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   8374篇
  免费   541篇
  国内免费   764篇
林业   1471篇
农学   1455篇
基础科学   362篇
  672篇
综合类   3069篇
农作物   614篇
水产渔业   352篇
畜牧兽医   1098篇
园艺   292篇
植物保护   294篇
  2024年   46篇
  2023年   134篇
  2022年   193篇
  2021年   216篇
  2020年   249篇
  2019年   284篇
  2018年   170篇
  2017年   254篇
  2016年   279篇
  2015年   271篇
  2014年   418篇
  2013年   418篇
  2012年   642篇
  2011年   603篇
  2010年   553篇
  2009年   509篇
  2008年   481篇
  2007年   508篇
  2006年   427篇
  2005年   374篇
  2004年   326篇
  2003年   282篇
  2002年   213篇
  2001年   180篇
  2000年   175篇
  1999年   150篇
  1998年   136篇
  1997年   149篇
  1996年   128篇
  1995年   124篇
  1994年   117篇
  1993年   102篇
  1992年   103篇
  1991年   101篇
  1990年   88篇
  1989年   57篇
  1988年   55篇
  1987年   40篇
  1986年   30篇
  1985年   15篇
  1984年   15篇
  1983年   3篇
  1982年   12篇
  1981年   8篇
  1980年   7篇
  1979年   9篇
  1978年   9篇
  1977年   5篇
  1976年   5篇
  1956年   3篇
排序方式: 共有9679条查询结果,搜索用时 12 毫秒
971.
结合当前国家对于改善城市水生态环境的指导策略和因地制宜的生态修复策略,从单一的纯林修复到复合型山体生态修复,城市山体修复不仅应满足生态适宜性,同时还应注重城市景观的多样性。根据山体的现状做出综合的立地条件分析,对乡土植物和适生植物全面了解,选择出适用于本城市山体修复的植物种类,同时结合最新的理念和工程技术,达到好看、好建、好管、安全的城市山体修复的目标,最终探寻出对于城市山体修复的普适性办法。  相似文献   
972.
Current approaches for rapid assessment of carbon source utilization by whole soil communities (i.e., community-level physiological profiling or CLPP) provide a limited, biased view of microbial communities with little connection to in situ activities. We developed an alternative CLPP approach based upon fluorometric detection of dissolved oxygen consumption in a microtiter platform which offers flexible manipulation of experimental factors. In the attempt to reduce oxygen re-dissolution, the wells were filled with liquid to very near the top and sealed. We found that filling the wells with 240 vs. 150 μl of sample improved the sensitivity of the system to discern both the response to a substrate amendment as low as 10 mg l−1 and un-amended, endogenous respiration. The preparation of a soil slurry facilitates inoculation into the microplate. Disruption of soil samples had a limited effect on the endogenous respiration in comparison to intact soil microbags in a 24-well microplate. Storage time (up to 33 days) reduced the level of activity in intact soil microbags but not in disrupted samples. A microcosm fertilization experiment was set to study the effects of N availability on the respiratory response in the plates. The use of soil organic carbon (SOC) and amended C-substrates (50 mg l−1) was increased by the addition of nitrogen (N) in the plate, and appeared N-limited shortly after microcosm fertilization. The addition of the eukaryotic inhibitor cycloheximide delayed the initial increase in fluorescence (time to minimum response) of several C sources (casein, acetate, asparagine, coumaric acid), varying among soils, which could be explained by the fungal use of these compounds. However, the extent of the inhibition caused by cycloheximide did not increase at higher fungal to bacteria ratios as estimated by PLFA analysis, indicating that the direct estimation of the fungal biomass from cycloheximide addition is not feasible. This paper provides an optimized, standardized protocol for soil analysis, and sets the basis for further validation studies that will continue to define the underlying capabilities/biases of this approach.  相似文献   
973.
随着虚拟实验技术的飞速发展和普遍应用,利用虚拟实验技术进行实验教学已成为一种趋势。本文从分析虚拟实验技术的理论基础出发,阐述了虚拟实验技术的分类,论述了它在实验教学中的重要意义,并对虚拟实验技术在实验教学中的应用进行了探索。  相似文献   
974.
针对传统玉米种子活力等级分类方法耗时长、环境要求严格、对种子产生损伤等问题,利用红外热成像技术结合SVM算法,建立了快速、无损、高效的玉米种子活力等级分类方法。首先采用人工老化的方法将1 200粒玉米种子分组分别老化0 h,72 h,144 h。利用不同老化时间玉米种子具有不同的生理特性,通过红外热成像仪采集温度胁迫后自然冷却的玉米种子红外热像图,提取温度值作为特征。随后对玉米种子进行标准萌发实验,根据实验结果,将玉米种子分为高活力,中活力和低活力3个活力等级。将温度值作为特征,活力等级作为标签分别建立K最近邻(KNN)和支持向量机(SVM)模型并进行训练,以模型分类准确率和训练时间作为评价指标,确定较佳模型,最终通过网格搜索对选择的模型参数进行优化。结果表明基于红外热成像技术结合支持向量机(SVM)建立的模型,训练集准确率达到了92.4%,测试集准确率为91%,训练用时0.12s。该模型经过优化后训练集准确率达到了97.1%,测试集准确率达到了96.5%。  相似文献   
975.
为选育柔嫩艾美耳球虫的早熟株和了解其免疫原性,本试验通过鸡体的传代培养,初步得到了柔嫩艾美耳球虫早熟株P9。以早熟株免疫雏鸡后,以亲本株P0半数致死量攻毒,以确定其免疫原性。结果表明:P9的潜隐期由P0的141h缩短至131.5h,免疫后攻毒显示该早熟株具有良好的免疫原性。  相似文献   
976.
This case report describes tuberculosis (TB) due to infection with Mycobacterium bovis (M. bovis) in alpaca (Lama pacos) on a farm in Ireland. Two severely debilitated alpaca were presented to the University Veterinary Hospital, University College Dublin in November 2004. Bloods were taken, and haematology and biochemistry results were indicative of chronic infection. Radiological examination showed evidence of diffuse granulomatous pneumonia suggestive of tuberculosis. On necropsy there were granulomatous lesions present throughout many body organs including lung, liver, kidney, intestine as well on peritoneum and mesentery. Culture of acid-fast bacilli from lesions led to a diagnosis of tuberculosis due to M. bovis. The use of intradermal skin testing proved inefficient and unreliable for ante mortem diagnosis of tuberculosis in alpaca. Infection due to M. bovis should be considered among the differential diagnoses of debilitating diseases in alpaca, particularly those farmed in areas known to be traditional black spots for tuberculosis in cattle.  相似文献   
977.
BACKGROUND: Famotidine administered IV has been associated anecdotally with hemolysis in cats, and some veterinarians recommend using injectable famotidine only by the subcutaneous (SC) route for cats. However, the actual risk of such a reaction is not known. HYPOTHESIS: We hypothesized that famotidine, when given IV slowly, would not be associated with a clinically significant drop in packed cell volume (PCV) in hospitalized cats. ANIMALS: One hundred and forty-two hospitalized cats. METHODS: A retrospective medical record review was performed for hospitalized cats prescribed famotidine IV (n = 56), famotidine SC (n = 48), or no famotidine (n = 38) at a veterinary medical teaching hospital over the period from January 2004 through December 2006. RESULTS: Baseline signalment, observation times, and famotidine dosage (in treated cats) were similar among groups. Median baseline PCVs were significantly lower in the IV (31.5%) and SC (32.0%) groups compared with the control group (35.0%; P= .04). The median percent drop in PCV (3-4%), however, was no different in cats that received famotidine by either route compared with the control group (P= .90), and no cats in either famotidine group were observed to have any clinical signs of hemolysis. CONCLUSIONS AND CLINICAL IMPORTANCE: We conclude from this retrospective study that famotidine IV was given to 56 hospitalized cats without evidence of hemolysis, and that the IV route appeared safe when famotidine was administered over 5 minutes. We could not document a safety advantage of SC versus IV administration in this group of cats.  相似文献   
978.
对青南地区玉树县境内2牧场的草地植被及成年母牦牛的采食行为和选择性采食习性及其关系进行了探讨,结果表明:B牧场禁牧区的生物量显著高于A牧场的禁牧区及A,B牧场的放牧区(P〈0.05),B牧场的可食植物量显著高于A牧场(P〈0.05),A牧场的可食植物量只占地上生物量的一半以下;B牧场分别以小嵩草,矮嵩草,披碱草,针茅,蒲公英等优良牧草为优势种,次优势种,而A牧场以线叶嵩草为优势种,凤毛菊,狼毒,黄帚橐吾等毒杂草及早熟禾等为次优势种;B牧场以小嵩草、矮嵩草等莎草科和为主要采食植物,而在A牧场除以莎草科植物为主要采食植物外,由于可食牧草量的不足,还采食一些本不喜食的植物。矮嵩草和小嵩草是牦牛最喜食,选择性最高的植物;斑块内采食口数A牧场为50.6 bite/patch显著少于B牧场的81.1 bite/patch(P〈0.01);斑块内采食站数A牧场为8.4 FS/patch显著多于B牧场的5.2 FS/patch(P〈0.001);采食站内采食口数A牧场为6.6 bite/FS显著少于B牧场的17.4 bite/FS(P〈0.001);采食速度A牧场为1.32 bites/sec显著慢于B牧场的1.46 bites/sec(P〈0.01)。放牧草地的食物资源的匮乏,使放牧牦牛的食物选择性降低,采食行为的内容变劣,A牧场草地处于严重退化状态;牦牛处于缺营养状态,草地的退化对牦牛的生产性能有很大的影响。  相似文献   
979.
980.
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