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   307篇
  免费   7篇
林业   42篇
农学   7篇
基础科学   2篇
  116篇
综合类   31篇
农作物   9篇
水产渔业   13篇
畜牧兽医   75篇
园艺   11篇
植物保护   8篇
  2023年   1篇
  2022年   1篇
  2021年   1篇
  2020年   3篇
  2019年   3篇
  2018年   7篇
  2017年   4篇
  2016年   3篇
  2015年   3篇
  2014年   4篇
  2013年   22篇
  2012年   14篇
  2011年   11篇
  2010年   13篇
  2009年   15篇
  2008年   27篇
  2007年   21篇
  2006年   18篇
  2005年   25篇
  2004年   16篇
  2003年   17篇
  2002年   19篇
  2001年   7篇
  2000年   4篇
  1999年   5篇
  1998年   3篇
  1997年   3篇
  1996年   4篇
  1995年   2篇
  1994年   2篇
  1993年   6篇
  1992年   1篇
  1991年   4篇
  1990年   3篇
  1989年   3篇
  1988年   1篇
  1985年   1篇
  1984年   2篇
  1982年   1篇
  1981年   3篇
  1979年   1篇
  1978年   1篇
  1977年   2篇
  1976年   1篇
  1975年   2篇
  1972年   1篇
  1965年   2篇
  1957年   1篇
排序方式: 共有314条查询结果,搜索用时 31 毫秒
71.
72.
The potential for development in decision support for forest management is set by decision theory, available technology and methods. Demands for decision support are emerging from contemporary challenges and problems of forest management which act as stimuli for the science community. Objectives and approaches in forest management as well as technologies have been changing throughout history. Accordingly, the demand for tools to support planning and decision-making has evolved. In this contribution, the authors review the historic development of decision support systems (DSS) for forest management and discuss past, current and future drivers. Based on evidence from scientific literature, case studies in the frame of the Forest Management Decision Support Systems (FORSYS) action, as well as experiences of the authors some hypotheses about the future of DSS are drawn. It is shown that in the past, the drivers evolving from forest management as well as decision support technologies have influenced the way of how models and methods have been applied as well as how DSS architectures have been designed. It is concluded that in the future, the challenges for DSS development will increase, as the complexity of decision-making processes and the related models will compete with the user demands which ask for simplicity.  相似文献   
73.
A high-resolution oxygen-isotope record from a thorium-uranium-dated stalagmite from southern Oman reflects variations in the amount of monsoon precipitation for the periods from 10.3 to 2.7 and 1.4 to 0.4 thousand years before the present (ky B.P.). Between 10.3 and 8 ky B.P., decadal to centennial variations in monsoon precipitation are in phase with temperature fluctuations recorded in Greenland ice cores, indicating that early Holocene monsoon intensity is largely controlled by glacial boundary conditions. After approximately 8 ky B.P., monsoon precipitation decreases gradually in response to changing Northern Hemisphere summer solar insolation, with decadal to multidecadal variations in monsoon precipitation being linked to solar activity.  相似文献   
74.
The naturally occurring betulinic acid (BA) and its derivative NVX‐207 show anticancer effects against equine malignant melanoma (EMM) cells and a potent permeation in isolated equine skin in vitro. The aim of the study was to determine the in vivo concentration profiles of BA and NVX‐207 in equine skin and assess the compounds’ local and systemic tolerability with the intent of developing a topical therapy against EMM. Eight horses were treated percutaneously in a crossover design with 1% BA, 1% NVX‐207 or a placebo in a respective vehicle twice a day for seven consecutive days with a seven‐day washout period between each formulation. Horses were treated at the neck and underneath the tail. Concentration profiles of the compounds were assessed by high‐performance liquid chromatography in the cervical skin. Clinical and histopathological examinations and blood analyses were performed. Higher concentrations of NVX‐207 were found in the skin compared to BA. Good systemic tolerability and only mild local adverse effects were observed in all three groups. This study substantiates the topical application of BA and NVX‐207 in further clinical trials with horses suffering from EMM; however, penetration and permeation of the compounds may be altered in skin affected by tumors.  相似文献   
75.
76.
Potassium (K) delivery to the roots of cereal plants in soils is determined by both soil and plant factors, root growth and root length density in particular, being of major importance. In Part A of this paper an evaluative framework is shown for sandy and loamy-silty soils for determination of root length density based on available field capacity, soil depth in 10 cm steps and the climatic water balance. A good correlation (r2 = 0.86) was found when the root length densities were determined at ear emergence. With the help of correction functions root length densities can be subsequently estimated for the other phenological stages (stem elongation, flowering and yellow ripeness). The method described is suited for determining root length densities in water and nutrient transport simulation models. In Part B exchange resins were used to simulate the K delivery from the soil to the root. Measurements were made for different soils and water contents. The depletion zone near the exchange resin was 6 mm thick after 4 days. This corresponds to results obtained from root measurements. Comparisons between a sandy and a loamy-silty soil show, that K delivery by diffusion for equal concentrations depends primarily on soil moisture and pore tortuosity. This leads to the conclusion, that soil physical properties must be taken into consideration when evaluating chemical soil analysis and recommendating fertilizer applications.  相似文献   
77.
78.
Water vapor adsorption isotherms were used for estimation of (apparent) surface areas of samples of limed and unlimed plots of an acidic sandy forest soil. Samples were taken at two microrelief (ridge and furrow) positions from five subsequent 10 cm layers. Values of surface area and CEC correlated linearly with organic matter content but only for four bottom layers. Surface areas and CEC values calculated per the unit mass of organic carbon were higher in upper layers than in lower layers for control samples. For limed samples an opposite trend was observed. The estimated average (apparent) charge densities of organic matter showed a better correlation with humic to fulvic acids ratio. Values of surface charge densities for every investigated profile increased with depth and they were lower in limed than in unlimed profiles.  相似文献   
79.
In a 40 year old pine stand, soil solution was collected continuously with suction cups from 50 and 200 cm depth over a 42-month period following the surface application of lime and K/Mg-fertilizer. K and Mg showed a much higher mobility than Ca, which hardly increased in 200 cm depth even after 42 months. After an initial nitrate peak, concentrations decreased but remained elevated throughout the entire study period. In 200 cm, Al-, Mn, and Cd-concentrations increased due to exchange processes in the top-soil. Continuously elevated sulfate concentrations in 200 cm indicate S-mineralisation from humus decomposition.  相似文献   
80.
Phytoestrogens of the lignan type are widely distributed in plant-derived food items and are believed to protect against hormone-dependent cancer. The richest known dietary source of lignans is flaxseed. Flaxseed has been reported to contain glycosides of secoisolariciresinol as the major lignan, together with small amounts of matairesinol, isolariciresinol, and pinoresinol. Secoisolariciresinol, but none of the other lignans, has so far been identified in pumpkin seeds. In the present study, two different methods for the hydrolysis of lignan glycosides are compared. Artifact formation and loss of lignans under acidic conditions were observed. Lariciresinol was identified by GC-MS analysis in two different types of flaxseed (Linum usitatissimum L. and Linum flavum L.) and in pumpkin seeds (Cucurbita pepo L.) for the first time. Likewise, the novel lignan demethoxy-secoisolariciresinol was tentatively identified in the flaxseed samples. Stereochemical analysis by chiral HPLC of several lignans isolated from flaxseed showed that secoisolariciresinol, matairesinol, and lariciresinol consisted predominantly of one enantiomer.  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