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   16433篇
  免费   0篇
林业   3634篇
农学   1294篇
基础科学   137篇
  2733篇
综合类   717篇
农作物   2095篇
水产渔业   1778篇
畜牧兽医   1064篇
园艺   1111篇
植物保护   1870篇
  2023年   1篇
  2022年   2篇
  2021年   3篇
  2020年   1篇
  2019年   3篇
  2018年   2744篇
  2017年   2703篇
  2016年   1178篇
  2015年   66篇
  2014年   21篇
  2013年   8篇
  2012年   790篇
  2011年   2123篇
  2010年   2102篇
  2009年   1253篇
  2008年   1313篇
  2007年   1576篇
  2006年   29篇
  2005年   96篇
  2004年   103篇
  2003年   150篇
  2002年   58篇
  2001年   5篇
  2000年   40篇
  1999年   2篇
  1996年   1篇
  1995年   1篇
  1994年   1篇
  1993年   13篇
  1992年   8篇
  1991年   2篇
  1990年   4篇
  1989年   5篇
  1988年   11篇
  1987年   1篇
  1986年   1篇
  1983年   1篇
  1982年   1篇
  1977年   4篇
  1972年   1篇
  1969年   1篇
  1968年   4篇
  1967年   1篇
  1965年   2篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
992.
This article discusses the determinants of profitability expressed as return on capital among Japanese pulp and paper companies. Return on capital is measured as income before tax related to stockholders’ equity. Return on capital has been divided into profit margin and asset utilization rate. The hypothesis is that return on capital can be explained by mill size, productivity, product line, financial situation, and use of wood resources. The dataset consists of 13 larger Japanese pulp and paper companies listed in Pulp &; Paper Statistics. Data on financial performance, production, and use of wood fibers were collected for the period 1991–2001. The average return on equity for the studied companies is 4.2%. Labor productivity has increased quite significantly, from about 600 to 1,000 kg/employee and year. An econometric analysis indicates that the best model fit is found when the asset utilization rate is used as the dependent variable. Significant variables are, among others, asset utilization rate lagged one time period (?), labor productivity (+), capital productivity (?), paper production as a share of total paper and board production (?), total value of assets on the books (?) and solidity (+). This may indicate that the total value of assets on the books is large in relation to the total production of paper, paperboard and pulp.  相似文献   
993.
994.
A survey of forestry practices and attitudes was undertaken in four communities in Leyte, the Philippines, to improve understanding of the social and economic factors affecting small-scale forestry development. The survey had three main data collection activities — initial focus group discussions (FGDs), household interviews, and reporting and validation FGDs. A team of enumerators was selected for household interviews which consisted of both males and females, to avoid potential problems of unwillingness of people to talk with those of the opposite gender. The interviewers were also required to be able to speak local dialects (Cebuano and Waray Waray), the survey questionnaires being administered in these dialects. Various methods were used to gain the support and assistance of local government units and barangay captains. Some difficulty was experienced by the survey team in the first community due to barangay elections at the time of the survey, and the requirement by the University of Queensland Ethics Committee that respondents sign a consent form. This requirement was found to be not culturally appropriate for the Leyte smallholder communities. Offering goods at the end of the interview was found to be of limited value for encouraging participation in the survey. Provision of food and drinks were found to encourage FGD participants to express their views, but too much alcohol had a negative effect. The importance of providing comprehensive feedback to respondents and involving them and other stakeholders in development of policy recommendations was apparent. These survey experiences provide valuable insights which are not generally available in textbooks on sample surveys, and provide lessons for planning and conducting smallholder community survey into natural resource management issues.  相似文献   
995.
We evaluated differences between the forest floors and the establishment and growth of coniferous seedlings in fenced (13 years) and unfenced plots on Mt Ohdaigahara where the sika deer (Cervus nippon) population density is high. Large coniferous seedlings (height > 0.05 m) were less abundant in the unfenced plot, as a result of deer browsing. Small coniferous seedlings (height < 0.05 m), however, were more abundant in the unfenced plot, where most seedlings of Abies homolepis were found on bare ground and those of Picea jezoensis var. hondoensis were found on buttresses and fallen logs. The large area of bare ground in the unfenced plot was caused by deer browsing. Deer therefore have an indirect effect on the emergence and growth of small coniferous seedlings by modifying the forest floor.  相似文献   
996.
In order to develop a fireproof wooden material, the synergic effect of fire-retardant chemicals and wood coatings was studied. The fire performance was evaluated by cone calorimeter. Impregnation of fire retardants including polyphosphatic carbamate, and ceramic coatings including alkoxy metal salt improved the fire performance of wooden materials. This treatment made it possible to meet the guidelines for fire performance of noncombustible materials in Japan. In addition to the vacuum-pressure impregnation treatment, hot-and-cold-bath impregnation treatment is an effective way to develop fire-retardant wood by impregnating fire retardant and ceramic coating. The weatherability of the developed material was also investigated. The ceramic coating was resistant to light and moisture. Part of this report was presented at 54th Annual Meeting (Sapporo, August 2004) and the 55th Annual Meeting (Kyoto, March 2005) of the Japan Wood Research Society  相似文献   
997.
The optimal formula of Beta-Cypermethrin aqueous capsule suspension was screened out through the experiment,preparation and manufacture of basic formulas, which has no sedimentation, suspended drops of oil, peels and agglomerating and the diameter of capsule is in range of 10-30μm, conformed to the technical requirement. The aftereffect of the 3.3% Beta-Cypermethrin aqueous capsule suspension which was manufactured according to the optimal formula was up more than 25 days. The experiments on controlling the larvae of Dendrolinmus superans Butler and the adults of Xylotrechus rusticus L were carried out with different concentrations of this chemical. The death rate reached 80% when 250 times solution of the chemical was sprayed on stem to control the larvae of D. superans. For control of the adults of X. rusticus, 200, 400 and 600 times solution of the chemical were applied and their control effects (death rate) reached 85.23%, 74.21% and 66.59% respectively. Two kinds of solution (200 times and 300 times) of the chemical were used to control the larvae of D. superans in large area, and the control effect for both concentrations was over 90%.  相似文献   
998.
Little is known about the extent of work-related accidents occurring during the performance of forestry work by the non-industrial private forest owner or their assistant (hereafter called self-employed NIPF). The absence of an effective accident registration system largely excludes this group from accurate representation in official statistics. In this study, data from 1996 to 2001 were collected from hospital injury records managed by the Umeå Accident Analysis Group at the University Hospital in Umeå. During that period, it was found that 485 persons in the Umeå region were given medical attention due to injuries possibly acquired during self-employed forestry work. Questionnaires about the accidents were sent to each of the 485 injured persons and 80% were returned. Responses revealed that 225 of the respondents were injured during self-employed forestry work. Out of these, 81% performed fewer than 31 days of forestry work annually. Due to the injury, 29% had been on sick leave for some period of time and 42% had persistent symptoms. On average, each injury led to 13 days of sick leave and 24 hours of institutional care. Firewood production was shown to be a key factor behind the large number of accidents for self-employed NIPF. Further studies on the lack of knowledge about how the work should be done and conscious risk taking should be considered as an important link to the development of effective accident prevention strategies.  相似文献   
999.
The amount of photosynthetic radiation intercepted by a crop is a function of the incident solar radiation on the plants, the leaf area index (L AI), and the light extinction coefficient (k). We quantified L AI and k in stands of black wattle (Acacia mearnsii De Wild.) over a 7-year growth cycle at two locations in the state of Rio Grande do Sul, Brazil. Our study was conducted in commercial stands in agroecological regions with high densities of black wattle plantations. L AI was calculated as the ratio between the leaf area of a tree and its planting space, and k was derived from Beer’s law. L AI depends on the planting site and stand age. Between the two sites, the L AI was similar over time, the amount of variation differed. Values of k depended only on stand age, with the highest average observed for stands up to 5 years old. The trend of k during the plantation cycle was inversely proportional to L AI and was correlated with L AI, leaf area, leaf dry mass, canopy volume, height, branches dry mass, total dry mass, and crown diameter.  相似文献   
1000.
Tea tree oil is extracted from the leaves and twigs of Melaleuca alternifolia (Maiden & Betche) Cheel, and it is widely used in medicines, food preservatives, cosmetics and health care products. Traditional propagation of M. alternifolia from seeds does not necessarily transfer the desired characteristics from their mother trees, the seedlings are not uniform, and the multiplication rate from cuttings is relatively low. For these reasons, it is necessary to develop tissue culture techniques for this species. This study showed that an efficient explant initiation medium for M. alternifolia was MS 1/2 + BA 0.6 mg L?1 + NAA 0.1 mg L?1 + sucrose 30 g L?1, which yielded a 75.9 % initiation rate. An efficient multiplication medium was MS + BA 0.3 mg L?1 + NAA 0.15 mg L?1 + sucrose 30 g L?1, which yielded a 4.3 multiplication rate and 3.2 cm shoot length. The rooting medium was MS 1/2 + IBA 0.1–0.25 mg L?1 + sucrose 15 g L?1, which yielded a 100 % rooting rate, 2.94–3.32 roots per individual and 1.36–1.44 cm root length. Local red-core soil was suitable as a transplant medium, and yielded 98 % survival. This study improved the tissue culture technique for mass-propagation of M. alternifolia, enabling the production of high quality plants for market.  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