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   1438篇
  免费   64篇
  国内免费   112篇
林业   187篇
农学   117篇
基础科学   179篇
  170篇
综合类   657篇
农作物   71篇
水产渔业   57篇
畜牧兽医   113篇
园艺   18篇
植物保护   45篇
  2024年   8篇
  2023年   17篇
  2022年   30篇
  2021年   37篇
  2020年   36篇
  2019年   34篇
  2018年   33篇
  2017年   42篇
  2016年   49篇
  2015年   58篇
  2014年   64篇
  2013年   73篇
  2012年   122篇
  2011年   105篇
  2010年   102篇
  2009年   74篇
  2008年   79篇
  2007年   96篇
  2006年   73篇
  2005年   66篇
  2004年   48篇
  2003年   47篇
  2002年   44篇
  2001年   40篇
  2000年   43篇
  1999年   31篇
  1998年   16篇
  1997年   25篇
  1996年   21篇
  1995年   15篇
  1994年   16篇
  1993年   12篇
  1992年   12篇
  1991年   7篇
  1990年   9篇
  1989年   14篇
  1988年   10篇
  1987年   2篇
  1986年   1篇
  1985年   2篇
  1981年   1篇
排序方式: 共有1614条查询结果,搜索用时 15 毫秒
11.
为探索土地利用方式变化对土壤理化性状的影响,以青藏高原青海省海北州原生矮嵩草(Kobresia humil)草甸、原生灌丛草甸、退化矮嵩草草甸、退化灌丛草草甸、人工草地、农田地为研究对象,对各利用方式下土壤的基本理化性状进行研究.结果表明:退化导致0~10 cm土层土壤容重增大,而对下层土壤容重没有明显影响,2种退化草地0~10 cm土层的土壤容重均大于10~20 cm土层,其中退化灌丛草甸与原生灌丛草甸之间差异显著(P<0.05),人工种植也导致表层土壤容重有升高的趋势,人工种草和开垦均导致0~20 cm土层土壤容重的变异消失.草地退化和开垦均导致土壤有机碳和全氮含量降低,草地退化还导致土壤  相似文献   
12.
The heat conduction characteristic of glass curtain wall was investigated. Based on one dimensional steady heat conduction theory, the numerical calculation model of glass system thermal transmittance was set up by using insulating glass as an example. Based on two dimensional steady heat conduction theory and finite element method, both the thermal transfer matrix and temperature load array of 3 DOFs triangular element were derived. Then the revised formulas about the heat transfer matrix and temperature load array under the boundary conditions of constant heat flux, heat convective, radiation and coupled of them were also derived. Then the numerical calculation models of glass curtain wall frame and linear thermal transmittance were set up. The program TJCW based on the Visual C++ and ObjectARX was developed for calculating the thermal transmittance of glass curtain wall and was validated by contrast with the results calculated by thermal software LBNL. Finally, TJCW program was used to calculate and check the thermal transmittance of glass curtain wall in a practical project. The results indicate that the thermal transmittance of glass curtain wall can be calculated correctly by adopting the presented model, and the program based on this model can be used in energy conservation calculation and analysis in the actual projects.  相似文献   
13.
肯定了国营林场营基础地位与作用后,揭示了当前国营林场营林工作所面临的问题主要是营林投资严重不足、林区 生产工人文化素质低、二代林更新困难,最后提出了发展营林生产途径与一些必要措施。  相似文献   
14.
探测树势的电测技术--电容法   总被引:8,自引:0,他引:8  
1994-1999年,对油松、侧柏、桧柏、白皮松、黑松、银杏等树种进行了树林电容测定。结果表明:树体电容参数不仅能准确地反映树木当时的生理状况,而且可将一个抽象的生长势概念变成一个具体数量,来进行数量比较与统计分析,在林业生产管理上有广泛的应用前景。  相似文献   
15.
果树产业是振兴农村经济的主要产业之一。近十几年来 ,山西省果树产业发展迅速 ,但在发展过程中 ,也出现了许多新的问题。为了促进山西省果树事业的发展 ,特提出一些发展建议 ,即优化树种品种结构 ,加快科技推广 ,提高果农素质 ,加速果树产业化进程等。  相似文献   
16.
17.
菌渣添加量对污染土壤Pb、Zn形态和水稻产量的影响   总被引:1,自引:0,他引:1  
通过测定3种菌渣添加量对水稻3个生育期土壤Pb、Zn形态的变化、水稻籽粒Pb、Zn含量及水稻产量的影响,以期得出菌渣的适宜添加量,为重金属污染土壤的修复应用和作物安全生产提供理论依据。结果表明:三种菌渣处理可以降低土壤交换态Pb、Zn含量,且随着生育期逐步减少,提高土壤碳酸盐结合态Pb、Zn含量,以15%菌渣处理的效果最好,其次是10%菌渣处理,再次是5%菌渣处理;三种菌渣添加量可以显著提高水稻的千粒重和产量,但是不随菌渣添加量的增加而提高千粒重和产量,添加10%菌渣处理的千粒重和产量均达最高,其次是15%的菌渣处理;三种菌渣添加量对降低水稻籽粒Pb、Zn含量效果明显,15%菌渣处理的效果最好,10%菌渣处理次之。  相似文献   
18.
Energetic status and body size are inconstant bodily states often considered to have feedback effects on behaviour. Feedbacks can be negative, like starvation‐threshold feedbacks (lower state values = increased risk taking due to higher energy need, and vice versa), or positive, like state‐dependent safety feedbacks (higher state values = lower vulnerability when expressing risky behaviours, and vice versa). Few studies have investigated feedback effects during the early‐life survival bottleneck of territorial species, when rapid growth is particularly important to maintain competitive ability. We investigated effects of body size and energetic status on the emergence latency into an unknown environment in highly territorial brown trout fry. We manipulated energetic status using high and low food rations in different combinations over two consecutive periods. In contrast to results from other small‐sized fishes, no general behavioural effects of energetic state were detected, but larger fish had on average shorter emergence latency. Starvation‐threshold feedbacks were not supported, but the size effect favours the presence of a state‐dependent safety feedback. When individuals were split into clusters characterised by long (>34 s) and short (<34 s) emergence latency, a state‐dependent safety feedback, based on energetic status, was found in the short‐latency cluster. Positive state‐dependent feedbacks may reflect the high level of competition among wild trout fry during the first summer, particularly among the more active individuals (i.e., from the short latency cluster). High state levels could be utilised to gain advantages over lesser competitors (e.g., to establish territories), particularly if the overall risk of starvation is higher than the risk of predation.  相似文献   
19.
水流对杂交鲟幼鱼游泳行为的影响   总被引:3,自引:1,他引:3  
在26℃水温下,使用特制的鱼类游泳行为测定装置研究了杂交鲟(Huso duricusGeorgi♂×Acipenser schrenc-kiBrandt♀)幼鱼在0 m/s、0.1 m/s、0.3 m/s、0.5 m/s 4种流速条件下的游泳行为。结果显示:杂交鲟幼鱼随着流速的增加趋流率增大,且在90min内趋流率达到100%所需的时间明显缩短。实过过程中0.3 m/s和0.5 m/s两组摆尾频率都高于0.1 m/s和静水组,但在各个时段内,摆尾频率与趋流率均没有显著的相关。杂交鲟幼鱼的游泳状态明显受到所处流速的影响:0.1 m/s流速下以逆流前进为主,约占时间比例56.3%;0.3 m/s流速下以逆流静止为主,约占时间比例58.1%;0.5 m/s流速下以逆流后退为主,约占时间比例80.7%。在逆流前进和逆流静止两种游泳状态下,杂交鲟幼鱼的游泳速度和摆尾频率呈线性相关。但在逆流后退和顺流而下两种状态下,两者之间却没有显著的相关,且此两种状态下其游泳速度和摆尾频率随流速增加的变化趋势也不一致。  相似文献   
20.
To investigate the hepatic metabolism of the new insecticide flupyrazofos [O,O-diethyl O-(1-phenyl-3-trifluoromethylpyrazol-5-yl) phosphorothioate], isolated rat liver was perfused with flupyrazofos under single-pass conditions. In outflow perfusate and bile, 1-phenyl-3-trifluoromethyl-5-hydroxyprazole (PTMHP), PTMHP-sulfate and PTMHP-glucuronide conjugates were identified as the metabolites of flupyrazofos. However, O,O-diethyl O-(1-phenyl-3-trifluoromethylpyrazol-5-yl) phosphate (flupyrazofos oxon) was not detected. A HPLC method with UV detection was used to investigate the hepatic disposition of flupyrazofos and its metabolite PTMHP. The concentrations of flupyrazofos, PTMHP and PTMHP conjugates in outflow perfusate reached steady-state levels within 20 min after commencing perfusion of 7.3 microM flupyrazofos. At steady state, the mean extraction ratio of flupyrazofos was 0.93 (+/- 0.01) and clearance was 26.1 (+/- 0.2) ml min-1 which nearly approached perfusate flow rate (28 ml min-1). PTMHP accounted for 55.7 (+/- 5.8)% of eliminated flupyrazofos and was recovered as unchanged PTMHP, PTMHP-sulfate and PTMHP-glucuronide in the bile as well as the outflow perfusate.  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