003年   122篇
  2002年   139篇
  2001年   141篇
  2000年   117篇
  1999年   40篇
  1998年   3篇
  1997年   2篇
  1996年   3篇
  1995年   1篇
  1994年   2篇
  1993年   4篇
  1992年   2篇
  1991年   4篇
  1990年   2篇
  1989年   1篇
  1988年   1篇
  1987年   1篇
  1986年   2篇
  1985年   1篇
  1981年   2篇
  1979年   1篇
  1965年   1篇
  1962年   3篇
  1956年   3篇
排序方式: 共有8345条查询结果,搜索用时 296 毫秒
101.
以新疆伊犁新源县的4科10种药用植物为试材,采用植物染色体标本制备的去壁低渗方法,研究了染色体数目并进行了核型分析。结果表明:大麻(Cannabis sativa L.)2n=20=12m+8sm、苦豆子(Sophora alopecuroides L.)2n=36=26m+8sm+2st、毛牛蒡(Arctium tomentosum Mill.)2n=36=6m+28sm+2st、牛蒡(Arctium lappaL.)2n=36=8m+26sm+2st、千叶蓍(Achillea millefolium L.)2n=36=26m+10sm、牛至(Origanum valgare)2n=30=12m+18sm、菊苣(Cichorium intybus L.)2n=18=12m+6sm、新疆鼠尾草(Salvia deserta Schang)2n=14=4m+10sm、草原糙苏(Phomis protensis Kar)2n=22=12m+6sm+4st、乌拉尔甘草(Glycyrrhiza uralensis Fish)2n=16=6m+10sm,其染色体数目与前人报道一致。千叶蓍(Achillea millefolium L.)、牛至(Origanum valgare)、新疆鼠尾草(Salvia deserta Schang)3种植物为首次报道。  相似文献   
102.
通过3年来对朝北的山坡或山岙地的笋竹两用林林分采取增加竹林密度和选粗壮的竹笋留养新竹的技术培育毛竹大径竹材,结果表明:新竹平均胸径达11.76 cm,比实施前提高了0.93 cm;胸径≥12 cm立竹株数达55.8%,比实施前提高了近18百分点;林分立竹竹材产量比实施前增加34.3%;胸径≥12 cm立竹竹材产量提高了14.8%;每度新竹立竹产值比实施前增加36.6%.  相似文献   
103.
王建  裘敏浩 《湖南农机》2012,(11):130-131
在华中世纪星数控车床上用G71粗车复合循环程序加工凸凹件的配合过程中,因凸凹配合件尺寸过大或者过小,需要再加工来满足配合要求,此时若屏蔽G71粗车复合循环程序段,之后加工精加工程序段就能满足配合要求,这样就能减少空运行加工时间,从而大大提高工件加工效率。  相似文献   
104.
Water and Energy Consumption by Agriculture in the Minqin Oasis Region   总被引:1,自引:0,他引:1  
Water used in agriculture consumes much energy, mainly due to pumping water for irrigation, but the water-energy nexus is always neglected in arid and semi-arid areas. Based on hydrological observation data, irrigation data and socio- economic data over the past 50 yr, this study has derived a detailed estimate of greenhouse gas (GHG) emissions from agricultural water use in the Minqin Oasis. Results show that the decreasing water supply and increasing demand for agriculture has caused severe water deficits over the past 50 yr in this region. The groundwater energy use rate rose by 76% between 1961 and 2009 because of the serious decline in groundwater levels. An increase in pump lift by an average 1 m would cause GHG emission rates to rise by around 2%. Over the past 10 yr, the GHG emissions from groundwater accounted for 65-88% of the total emissions from agricultural water. GHG emissions for diverted water varied from 0.047 to 0.074 Mt CO 2 e as the water input increased. Long distance conveyance and high pump lifts need more electricity input than groundwater abstraction does. Government policies have had a favorable effect on total emissions by reducing water abstraction. But groundwater depletion, exacerbated by a growing population and an expansion in arable land, remains the principal energy-water nexus challenge in the region. In response to the increasing water-energy crisis, energy-saving irrigation technology, matching to cost efficiencies, and better coordination between different infrastructural agencies could be feasible ways of rendering the water and energy sectors more sustainable over the long term.  相似文献   
105.
Although water and energy resources are well-recognized concerns regarding economic and social development sustainability, little specific research has focused on both water and energy problems at the same time. This study analyzed the water and electricity-use patterns in Shenzhen, South China during 2001-2009. A curve regression method was used to examine the relationship between water and electricity use per gross domestic product (GDP) in Shenzhen and its three sectors, i.e., agriculture, industry & construction, and residential life & services. Results showed that agriculture only covered less than 10% of water and electricity use in Shenzhen, while industry & construction and residential life & services accounted for more than 90% of water and electricity use in Shenzhen, which coincided with the city’s industrial structure. The water and electricity use per GDP in agriculture was the biggest among three sectors in Shenzhen during 2001-2009, which means inefficiency of water and electricity use in agriculture. Due to transitioning to advanced materials and manufacturing, both water and electricity use per GDP in industry & construction decreased during 2001-2009 and their utilization efficiencies gradually increased over time. The same held true for those in residential life & services transformed toward modern business, creative culture, finance services, etc. Derived from the survival of the fittest in competing for limited water and electricity resources, agriculture in Shenzhen has been gradually substituted by industry & construction and residential life & services, with much higher efficiencies of water and electricity use. And traditional agriculture will not be sustainable in the process of urbanization and industrialization, except high-tech intensive agriculture with low water and energy cost. Furthermore, by means of curve regression, we found that there was a significant quadratic relationship between water use per GDP and electricity use per GDP in the entire city and its three sectors. Suitable industrial transformation and advancement was a very effective way to save water and energy for modern cities. This can provide some reference for systematic planning and design of water and electricity allocation and use in agriculture, industry & construction and residential life & services in a city.  相似文献   
106.
花生壳残渣制备活性炭及吸附性能测定   总被引:1,自引:0,他引:1  
杨莉  谢宇  邱贤华  邓磊 《花生学报》2010,39(2):16-20
用提取黄酮后废弃的花生壳做原料,选择不同的活化剂在一定温度下制备活性炭,并且测定其吸附性能。结果表明:磷酸作为活化剂时活性炭产率最高,达39.5%;当炭化温度为500℃、活化剂为氢氧化钾或磷酸、活化剂浓度为10%时,碘吸附值最高,为966.7mg/g;当炭化温度为500℃、活化剂浓度为10%时,几种活化剂制备出来的活性炭亚甲基蓝脱色力均达到40mL/g左右;与几种市售活性炭比较,花生壳活性炭碘吸附值能够满足市场需要,但是亚甲基蓝脱色力偏低;相同条件下,盐作为活化剂所制备出的活性炭对镍离子的吸附能力比较稳定。  相似文献   
107.
建立转基因奶牛多重PCR快速检测方法,为转基因动物及产品进出境监测技术平台的建立提供技术支持,并为转基因动物及产品检测技术标准的制定提供参考。根据转基因奶牛内源基因β2-微球蛋白基因(β2-microglobulin,B2 M),外源基因人血清白蛋白基因(Human α-lactalbumin,α-LA)及标记基因绿色荧光蛋白基因(EGFP)和新霉素磷酸转移酶基因(NPTII)设计特异性引物,优化反应条件,建立转基因奶牛多重PCR检测方法。该方法敏感、快速、特异,一个反应可以检测多个基因片段,可有效用于转基因奶牛外源基因的检测。  相似文献   
108.
莆田老鹰尖自然保护区野生食用菌资源调查   总被引:2,自引:0,他引:2  
首次对莆田老鹰尖自然保护区的野生食用菌资源进行实地调查、收集、鉴定,得出该区有22科、95种野生食用菌资源。  相似文献   
109.
对云南蓝果树的种子采集处理、育苗试验和对苗木生长观测调查,初步研究了云南蓝果树的育苗技术、苗期生长规律。结果表明:云南蓝果树种子无休眼期,宜随采随播,播种1周后开始萌动,25 d出土,种子贮藏方式以混沙贮藏为好。培育的云南蓝果树容器苗210 d即可出圃造林,1 a生苗高和地径的生长节律呈现出"慢-快-慢"的生长过程,其幼苗的生长高峰为6月~7月,这期间应加强水肥管理,促进苗木生长。  相似文献   
110.
采访是图书馆的基础性工作,它关系到整个图书馆藏书的质量和专业水平。为此,图书馆采访馆员担负着重要职责,如何将采访馆员的基本素质在工作范畴中得以体现并发挥其优势,本文将对此与同仁做出探讨。  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