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   38242篇
  免费   2300篇
  国内免费   4058篇
林业   2752篇
农学   4033篇
基础科学   596篇
  4642篇
综合类   17648篇
农作物   2764篇
水产渔业   3240篇
畜牧兽医   5142篇
园艺   1603篇
植物保护   2180篇
  2024年   57篇
  2023年   423篇
  2022年   810篇
  2021年   1018篇
  2020年   1133篇
  2019年   1225篇
  2018年   944篇
  2017年   1612篇
  2016年   2041篇
  2015年   1749篇
  2014年   1362篇
  2013年   2053篇
  2012年   3001篇
  2011年   2910篇
  2010年   2299篇
  2009年   2100篇
  2008年   2377篇
  2007年   2855篇
  2006年   2586篇
  2005年   1948篇
  2004年   1632篇
  2003年   1152篇
  2002年   919篇
  2001年   817篇
  2000年   749篇
  1999年   626篇
  1998年   500篇
  1997年   455篇
  1996年   440篇
  1995年   430篇
  1994年   361篇
  1993年   380篇
  1992年   355篇
  1991年   305篇
  1990年   274篇
  1989年   194篇
  1988年   162篇
  1987年   100篇
  1986年   105篇
  1985年   47篇
  1984年   22篇
  1983年   13篇
  1982年   4篇
  1981年   9篇
  1980年   7篇
  1979年   11篇
  1978年   4篇
  1963年   5篇
  1956年   3篇
  1955年   5篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
Heritage apple (Malus domestica Borkh. hybrids) and pear (Pyrus communis L. hybrid) trees grow in villages throughout Terceira Island, Azores, Portugal. Some of these pears have different names but similar morphology. The objective of this study was to determine synonymy, homology, and phylogeny of apples and pears from Terceira and to examine potential relationships of the island pears with standard apples and pears of Portuguese or American descent. Nine apple microsatellite markers were used to determine genetic relationships. Distance- and parsimony-based cluster analysis grouped these genotypes into separate apple and pear clades. The Terceira apples were divided into two clades: the maçā and the reineta-reinette. Among the 17 heritage apple genotypes, seven unique accessions were identified and four groups of synonyms, or possibly clones, were detected including: ‘Reineta Agosto’ and ‘Reineta Verde’ from Altares; ‘Reineta Castanha’ and ‘Reineta Verde Miuda’; ‘Maçā Pêra,’ ‘Maçā Calhau’, ‘Pêro Branco’ from Salga and from Terra-Chā and ‘Maçā Marmelo’; and the five genotypes ‘Maçā Sao Joao’, ‘Malápio Rosa’, ‘Maçā Gaspar’, ‘Maçā Branca’ and ‘Maçā Pato’. In addition, two homonyms were detected. ‘Pêro Vermelho’ from Terra Chā was a separate genotype from a tree from Doze Ribeiras of the same name, but Pêro Branco from Terra Chā appears to be a clone that can be distinguished by an additional allele at CH1F07a from a tree with that name from Salga. One pair of apple clones, ‘Reineta Agosto’ and ‘Reineta Verde’ from Altares appear to be derived from an unreduced gamete of ‘Golden Delicious.’ Another apple genotype ‘Maçā Acida’ could be a sibling of the ‘Maçā Pêra’ clonal group. Other tested standard apples from the US genebank were unrelated to Terceira genotypes. Of the seven heritage pears, five unique genotypes and one pair of synonyms were detected. ‘Pêra Papo Pintassilgo’ from Raminho and ‘Pêra Vermelha’ from the nursery of Serviço de Desenvolvimento Agario da Terceira (SDAT) were synonyms. ‘Passans du Portugal’ was related to ‘Pêra Cabaca’ but other standard pears from the US genebank were unrelated to Terceira genotypes. Future studies will include additional apple and pear cultivars from other Islands of the Azores and continental Portugal, and wild Asian species to further explore genetic relationships.  相似文献   
992.
实体水-虚拟水"二维三元"耦合流动理论基本框架   总被引:3,自引:1,他引:3  
在变化环境下,区域水资源系统演变规律正在发生深刻变化。解决新形势下出现的水问题单单依靠传统的实体水资源规划、调度和调控手段还不够。虚拟水概念和理论的提出拓展了水文水资源的认知范畴,丰富了水问题的解决手段。然而,如何搭建虚拟水理论与传统水文水资源认知体系的统一框架,认识自然-经济-社会连续系统中水资源演化的新规律,目前学界还没有成熟统一的看法。该文首先提出了实体水-虚拟水耦合流动效应的基本理论框架,指出实体水-虚拟水耦合流动是现代环境下自然-经济-社会水资源系统呈现的新特征。其次,从文明进步和生产力发展角度,论述了人类社会水文水资源系统的演化历史,将水文水资源系统演进大体分为三个阶段:早期的实体水"一维一元"自然循环阶段,近现代的实体水"二元"水循环阶段和当前的实体水-虚拟水"二维三元"耦合流动阶段。最后,该文详细论述了实体水-虚拟水耦合流动过程的路径结构,并针对其流动过程和状态表征提出了定量表达方程,初步构建了实体水-虚拟水耦合流动基本理论框架。  相似文献   
993.
利用1996—2000年统计资料的11项指标,采用典型相关分析(CCA)方法,以中国北方农牧交错带为例,对其农林牧业生产结构类型的空间分异特征进行了分析。首先,确定了以种植业比重、林业比重、牧业比重为自变量组,其余指标为因变量组;其次,利用CANOCO4.5和CANODRAW4.1软件,将研究区划分为偏农、偏林、偏牧、农牧、牧农等5种农林牧业生产结构类型;最后,结合GIS技术,分析研究区农林牧业生产结构的空间分异特征。研究结果表明,由东南向西北,5种类型分布大体为偏农型、农牧型、牧农型、偏林型和偏牧型。  相似文献   
994.
彭倩  周青 《中国农业气象》2007,28(3):285-288
采用水培实验方法研究了稀土元素La(Ⅲ)对紫外辐射(UV-B:0.15Wm-2,0.45Wm-2)胁迫下大豆(Glycine max)幼苗的δ-氨基乙酰丙酸(ALA)、胆色素原(PBG)、原叶绿素(酸)(Pchl)、原卟啉IX(Proto-IX)、镁原卟啉(Mg-Proto)等5种叶绿素(Chl)合成中间产物含量及Chl降解关键酶-叶绿素酶(Chlase)活性的影响,从叶绿素的生物合成及降解角度诠释La(Ⅲ)对UV-B辐射胁迫下大豆幼苗叶绿素含量影响的机理。实验结果表明:La(Ⅲ)具有提高叶绿素生物合成中间产物——ALA(δ-氨基乙酰丙酸)、PBG(胆色素原)、Proto-IX(原卟啉IX)、Mg-Proto(镁原卟啉)含量的作用,促进Pchl(原叶绿素)转化为叶绿素,进而缓解UV-B辐射胁迫下中间产物PBG、Proto-IX和Mg-Proto的形成,使叶绿素的生物合成受阻,受阻位点为ALA→PBG。  相似文献   
995.
近红外光谱技术及其在农产品品质分析中的应用   总被引:1,自引:0,他引:1  
近红外光谱技术是一种高效、快速的现代分析技术,已在很多领域得到广泛应用。文章对近红外光谱分析的技术原理、技术方法、技术特点作了简要介绍,并对其在农产品品质分析中的应用现状和应用前景进行了综述。  相似文献   
996.
RZ-SHAW is a hybrid model, comprised of modules from the Simultaneous Heat and Water (SHAW) model integrated into the Root Zone Water Quality Model (RZWQM) that allows more detailed simulation of different residue types and architectures that affect heat and water transfer at the soil surface. RZ-SHAW allows different methods of surface energy flux evaluation to be used: (1) the SHAW module, where evapotranspiration (ET) and soil heat flux are computed in concert with a detailed surface energy balance; (2) the Shuttleworth–Wallace (S–W) module for ET in which soil surface temperature is assumed equal air temperature; and (3) the PENFLUX module, which uses a Penman transformation for a soil slab under incomplete residue cover. The objective of this study was to compare the predictive accuracy of the three RZ-SHAW modules to simulate effects of residue architecture on net radiation, soil temperature, and water dynamics near the soil surface. The model was tested in Akron, Colorado in a wheat residue-covered (both standing and flat) no-till (NT) plot, and a reduced till (RT) plot where wheat residue was incorporated into the soil. Temperature difference between the soil surface and ambient air frequently exceeded 17 °C under RT and NT conditions, invalidating the isothermal assumption employed in the S–W module. The S–W module overestimated net radiation (Rn) by an average of 69 Wm−2 and underestimated the 3-cm soil temperature (Ts3) by 2.7 °C for the RT plot, attributed to consequences of the isothermal assumption. Both SHAW and PENFLUX modules overestimated midday Ts3 for RT conditions but underestimated Ts3 for NT conditions. Better performances of the SHAW and PENFLUX surface energy evaluations are to be expected as both approaches are more detailed and consider a more discretized domain than the S–W module. PENFLUX simulated net radiation slightly better than the SHAW module for both plots, while Ts3 was simulated the best by SHAW, with a mean bias error of +0.1 °C for NT and +2.7 °C for RT. Simulation results for soil water content in the surface 30 cm (θv30) were mixed. The NT conditions were simulated best by SHAW, with mean bias error for θv30 within 0.006 m3 m−3; RT conditions were simulated best by the PENFLUX module, which was within 0.010 m3 m−3.  相似文献   
997.
Identifying the transformation process of amino acid enantiomers was essential to probe into the fate, turnover and aging of soil nitrogen due to their important roles in the biogeochemical cycling. If this can be achieved by differentiating between the newly biosynthesized and the inherent compounds in soil, then the isotope tracer method can be considered most valid. We thereby developed a gas chromatography/mass spectrometry (GC/MS) method to trace the 15N or 13C isotope incorporation into soil amino acid enantiomers after being incubated with 15NH4+ or U-13C-glucose substrates. The most significant fragments (F) as well as the related minor ions were monitored by the full scan mode and the isotope enrichment in amino acids was estimated by calculating the atom percentage excess (APE). 15NH4+ incorporation was evaluated according to the relative abundance increase of m/z F+1 to F for neutral and acidic amino acids and F+2 to F (mass 439) for lysine. The assessment of 13C enrichment in soil amino acids was more complicated than that of 15N due to multi-carbon atoms in amino acid molecules. The abundance ratio increment of m/z F+n to F (n is the original skeleton carbon number in each fragment) indicated the direct conversion from the added glucose to amino acids, but the total isotope incorporation from the added 13C can only be calculated according to all target isotope fragments, i.e. the abundance ratio increment summation from m/z (Fa+1) through m/z (Fa+T) represented the total incorporation of the added 13C (Fa is the fragment containing all original skeleton carbons and T is the carbon number in the amino acid molecule). This method has a great advantage especially for the evaluation of high-abundance isotope enrichment in organic compounds compared with GC/C/IRMS. And in principle, this technique is also valid for amino acids besides enantiomers if stereoisomers are not concerned. Our assessment approach could shine a light on investigating the biochemical mechanism of microbial transformation of N and C in soils of terrestrial ecosystem.  相似文献   
998.
橡胶油管式车辆行驶称重技术的研究   总被引:1,自引:0,他引:1       下载免费PDF全文
提出了一种橡胶油管的车辆行驶称重技术,并研制了一种便携式车辆行驶称重系统。针对多种因素会对行驶称重系统的输出值产生影响,提出一种与BP神经网络相结合的计算模型,进行数据处理,同时结合信息融合技术建立了车型识别模型及专家系统。经一系列试验证明,该行驶称重系统可实现不同路面的超载检查的要求,为车辆超载检测及交通数据统计提供了一种经济有效的手段。  相似文献   
999.
为了高效去除循环水养殖系统中的固体悬浮颗粒物,研发了多向流重力沉淀装置,并对其水力特性及相关结构参数进行试验研究。该文以固体悬浮颗粒物去除率为试验指标,设计了5~40 min的水力停留时间(hydraulic retention time,HRT)及斜管填料规格参数的双因素试验。结果显示,随着HRT增加,多向流重力沉淀装置的固体悬浮颗粒物去除率呈显著上升的阶段性变化特征,但以HRT为20 min时沉淀装置的综合处理效果最优,最高平均去除率可达(58.57 %±10.12 %),其中粒径小于20 μm的微颗粒去除率为19.5%,粒径大于60 μm的微颗粒去除率高达90.3 %;较小的斜管直径可显著提高多向流重力沉淀装置的固体悬浮颗粒物去除率,而斜管高度对去除率无显著影响。多向流重力沉淀装置用于去除循环水养殖系统中的固体悬浮颗粒物,是一种行之有效的方法。  相似文献   
1000.
水稻产量对长期不同施肥和环境的响应   总被引:1,自引:1,他引:1  
以开始于1982年的不同施肥长期定位试验为平台,选择氮磷钾肥(NPK)、有机肥(牛粪,M)、氮磷钾肥+有机肥(NPKM)、氮磷肥+有机肥(NPM)、氮钾肥+有机肥(NKM)和磷钾肥+有机肥(PKM)共6个处理,采用产量年际变化、变异系数(CV)、产量可持续指数(SYI)、肥料增产贡献率及AMMI模型对影响双季稻稻谷总产量稳定性的施肥处理、环境和二者互作进行综合分析,研究历年稻谷产量(1982—2012年)对长期不同施肥和环境的响应特征。结果表明:NPKM处理稻谷产量最高;在养分投入量相同的情况下,单施有机肥和单施化肥对于早稻产量的影响效果一致,但单施有机肥较单施化肥有利于提高晚稻稻谷产量。不同施肥处理稻谷产量CV随试验时间延长逐渐降低,有机无机配施相比较单施化肥能够减小产量CV,各处理晚稻产量的CV大于早稻,但其SYI值小于早稻。NPKM处理的SYI值最高,为0.51;M和NPK处理的SYI值分别为0.44和0.42。肥料对于产量的贡献率表现为:有机肥化肥氮化肥磷化肥钾。AMMI模型交互效应主成分(IPCA)表明,不同施肥处理在不同试验年份对环境的响应不一样。综合以上分析结果,氮磷钾完全肥基础上配施有机肥(NPKM)是该区域双季稻高产和稳产的最佳施肥措施。  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