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   134309篇
  免费   7691篇
  国内免费   65篇
林业   5556篇
农学   4665篇
基础科学   861篇
  16606篇
综合类   18794篇
农作物   4945篇
水产渔业   7086篇
畜牧兽医   72835篇
园艺   1801篇
植物保护   8916篇
  2020年   1182篇
  2019年   1414篇
  2018年   2237篇
  2017年   2508篇
  2016年   2272篇
  2015年   1965篇
  2014年   2437篇
  2013年   5704篇
  2012年   4341篇
  2011年   5144篇
  2010年   3479篇
  2009年   3512篇
  2008年   5098篇
  2007年   4942篇
  2006年   4607篇
  2005年   4226篇
  2004年   4071篇
  2003年   4130篇
  2002年   3749篇
  2001年   4716篇
  2000年   4554篇
  1999年   3645篇
  1998年   1389篇
  1997年   1379篇
  1996年   1305篇
  1995年   1594篇
  1994年   1325篇
  1993年   1349篇
  1992年   2753篇
  1991年   2902篇
  1990年   2858篇
  1989年   2893篇
  1988年   2610篇
  1987年   2592篇
  1986年   2540篇
  1985年   2401篇
  1984年   1956篇
  1983年   1715篇
  1982年   1154篇
  1979年   1649篇
  1978年   1285篇
  1977年   1135篇
  1976年   1100篇
  1975年   1164篇
  1974年   1356篇
  1973年   1448篇
  1972年   1377篇
  1971年   1194篇
  1970年   1247篇
  1969年   1146篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
Journal of Plant Diseases and Protection - Gerbera is an important cut flower crop popular throughout the world. In summer months of 2016, gerberas grown in polyhouses of RHREC, GKVK, Bengaluru,...  相似文献   
2.
Journal of Plant Diseases and Protection - The neotropical brown stink bug Euschistus heros (F.) is an important pest for many crops in several countries and its ability to survive on associated...  相似文献   
3.
4.
Journal of Plant Diseases and Protection - Fall armyworm, Spodoptera frugiperda (Noctuidae: Lepidoptera), is an economically important invasive pest infesting maize. Habitat manipulation as a...  相似文献   
5.
Dabrin  A.  Bégorre  C.  Bretier  M.  Dugué  V.  Masson  M.  Le Bescond  C.  Le Coz  J.  Coquery  M. 《Journal of Soils and Sediments》2021,21(2):1256-1274
Journal of Soils and Sediments - Suspended particulate matter (SPM) transport through rivers is a major vector of nutrients and pollutants to continental shelf areas. To develop efficient sediment...  相似文献   
6.

Over the past decades, the general trend towards shade reduction and intensification of cacao management has led to biodiversity losses. In the Peruvian Amazon, the regional government is heavily promoting crop conversion to shift from regionally marketed foods towards cacao (Theobroma cacao) and copoazu (Theobroma grandiflorum). While this shift is already visibly impacting the farming landscape and the lives of many smallholder farmers, little is known about the reasons that drive farmers to choose certain types of Theobroma species or cacao varieties over others. In this paper, we addressed how cacao farmers perceive and manage specific and varietal Theobroma diversity. We interviewed cacao farmers (n?=?20) during a seed-exchange fair and adapted a version of the four-square analysis to explore which Theobromas are currently adopted by farmers and why. The native cacao variety (cacao chuncho) was the one cultivated by most farmers, followed by the more industrial clonal varieties. The source of seeds and seedlings for the most cultivated varieties was a mix of donations by public institutions (for clonal varieties) and informal exchange among farmers (mainly for native and criollo varieties, and species i.e. macambo (Theobroma bicolor) and cacauillo (Theobroma speciosum)). The cacao varieties incentivized by public institutions were the least desired for future investment. The motivations for farmers to plant Theobromas was mainly based on the perceived current and potential market, but their desire to invest in a given species or variety was also based on pest resistance, traditional use, farm diversification and taste. Copoazu and CCN cacao varieties are produced by many households in large areas, while macambo, cacauillo and the cacao varieties TSH and porcelana are produced by few households in small areas. Our study suggests that agroforestry systems that include alternative Theobroma species are multi-strata and more diverse than cacao-based systems. It highlights the risks of agrobiodiversity loss associated with the promotion of industrial cacao varieties, and the importance of seed and seedling access for the development of diverse farming systems. We recommend the expansion of local seed-sharing networks and the extension of public cacao-donation campaigns to neglected and underutilized Theobromas with socio-economic and environmental benefits, in order to make the local farming systems more diverse and resilient.

  相似文献   
7.
New Forests - Resin canal features are associated with significant economic losses to appearance grade Pinus radiata timber. This study investigated the variation in resin canal features in young,...  相似文献   
8.
Eurasian Soil Science - To assess the state of plants and their response to changes in soil properties, the elemental composition of leaves of widespread and pollution-tolerant species Betula...  相似文献   
9.
10.
  1. Isoëtes sabatina is the rarest aquatic quillwort in Europe. Although recently found (2013) in Lake Bracciano (central Italy), the species is just one step away from extinction with an estimated population not exceeding 400 individuals and a spatial range of a few hundred square metres.
  2. Lake Bracciano is a deep, oligo-mesotrophic Mediterranean volcanic lake that has been subjected to human activities. From January to October 2017, the lake experienced a dramatic water level decrease (up to −1.50 m), which significantly affected the littoral zone and the habitat of I. sabatina.
  3. To improve the chances of survival of I. sabatina, the first eco-taxonomic investigation on this species was carried out to describe its genetic distinctness, physical and chemical requirements and companion species.
  4. The phylogenetic position of I. sabatina was investigated by applying standard DNA barcoding methods. Simultaneously, during summer 2019, the physical and chemical features of water and sediments of the I. sabatina population and five small Alpine lakes colonized by Isoëtes echinospora – a supposed close relative – were characterized. These data were then compared with the available data on the trophic requirements of the target obligate aquatic Isoëtes, together with Isoëtes lacustris and Isoëtes malinverniana.
  5. The present survey confirmed the taxonomic and ecological distinctness of I. sabatina – providing the first evidence of genetic differentiation from I. echinospora. Isoëtes sabatina grows in waters with temperature, conductivity and total alkalinity up to 30°C, 561 μS cm−1 and 3.45 meq L−1, respectively.
  6. The edaphic requirements of I. sabatina confirm its outstanding conservation value, and this study offers a basic understanding of how to prevent its extinction. Now, all possible actions must be taken immediately to save this species.
  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