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   29527篇
  免费   1893篇
  国内免费   3128篇
林业   1937篇
农学   1531篇
基础科学   1470篇
  3263篇
综合类   14198篇
农作物   2136篇
水产渔业   1267篇
畜牧兽医   5182篇
园艺   2163篇
植物保护   1401篇
  2024年   206篇
  2023年   644篇
  2022年   1553篇
  2021年   1432篇
  2020年   1336篇
  2019年   1339篇
  2018年   976篇
  2017年   1492篇
  2016年   1010篇
  2015年   1461篇
  2014年   1529篇
  2013年   1899篇
  2012年   2524篇
  2011年   2738篇
  2010年   2462篇
  2009年   2041篇
  2008年   2135篇
  2007年   1951篇
  2006年   1577篇
  2005年   1230篇
  2004年   743篇
  2003年   471篇
  2002年   526篇
  2001年   466篇
  2000年   430篇
  1999年   170篇
  1998年   19篇
  1997年   24篇
  1996年   15篇
  1995年   13篇
  1994年   21篇
  1993年   22篇
  1992年   12篇
  1991年   10篇
  1990年   11篇
  1989年   8篇
  1988年   7篇
  1987年   7篇
  1986年   6篇
  1985年   1篇
  1982年   1篇
  1981年   4篇
  1980年   2篇
  1965年   1篇
  1962年   4篇
  1956年   16篇
  1955年   3篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
101.
Ecological ravages, environmental pollution, and the developmental problems of regional ecology, economy, and society are not geographical restrictions. The synergistic development of the ecology-economy-society system in forestry was quantitatively assessed by system theory and synergistic dynamic models, illustration of the metering principle and corresponding theory, assessing and quantizing synergistic development status of ecology-economy-society system, and the orderliness of the subsystem of the last decade. The orderliness of the ecology and ecological subsystems developed unstably with an increasing trend. The eco-environment construction and eco-conservation achievement of forestry in China are effective and need to be maintained. The industry structure should be improved to promote the growth of the forestry economy. The society subsystem development is increasing but is not at a high level, which indicates that the public demand to ecological service was not satisfied. The theoretical analysis and numerical examples show that the development of the forestry system has a good development state. The synergy changes of forestry system in the 0 to 0.951 range, which is in the intermediate degree of synergy during this period. It still needs much improvement on efficiency of synergy. The orderliness and the relationship among the subsystems of forestry should be improved in future.  相似文献   
102.
火龙果镰刀菌果腐病病原菌鉴定及生物学特性研究   总被引:2,自引:0,他引:2  
采用形态学特征观察、rDNA-ITS序列、EF-1α序列比对和系统发育树分析的方法对引起火龙果镰刀菌果腐病病原菌进行鉴定,并初步研究其生物学特性。结果表明:引起火龙果镰刀菌果腐病是单隔镰刀菌(Fusarium dimerum Penzig in Saccardo),这是该菌引起火龙果果腐病在国内的首次报道。该菌菌丝生长的适宜温度25~35℃,最适温度30℃,适宜产孢温度25~35℃,最适产孢温度30~35℃,致死温度75℃(10 min);适宜pH5~9,最适pH7,最适产孢pH4;连续光照、D-果糖为碳源、牛肉膏和蛋白胨为氮源时最有利于该菌菌丝生长;完全黑暗、D-半乳糖为碳源、尿素为氮源时最有利于该菌产孢。综合分析认为,该菌耐高温,光照充足、弱酸和富含有机营养的环境有利于该菌的生长和繁殖。  相似文献   
103.
旨在揭示大肠埃希菌内毒素(ET)对大鼠小肠黏膜的结构、绒毛长度、上皮内淋巴细胞(IEL)的数量和分布的影响,并探讨多价阳离子A(CA)对上述指标的保护效应。选用72只140g~150g SPF级SD大鼠随机分为3组,即对照组、ET组和CA保护组,经相应处理后分别在3、4、8、12h采集十二指肠、空肠组织作为检测样本,制备病理组织切片,HE染色并利用图像分析系统进行分析。ET组十二指肠和空肠绒毛长度在3、4、8、12h均显著低于对照组和CA保护组(P0.01),ET组十二指肠、空肠IEL数量在3、4、8、12h均显著低于对照组(P0.01);CA保护组十二指肠和空肠IEL数量在4、8、12h均显著高于ET组(P0.01)。结果显示,ET在不同程度上能够破坏小肠黏膜的正常组织结构,降低小肠绒毛长度,减少IEL的数量,从而影响小肠正常的吸收和免疫功能,而CA则能明显降低ET所导致的毒性作用,发挥其保护效应。  相似文献   
104.
随着经济的快速发展,水资源短缺矛盾日益突出,通过对国内外雨洪利用情况进行介绍,着重阐述了坑塘雨洪利用工程及沟道雨洪利用工程,并以房山雨洪利用工程为例,详细介绍了2种工程的设计方法、内容和工艺。  相似文献   
105.
为搞清湿地土壤驱动N2O排放的关键氮源类型,有效减少湿地N2O的排放,本文通过室内控制温湿度,用气相色谱法分析不同外源氮素对湿地N2O排放的影响。结果表明:外加氮源组总是高于对照组N2O排放量(4.4 mg·m-3)。在设定的剂量范围内,单独添加尿素或尿素与硝酸铵1∶1配合时N2O排放量呈现先增后减的单峰分布趋势,峰值分别为10.6 mg·m-3和229.0 mg·m-3;单独添加硝酸铵时N2O排放量(32.6~111.0 mg·m-3)随着氮素添加量增加呈现持续上升趋势。单独添加尿素或硝酸铵、尿素与硝酸铵1∶1配合均促进N2O的排放,但硝酸铵尿素混合添加对N2O排放量的贡献单独添加硝酸铵单独添加尿素。这为预测内蒙古高原区农牧交错带湿地氮素输入可能带来的温室效应和有效减排提供科学依据。  相似文献   
106.
以‘克新1号’马铃薯为试验材料,试验设置低、中、高3个施肥量处理,在复合肥和有机肥的基础上,每个处理中固体胶冻样类芽孢杆菌微生物菌剂设置4个不同的梯度处理,在马铃薯块茎膨大期冲施定量的液体胶冻样类芽孢杆菌微生物菌剂,探究增施胶冻样类芽孢杆菌微生物菌剂对马铃薯生长指标、产量、品质及经济效益等的影响,旨在找出适合蒲县当地的科学施肥方法,为其马铃薯的高产优质生产提供理论和生产实践依据。结果表明,适量增施微生物菌剂有利于提高马铃薯产量、品质和经济效益等指标,但高浓度的微生物菌剂却抑制马铃薯的生长,在本试验中以低肥量处理下的T2组效果最佳,其株高、出苗率、维生素C含量、产量及净利润比常规施肥组分别提高了6.83%、5.31%、92.11%、27.0%和16.94%,还原糖含量降低了18.42%,是高产优质马铃薯的科学施肥配方及标准化生产技术方案。  相似文献   
107.
AIM: To study the effect of remifentanil on monophasic action potential and transmural dispersion of repolarization (TDR) in the 3-layer myocardium of isolated rabbit hearts. METHODS: Adult rabbits (n=18, 2.0 ~ 2.5 kg) were used to isolate the hearts for preparing Langendorff perfusion model. The hearts were randomly divided into 3 groups after perfusion with K-H solution for 15 min: the perfusion in control group (C group) continued for 60 min; the hearts in remifentanil group (R group) were perfused with 12 μg/L remifentanil K-H solution for 60 min; the hearts in remifentanil+aminophylline group (RA group) were given 60-min perfusion of 12 μg/L K-H remifentanil+30 mg/L aminophylline. The HR and 3 layers of myocardial monophasic action potential (MAP) in the left ventricular anterior wall were recorded at time points after balanced infusion for 15 min (T0), and continued perfusion for 15 min (T1), 30 min (T2) and 60 min (T3). The monophasic action potential duration of repolarization at 90% (MAPD90) and the transmural dispersion of repolarization (TDR) were calculated. The early afterdepolarization, delay afterdepolarization and arrhythmia were also observed. RESULTS: In R group, slower HR and prolonger MAPD90 and TDR at T1~T3 were observed as compared with those at T0 (P<0.05). R group showed slower HR and longer MAPD90 and TDR than C group and RA group (P<0.05). CONCLUSION: Remifentanil slows the HR, extends the MAPD90 and increases the TDR, thus being prone to induce reentry. Aminophylline makes HR faster and MAPD90 shorter, thereby reducing the TDR.  相似文献   
108.
低温日灼对核桃腐烂病发生的影响   总被引:1,自引:0,他引:1  
以"辽宁1号"等6个核桃品种为试材,对参试树进行低温日灼及腐烂病发生的方位、病级、发病率等统计分析,研究了核桃腐烂病的发生与低温日灼的关系。结果表明:核桃腐烂病主要发生在成龄结果树枝干南或西南方向,发病处均伴有低温日灼现象;核桃不同品种间抗逆性存在着较大差异,低温日灼发生率低的品种腐烂病发生率也较低,但发生率均与发病程度无必然联系;腐烂病病情与低温日灼伤情密切相关。  相似文献   
109.
AIM: To investigate the effect of paricalcitol (P) on renal tubulointerstitial fibrosis and the underlying mechanisms in diabetic nephropathy (DN).METHODS: DN rat model was induced by a single intraperitoneal injection of streptozotocin after fasting. The animals were randomly divided into 2 groups:the DN rats in paricalcitol-intervened group (group P) were injected intraperitoneally with paricalcitol dissolved in propylene glycol after the day when the model was induced successfully at a dose of 0.4 μg/kg (3 times a week); the DN rats in DN group (group D) were given isopyknic propylene glycol. Normal control group (group C) was also set up. The samples of blood, urine and renal tissue were collected after intervention of paricalcitol for 12 weeks. The biochemical indexes were measured. The renal tissues were used for pathologic observation and determining the expression of transforming growth factor-β1 (TGF-β1), Wnt-4, β-catenin and Klotho by immunohistochemistry and Western blotting. In addition, the correlation among the above indexes was analyzed.RESULTS: (1) Scr, BUN and 24 h urine protein increased significantly in group D compared with group C, while decreased in group P compared with group D (P<0.05). (2) The area of renal tubulointerstitial fibrosis increased in group D compared with group C, while decreased in group P compared with group D (P<0.05). (3) The expression of Klotho decreased, while the expression of TGF-β1, Wnt-4 and β-catenin increased in group D compared with group C (P<0.05). Compared with group D, the expression of Klotho increased, while the expression of TGF-β1, Wnt-4 and β-catenin decreased in group P (P<0.05). (4) The expression of Klotho was negatively correlated with the fibrosis area, TGF-β1, Wnt-4 and β-catenin (P<0.05).CONCLUSION: Paricalcitol inhibits renal tubulointerstitial fibrosis in DN by promoting the expression of renal Klotho, and inhibiting Wnt/β-catenin signaling pathway activation and TGF-β1 synthesis.  相似文献   
110.
肠致病性大肠杆菌DPO-PCR快速检测方法的建立   总被引:1,自引:0,他引:1  
为建立肠致病性大肠杆菌(EPEC)的快速检测方法,本研究以EPEC bfp A基因为靶基因设计了一对双启动寡核苷酸引物(DPO),建立了EPEC的DPO-PCR快速检测方法。结果显示,退火温度在45℃~65℃范围内均能够高效地扩增出目的基因,表明DPO引物对退火温度不敏感。该方法对其他菌株的扩增结果均为阴性,特异性强;其灵敏度为97 cfu/m L。利用该方法和国标法分别对采集的230份临床样品进行检测,均共计检出5份EPEC阳性样品,两者符合率为100%。本研究建立的DPO-PCR方法设计简单、特异性强、灵敏度高,具有良好的实用性,为快速准确检测EPEC提供新的检测手段。  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