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   1630篇
  免费   116篇
  国内免费   1篇
林业   118篇
农学   62篇
基础科学   12篇
  234篇
综合类   267篇
农作物   64篇
水产渔业   211篇
畜牧兽医   608篇
园艺   58篇
植物保护   113篇
  2023年   16篇
  2022年   10篇
  2021年   35篇
  2020年   52篇
  2019年   50篇
  2018年   58篇
  2017年   52篇
  2016年   40篇
  2015年   40篇
  2014年   57篇
  2013年   75篇
  2012年   98篇
  2011年   138篇
  2010年   70篇
  2009年   57篇
  2008年   108篇
  2007年   97篇
  2006年   90篇
  2005年   95篇
  2004年   102篇
  2003年   104篇
  2002年   80篇
  2001年   22篇
  2000年   5篇
  1999年   13篇
  1998年   13篇
  1997年   15篇
  1996年   7篇
  1995年   14篇
  1994年   10篇
  1993年   4篇
  1992年   7篇
  1991年   5篇
  1990年   8篇
  1989年   6篇
  1988年   6篇
  1987年   5篇
  1986年   4篇
  1984年   6篇
  1983年   5篇
  1982年   5篇
  1981年   6篇
  1980年   6篇
  1978年   7篇
  1977年   8篇
  1976年   4篇
  1975年   5篇
  1973年   7篇
  1972年   3篇
  1970年   3篇
排序方式: 共有1747条查询结果,搜索用时 31 毫秒
991.
eBird: A citizen-based bird observation network in the biological sciences   总被引:1,自引:0,他引:1  
New technologies are rapidly changing the way we collect, archive, analyze, and share scientific data. For example, over the next several years it is estimated that more than one billion autonomous sensors will be deployed over large spatial and temporal scales, and will gather vast quantities of data. Networks of human observers play a major role in gathering scientific data, and whether in astronomy, meteorology, or observations of nature, they continue to contribute significantly. In this paper we present an innovative use of the Internet and information technologies that better enhances the opportunity for citizens to contribute their observations to science and the conservation of bird populations. eBird is building a web-enabled community of bird watchers who collect, manage, and store their observations in a globally accessible unified database. Through its development as a tool that addresses the needs of the birding community, eBird sustains and grows participation. Birders, scientists, and conservationists are using eBird data worldwide to better understand avian biological patterns and the environmental and anthropogenic factors that influence them. Developing and shaping this network over time, eBird has created a near real-time avian data resource producing millions of observations per year.  相似文献   
992.
Repeated fire is key to the viability of longleaf pine (Pinus palustris Mill.) ecosystems, but its acceptance as a management tool may depend on satisfactory longleaf pine growth. This is because longleaf pine establishment often has the dual-purpose of ecosystem restoration and stemwood production. Timely recovery of scorched foliage among longleaf pine seedlings and saplings supports maximum juvenile growth. We identified two means of foliage re-establishment in the growing season after prescribed fire regardless of the season of fire application. New foliage growth after spring or fall fire was correlated with un-scorched foliage biomass and the presence of lateral branches. After prescribed fire in spring, foliage biomass recovery also appeared to benefit from the mobilization of starch. The high carbon demand of foliage recovery after fall prescribed fire was associated with interruption of seasonal starch accumulation in the stem and taproot. The implication of low starch accumulation in stem and taproot tissues during the growing season after fall prescribed fire is unknown and warrants further investigation. Our results demonstrate a positive influence of residual foliage, lateral branches, and stored starch on timely foliage recovery of young longleaf pines after fire. Together with knowledge of longleaf pine development and fuel and climate conditions at the time of prescribed fire, this information will aid prescribed fire practitioners charged with maintaining longleaf pine stands of high vigor.  相似文献   
993.
994.
On November 5, 2015, after the collapse of the Fundão tailings dam, a massive amount of iron mine waste was released into the Doce River system in southeast Brazil. The aim of our study was to determine the mass fractions of potentially toxic elements in soil affected by the deposition of material by the waste wave. A preliminary screening was performed with portable X-ray fluorescence spectrometry (PXRF) and principal component analysis (PCA). The EPA 3050B method was further applied to digest the samples for quantitative determination of As, Ba, Cr, Cu, Mn, Ni, Pb, V, and Zn by inductively coupled plasma mass spectrometry. PCA was useful to classify the mine waste samples based on the Fe signal from the PXRF spectra, in spite of the heterogeneous nature of the material discharged into the Doce River system. The anomalous levels of As (up to 164 mg kg?1) and Mn (as high as 2410 mg kg?1) found in some mine waste and affected soil samples are within the background ranges typically observed in the soils of the Iron Quadrangle region. The toxicity characteristic leaching procedure shows no evidence of hazards regarding As, but a high natural background level of Mn was found in the mobile fraction. This preliminary environmental assessment highlights the importance of evaluation of long-term effects on soil directly impacted, as well as on the aquatic biota of the Doce River system and adjacent coastal environment given the large affected area, which includes regions with varying background levels of toxic elements.  相似文献   
995.
996.
Population diversity is a mechanism for resilience and has been identified as a critical issue for fisheries management, but restoration ecologists lack evidence for specific habitat features or processes that promote phenotypic diversity. Since habitat complexity may affect population diversity, it is important to understand how population diversity is partitioned across landscapes and among populations. In this study, we examined life history diversity based on size distributions of juvenile Central Valley Chinook salmon (Oncorhynchus tshawytscha) within the Yolo Bypass, a remnant transitional habitat from floodplain to tidal sloughs in the upper San Francisco Estuary (SFE). We used a generalized least squares model with an autoregressive (AR1) correlation structure to describe the distribution of variation in fish size from 1998 to 2014, and tested the effect of two possible drivers of the observed variation: (i) environmental/seasonal drivers within the Yolo Bypass, and (ii) the juvenile Chinook source population within the Sacramento River and northern SFE. We found that the duration of floodplain inundation, water temperature variation, season, and sampling effort influenced the observed time‐specific size distribution of juvenile Chinook salmon in the Yolo Bypass. Given the lack of seasonally inundated habitat and low thermal heterogeneity in the adjacent Sacramento River, these drivers of juvenile size diversification are primarily available to salmon utilizing the Yolo Bypass. Therefore, enhancement of river floodplain‐tidal slough complexes and inundation regimes may support the resilience of imperiled Central Valley Chinook salmon.  相似文献   
997.
Production of channel catfish, Ictalurus punctatus, female × blue catfish, Ictalurus furcatus, male hybrids has increased dramatically in the USA. Hybrid production requires surgical removal and maceration of blue catfish testes. Farmers report individual and seasonal variation in blue catfish testes development, and understanding the factors influencing testes development could improve hybrid catfish production. Effects of season (mid‐May vs. mid‐June), strain (D&B, Missouri, and Texas), and body weight on testes development in 5‐yr‐old blue catfish were determined. Males were measured in mid‐May and mid‐June for length, body weight, testes weight, and gonadosomatic index (GSI). Total sperm/g of testes, percent motility, and motile sperm/g of testes were determined. D&B strain males weighed more and had larger testes with more sperm than Missouri or Texas strain males in both months. Mature Texas strain males were larger than immature Texas strain males in both months. Testes were larger and had more sperm in May than in June for all strains; however, there were males from each strain with good quality testes in June. GSI, sperm/g of testes, percent motility, and motile sperm/g of testes were positively correlated. Information from this study provides a better understanding of factors influencing testes development in blue catfish.  相似文献   
998.
The importance of socio-economic impacts (SEI) from the introduction and use of genetically modified (GM) crops is reflected in increasing efforts to include them in regulatory frameworks. Aiming to identify and understand the present knowledge on SEI of GM crops, we here report the findings from an extensive study of the published international scientific peer-reviewed literature. After applying specified selection criteria, a total of 410 articles are analysed. The main findings include: (i) limited empirical research on SEI of GM crops in the scientific literature; (ii) the main focus of the majority of the published research is on a restricted set of monetary economic parameters; (iii) proportionally, there are very few empirical studies on social and non-monetary economic aspects; (iv) most of the research reports only short-term findings; (v) the variable local contexts and conditions are generally ignored in research methodology and analysis; (vi) conventional agriculture is the commonly used comparator, with minimal consideration of other substantially different agricultural systems; and (vii) there is the overall tendency to frame the research upon not validated theoretical assumptions, and to over-extrapolate small-scale and short-term specific results to generalized conclusions. These findings point to a lack of empirical and comprehensive research on SEI of GM crops for possible use in decision-making. Broader questions and improved methodologies, assisted by more rigorous peer-review, will be required to overcome current research shortcomings.  相似文献   
999.
Street sweeping is regularly performed within cities and residential communities to reduce roadway debris and ensure properly functioning storm water management systems. Given removal of plant and soil debris, street sweeping may also reduce nonpoint source pollution through removal of leachate source material. To assess the influence of street sweeping on storm water pollutants, 36 storm water collection devices were installed within six residential communities in Central Florida and subjected to varying municipal sweeping regimes. Additionally, precipitation and storm water retention pond leachate samplers were installed to quantify pollutant sources that may enter and leave selected urban communities. Despite high variability in percentage of impervious surfaces, population density, and volume of road debris among communities, no significant (P?≤?0.05) differences were observed for total Kjeldahl nitrogen (TKN), nitrate + nitrite (NOx), and total phosphorus (TP) concentrations in storm water among communities or between swept and unswept areas of roadways. Similarly, no significant differences were observed for TKN, NOx, and TP concentrations in precipitation and storm water. Significant differences in orthophosphate (ortho-P), however, were observed between communities and precipitation. Additionally, storm water TP concentrations were greater than discharge estimated to originate from communities within the study area. Although street sweeping may be effective at reducing volume of roadway debris, our data did not find it reduced N or P in storm water.  相似文献   
1000.
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