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   377篇
  免费   20篇
林业   71篇
农学   9篇
基础科学   1篇
  61篇
综合类   31篇
农作物   8篇
水产渔业   9篇
畜牧兽医   196篇
园艺   2篇
植物保护   9篇
  2021年   3篇
  2019年   5篇
  2018年   7篇
  2017年   4篇
  2016年   8篇
  2015年   4篇
  2014年   9篇
  2013年   24篇
  2012年   12篇
  2011年   19篇
  2010年   7篇
  2009年   8篇
  2008年   16篇
  2007年   22篇
  2006年   10篇
  2005年   14篇
  2004年   27篇
  2003年   16篇
  2002年   15篇
  2001年   11篇
  2000年   7篇
  1999年   4篇
  1998年   3篇
  1997年   7篇
  1996年   6篇
  1995年   3篇
  1992年   4篇
  1991年   3篇
  1990年   6篇
  1989年   5篇
  1988年   6篇
  1987年   6篇
  1986年   5篇
  1983年   3篇
  1981年   3篇
  1980年   3篇
  1979年   3篇
  1972年   4篇
  1971年   6篇
  1970年   3篇
  1969年   3篇
  1967年   3篇
  1966年   4篇
  1965年   6篇
  1956年   3篇
  1932年   4篇
  1926年   2篇
  1925年   2篇
  1911年   6篇
  1898年   2篇
排序方式: 共有397条查询结果,搜索用时 156 毫秒
91.
Antioxidant enzyme activities and glutathione status were determined in different tissues of two teleost species, the rainbow trout (Oncorhynchus mykiss) and the black bullhead (Ameiurus melas) to establish whether age-related changes exist between mature and immature individuals. Glutathione reductase and superoxide dismutase activities were significantly lower in hepatic and extrahepatic tissues of 3+ year than 1+ year trout and bullheads. Activities of glutathione peroxidase, catalase and glutathione S-transferase did not exhibit a clear pattern, with decreases in liver and kidney, but increases in gill and muscle tissues. Glutathione concentrations were significantly higher in most tissues of 3+ year than in 1+ year trout, but remained unchanged or decreased in tissues of older bullheads. The results imply an age- or maturation-dependent effect on key antioxidant enzymes in various tissues of these 2 teleost species. Thus, age and maturation may impact upon the use of oxidative stress parameters as indicators of contaminant exposure in environmental studies.  相似文献   
92.
Abstract

The outer facade constitutes a substantial volume of the total consumption of materials used in a building and the need for maintenance of the facade makes it especially interesting from a life cycle perspective. The range of wooden materials and products used for facades has different impacts over the life cycle, but the analyses so far have not included a time-adjusted global warming potential (GWP). Wooden facade materials were assessed with respect to their life cycle carbon footprint based on the environmental product declarations (EPD) and adjusted according to EN15804. The results showed low impacts of untreated Scots pine heartwood (≈1 kg CO2-eq. per square meter [m2]), medium for thermally modified Scots pine, coated Norway spruce and oil/copper–organic preservative-treated Scots pine (1–5 kg CO2-eq. per m2) and high for furfurylated Scots pine and acetylated Radiata pine (5–10 kg CO2-eq. per m2). The results with time-adjustment showed that these methods have a potential large effect on the carbon footprint of wooden claddings. The inclusion of biogenic carbon flows and timing seems to be more important than the difference between the product when biogenic carbon is not included. This fact highlights the importance time-adjusted GWP would have for wood products EPDs.  相似文献   
93.
94.
Institutional repositories manage and disseminate a University's scholarly output and provide a multitude of benefits to the organization and society. Rutgers University Libraries is actively expanding its repository to include materials with scholarly merit that are currently siloed in academic departments or otherwise unpreserved and unavailable to the public. This article describes a collaboration between Rutgers Libraries faculty and Rutgers teaching faculty which is enabling discovery of a significant collection of video data relating to equine behavioral responses. The article describes the research, the repository tools, and the process of developing and customizing metadata to bring this collection to light.  相似文献   
95.
96.

The influence of time of weaning on weight changes, and on hormone and electrolyte status in domestic mink (Mustela vison) dams raising at least five kits per litter was examined. The kits were weaned at either day 42 (group 1) or day 49 (group 2) after birth. The dams were weighed and urine and blood were sampled from day 29 to day 56 after delivery. A considerable loss of dams' body weight, constituting 6.3% in group 1 and 8.1% in group 2, was noticed on the day after weaning, indicating a greatly reduced food consumption on the day of weaning. No changes in plasma aldosterone and no significant decrease in urinary sodium concentration were seen in dams after removal from their litters, whereas, irrespective of time of weaning, urinary sodium and chloride concentrations were halved a few days after weaning compared with the other group. The plasma cortisol concentration was high before and during weaning and was nearly halved 1 week after weaning, implying less strain on the dams after weaning. It is concluded that the weaning period is a most vulnerable and stressful period to the dams irrespective of the actual time of weaning.  相似文献   
97.
Nitrification inhibitors (NIs), DCD (dicyandiamide), and DMPP (3,4‐dimethylpyrazole phosphate), in combination with urea (UR) and ammonium sulfate nitrate (ASN) fertilizers were studied under contrasting soil textures (sand, loam, and clay) from cultivated soils collected in Brazil and Germany. Soil samples were incubated over 50 days and the content of ammonium ( ), nitrate ( ), and soil pH were measured periodically. Applied NIs delayed the nitrification process across all soil textures. Correlation analysis indicated that combining ASN with NIs resulted in higher content and efficiency in delaying the nitrification process with high N‐conversion rate (r = –0.82). The combination of ASN+ DMPP increased the efficiency of the N‐conversion rate (r = –0.86) due to H+ release in soil, while UR+DCD (r = –0.50) had an efficiency of the N‐conversion rate similar to UR (r = –0.42). All the NIs had a better performance in reducing formation in sandy soils as compared to the loam and clay textured soils. Use of DMPP with an N fertilizer results in a soil pH decrease and can be an option to increase the efficiency of the N‐conversion rate, reducing N losses in soil. Overall, our results suggest that NIs have a better performance in reducing formation in sandy soils as compared to that of the loam and clay textured soils. Use of DMPP with ASN results in a soil pH decrease and can be an option to reduce N losses in soil.  相似文献   
98.
Pinus patula is a species commonly used for reforestation in Mexico.However,efficient methods for the mass production seedlings are required.Micropropagation particularly by somatic embryogenesis provides an option for the rapid multiplication of high-quality,genetically improved material.This study induces somatic embryogenesis in this species using the thin cell layer(TCL)technique.Two sources of explants(complete immature embryos;lTCL segments from immature embryos)were evaluated.The efficiency of TCL from longitudinal sections[lTCL]and transverse[tTCL]was evaluated.The results show using thin cell layers from immature embryos cultivated in 16 light/8 dark hours achieves induction of somatic embryos.A higher percentage of embryogenic callus was obtained when tTCL segments were used as an explant source.These results produced somatic embryos from tTCL segments of an immature embryo without germinating the seed,making the process more time efficient.In addition,this technique can be used to generate somatic embryogenesis in forest species that have low germination rates.  相似文献   
99.
The exchange of CO2 between the atmosphere and a beech forest near Sorø, Denmark, was measured continuously over 14 years (1996-2009). The simultaneous measurement of many parameters that influence CO2 uptake makes it possible to relate the CO2 exchange to recent changes in e.g. temperature and atmospheric CO2 concentration. The net CO2 exchange (NEE) was measured by the eddy covariance method. Ecosystem respiration (RE) was estimated from nighttime values and gross ecosystem exchange (GEE) was calculated as the sum of RE and NEE. Over the years the beech forest acted as a sink of on average of 157 g C m−2 yr−1. In one of the years only, the forest acted as a small source. During 1996-2009 a significant increase in annual NEE was observed. A significant increase in GEE and a smaller and not significant increase in RE was also found. Thus the increased NEE was mainly attributed to an increase in GEE. The overall trend in NEE was significant with an average increase in uptake of 23 g C m−2 yr−2. The carbon uptake period (i.e. the period with daily net CO2 gain) increased by 1.9 days per year, whereas there was a non significant tendency of increase of the leafed period. This means that the leaves stayed active longer. The analysis of CO2 uptake by the forest by use of light response curves, revealed that the maximum rate of photosynthetic assimilation increased by 15% during the 14-year period. We conclude that the increase in the overall CO2 uptake of the forest is due to a combination of increased growing season length and increased uptake capacity. We also conclude that long time series of flux measurements are necessary to reveal trends in the data because of the substantial inter-annual variation in the flux.  相似文献   
100.
Phytoplankton blooms characterize temperate ocean margin zones in spring. We investigated the bacterioplankton response to a diatom bloom in the North Sea and observed a dynamic succession of populations at genus-level resolution. Taxonomically distinct expressions of carbohydrate-active enzymes (transporters; in particular, TonB-dependent transporters) and phosphate acquisition strategies were found, indicating that distinct populations of Bacteroidetes, Gammaproteobacteria, and Alphaproteobacteria are specialized for successive decomposition of algal-derived organic matter. Our results suggest that algal substrate availability provided a series of ecological niches in which specialized populations could bloom. This reveals how planktonic species, despite their seemingly homogeneous habitat, can evade extinction by direct competition.  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