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   151596篇
  免费   5253篇
  国内免费   118篇
林业   6225篇
农学   4506篇
基础科学   963篇
  18221篇
综合类   26577篇
农作物   6041篇
水产渔业   7225篇
畜牧兽医   75859篇
园艺   1901篇
植物保护   9449篇
  2019年   1508篇
  2018年   2074篇
  2017年   2377篇
  2016年   2177篇
  2015年   1822篇
  2014年   2362篇
  2013年   5594篇
  2012年   4268篇
  2011年   5321篇
  2010年   3437篇
  2009年   3519篇
  2008年   5176篇
  2007年   4838篇
  2006年   4621篇
  2005年   4296篇
  2004年   4066篇
  2003年   4228篇
  2002年   3883篇
  2001年   4513篇
  2000年   4611篇
  1999年   3624篇
  1998年   1529篇
  1997年   1535篇
  1996年   1390篇
  1995年   1630篇
  1994年   1439篇
  1993年   1432篇
  1992年   2909篇
  1991年   3062篇
  1990年   2921篇
  1989年   2966篇
  1988年   2669篇
  1987年   2696篇
  1986年   2864篇
  1985年   2739篇
  1984年   2206篇
  1983年   1966篇
  1979年   2042篇
  1978年   1666篇
  1977年   1371篇
  1976年   1385篇
  1975年   1439篇
  1974年   1816篇
  1973年   1852篇
  1972年   1752篇
  1971年   1667篇
  1970年   1590篇
  1969年   1621篇
  1968年   1327篇
  1967年   1420篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
871.
872.
The rapid expansion of the world’s urban population is a major driver of contemporary landscape change and ecosystem modification. Urbanisation destroys, degrades and fragments native ecosystems, replacing them with a heterogeneous matrix of urban development, parks, roads, and isolated remnant fragments of varying size and quality. This presents a major challenge for biodiversity conservation within urban areas. To make spatially explicit decisions about urban biodiversity conservation actions, urban planners and managers need to be able to separate the relative influence of landscape composition and configuration from patch and local (site)-scale variables for a range of fauna species. We address this problem using a hierarchical landscape approach for native, terrestrial reptiles and small mammals living in a fragmented semi-urban landscape of Brisbane, Australia. Generalised linear modelling and hierarchical partitioning analysis were applied to quantify the relative influence of landscape composition and configuration, patch size and shape, and local habitat composition and structure on the species’ richness of mammal and reptile assemblages. Landscape structure (composition and configuration) and local-scale habitat structure variables were found to be most important for influencing reptile and mammal assemblages, although the relative importance of specific variables differed between reptile and mammal assemblages. These findings highlight the importance of considering landscape composition and configuration in addition to local habitat elements when planning and/or managing for the conservation of native, terrestrial fauna diversity in urban landscapes.  相似文献   
873.
874.
The genetic diversity of the 34 populations of wild rice Oryza meyeriana Baill.distributed in Yunnan Province,China was analyzed using 13 inter-simple sequence repeat(ISSR)markers.A total of 168 bands were amplified,of which 135 polymorphic bands were discovered and the percentage of polymorphic bands(PPB)was 80.36%.A genetic diversity was revealed as Nei's gene diversity(H)=0.2666 and Shannon information index(η)=0.4028 at population level.The 34 populations were divided into different groups based on administrative regions,latitude and longitudes,river areas,altitudes of their origins,and their indexes such as Na(number of alleles),Ne(effective number of alleles),H,I and PPB were calculated.Richer genetic diversity was found in the wild rice populations distributed in Simao Prefecture than that in Lingcang Prefecture or Xishuangbanna Prefecture whereas the least genetic diversity was in Baoshan Prefecture or Dehong Prefecture.Rich genetic diversity was also discovered in the wild rice populations originated from higher than 710 m altitude around the middle and lower reaches of the Lancang River belonging to the Pacific Ocean drainage system.The 34 populations could be classified into two groups,one group covered the wild rice distributing in Simao Prefecture only while the other group covered ones in Lingcang,Xishuangbanna and Oehong Prefectures.The issue on how to effectively conserve the wild rice germplasm was discussed.  相似文献   
875.
The aim of this research was to optimize mixtures of fibers from different sources and degree of processing meeting acceptable dough viscometric standards to design low‐calorie wheat bread formulations. Effects of soluble (inuline [FN]), partially soluble (sugar beet [FX]), pea cell wall (SW), and insoluble (pea hull [EX]) dietary fibers on wheat dough pasting and gelling profiles have been investigated. Impact of fibers added singly and in associated mixtures at different levels on the investigated viscometric parameters retrieved from a Rapid Visco Analyser curve has been assessed by response surface methodology, and the thermal parameters derived from the cooking and cooling functional profile were correlated. Flour replacement up to 34% by fibers significantly provided a deleterious effect on pasting and gelling viscosity profiles of the resulting hydrated high fiber‐flour blends. The magnitude of the reduction in dough viscometric characteristics during gelatinization, pasting, and setback closely depended on the nature of the fibers in the blend and on the extent of the flour substitution. A delayed and restricted swelling of starch granules and amylose leaching process preferentially achieved by the pair FN‐FX resulted in higher pasting temperatures and reduced peak viscosities during cooking and a sharp decrease of the setback on cooling. Single addition of FX, FN, and EX, respectively, provided a significant decrease in both breakdown viscosity and viscosity at the end of 95°C. Simultaneous presence of FN and EX that exhibit medium or low hydration properties allowed a partial restoration of initial breakdown viscosity and a simultaneous decrease in holding strength. Caution should be paid to the pairs FN‐FX and EX‐SW because of the adverse extra decline they induced in the viscosities of both hot paste and cold gel.  相似文献   
876.
The use of near-infrared spectroscopy (NIRS) for the prediction of whole-grain triticale moisture and protein content was evaluated. Because triticale is genetically close to wheat, commercially available wheat prediction models for Foss Infratec analyzers were applied in a year-by-year basis to triticale samples harvested in Iowa between 2002 and 2006. Wheat models were not applicable to moisture prediction (SEPavg = 0.37% pt; expected SEP on wheat samples 0.15% pt), but usable for screening for protein (SEPavg = 0.38% pt; expected SEP on wheat samples 0.25% pt). Dedicated triticale calibrations were developed from 2002 to 2005 data. Prediction results for 2006 samples only were compared. Triticale calibrations performed better than wheat calibrations for 2006 samples (moisture SEPtriticale = 0.29% pt, SEPwheat = 0.50% pt; protein SEPtriticale = 0.30% pt, SEPwheat = 0.68% pt).  相似文献   
877.
The cotton aphid (Aphis gossypii) is one of the major pests of okra. Eleven okra varieties collected from farmers' fields in Cameroon and four aphid-resistant accessions from The World Vegetable Center (AVRDC, Asian Vegetable Research and Development Center) were evaluated from October 2012 to March 2013, and another seven aphid-resistant accessions from AVRDC were evaluated from March to July 2013 at AVRDC's Nkolbisson station, Yaoundé, Cameroon. Accessions selected from these trials were screened in an advanced confirmatory screening in autumn (October–December) 2013 in Shanhua, Taiwan. Results showed that none of the farmers' landraces were resistant to aphids. Two accessions (VI041210 and VI033824) with the lowest aphid populations had the highest trichome density. Analysis of various phytochemicals in selected okra accessions revealed that higher nitrogen and potassium levels made the okra accessions susceptible to aphids during the reproductive stage of the plants. Similarly, the total phenol content was lower in moderately resistant accessions during their reproductive stage. Aphid feeding induced an increase in phenols, and to a lesser extent, tannins. Accessions VI041210 and VI033824 could be incorporated into an integrated pest management strategy to enhance productivity of okra.  相似文献   
878.
879.
The use of yeast autolysate as a stimulant to accelerate the development of seeds leads to activation of development processes in the seeds and in epiphytic and endophytic microorganisms. The method of seed treatment has a great influence on the direction of action of biofertilizers. A biofertilizer based on brewer’s yeast autolysate (BYA), containing 0.5–1% of live yeast cells, appears to be a promising stimulant of development during presowing seed treatment. A complex biofertilizer based on BYA and containing gibberellins and humates has been developed. Its suspensions have good operational properties and stable effectiveness in application.  相似文献   
880.
The tropical legume sunn hemp (Crotalaria juncea L.) cultivar ‘AU Golden’ has the potential to provide substantial nitrogen (N) to subsequent crops to reduce recommended application rates of synthetic N fertilizers. A mineralization field trial was conducted to measure mass decomposition and N and carbon (C) amounts remaining from sunn hemp residue following three planting dates (May, June, and July) during the 2013 growing season at the Tennessee Valley (TVS) and Coastal Plain (WGS) locations of AL. Residue from June and July plantings contained 50.0% and 61.1% N at WGS and 41.5% and 66.5% N at TVS by the end of their respective incubation periods compared to residue from the May planting, which contained 21.1% N at WGS and 47.8% at TVS. In order to create a more synchronous relationship between ‘AU Golden’ residue N mineralization and crop demand, termination must be delayed until approximate planting of the following crop.  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