全文获取类型
收费全文 | 34116篇 |
免费 | 1859篇 |
国内免费 | 66篇 |
专业分类
林业 | 3073篇 |
农学 | 1939篇 |
基础科学 | 276篇 |
6995篇 | |
综合类 | 3451篇 |
农作物 | 1588篇 |
水产渔业 | 2205篇 |
畜牧兽医 | 12241篇 |
园艺 | 1156篇 |
植物保护 | 3117篇 |
出版年
2023年 | 187篇 |
2022年 | 180篇 |
2021年 | 312篇 |
2020年 | 341篇 |
2019年 | 338篇 |
2018年 | 2101篇 |
2017年 | 2091篇 |
2016年 | 1289篇 |
2015年 | 715篇 |
2014年 | 745篇 |
2013年 | 1204篇 |
2012年 | 1739篇 |
2011年 | 2488篇 |
2010年 | 2133篇 |
2009年 | 1803篇 |
2008年 | 2073篇 |
2007年 | 2364篇 |
2006年 | 988篇 |
2005年 | 1141篇 |
2004年 | 950篇 |
2003年 | 969篇 |
2002年 | 773篇 |
2001年 | 1017篇 |
2000年 | 957篇 |
1999年 | 790篇 |
1998年 | 224篇 |
1997年 | 208篇 |
1996年 | 162篇 |
1995年 | 206篇 |
1994年 | 193篇 |
1993年 | 175篇 |
1992年 | 430篇 |
1991年 | 409篇 |
1990年 | 351篇 |
1989年 | 340篇 |
1988年 | 343篇 |
1987年 | 365篇 |
1986年 | 292篇 |
1985年 | 239篇 |
1984年 | 212篇 |
1983年 | 179篇 |
1982年 | 110篇 |
1981年 | 101篇 |
1980年 | 93篇 |
1979年 | 175篇 |
1978年 | 114篇 |
1977年 | 102篇 |
1974年 | 105篇 |
1969年 | 97篇 |
1968年 | 108篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
This paper empirically examines the key factors driving UK direct investment in the Spanish regions over the period 2000–2016 and, consequently, tries to unveil its main motivation. Applying a spatial Durbin panel model to capture spatial linkages, the results point to the existence of complementarity between the FDI received by a region and that of the remaining ones. This outcome, along with a positive and statistically significant spillover effect of market potential, reveals that complex-vertical FDI motivation with agglomeration economies prevails among UK MNEs investing in Spain. Additionally, our findings unveil the role played by some other FDI drivers, such as wages and infrastructure. Furthermore, the paper is unique in decomposing the average direct and spillover effects by region and pairs of regions, so that remarkable differences can be identified. This breakdown has strong significance from a policy perspective since it can guide regional policy makers. In short, our findings point out to the fact that FDI policy should be jointly designed by those regions presenting strong bilateral spillover effects. Thus, greater cooperation among policy makers would be welcome. 相似文献
2.
A. Pérez-Sierra P. Romón-Ochoa C. Gorton A. Lewis H. Rees S. Van Der Linde J. Webber 《Plant pathology》2019,68(4):727-737
Chestnut blight, caused by Cryphonectria parasitica, was identified in Devon, UK, in December 2016. Intensive surveys detected the disease at further sites in Devon (seven), Berkshire (one), Dorset (one), Derbyshire (four) and a cluster of eight sites in southeast London. Over 570 survey samples were tested, and 227 were positive for C. parasitica by isolation and real-time PCR. A total of 227 isolates were tested for mating type, and 197 screened for vegetative compatibility group (VCG) and compared with VCGs known from mainland Europe. The same isolates were also screened for the presence of Cryphonectria hypovirus 1 (CHV-1). Eleven VCGs were identified within the UK population. Five corresponded to already known European VCGs but six were unique. The European VCGs mainly came from the Devon, Dorset, Berkshire and Derbyshire disease outbreaks, whilst unique VCGs were almost exclusively from the southeast London cluster. Both mating types were detected, but only one mating type was present at each site, with the exception of a single Devon site. Perithecia of C. parasitica were never observed at any site. CHV-1 was found in seven isolates from three different locations and was always subtype-I, which has limited hypovirulence. Therefore, although CHV-1 is associated with C. parasitica at some outbreaks, it probably has limited impact on virulence. The diversity of VCGs and their distribution at outbreak sites, together with findings of CHV-1, suggests C. parasitica has been introduced to the UK multiple times over at least two decades through international plant trade. 相似文献
3.
Invasive plants may be more plastic than non‐invasive plants and maintain high fitness under various environmental conditions. Previous studies mainly focused on the comparisons between invasive and native plants, and comparisons between highly invasive and less invasive exotic species are still relatively rare, especially for comparisons at the subspecies level. This study examined the effects of nutrient addition and shading on the performance of the highly invasive Mimosa invisa and its less invasive subspecies M. invisa var. inermis under either isolated or competitive conditions. Nutrient addition increased biomass and plant height and decreased root‐to‐shoot ratio (R/S). Shading decreased biomass and R/S and increased plant height. Under isolated conditions, the two invaders did not differ in R/S, plant height and plasticity of these traits in response to nutrient addition or shading, and the two invaders also did not differ in biomass production under each of the nutrient and light treatments. When the two invaders competed with each other, M. invisa outcompeted M. invisa var. inermis under high soil nutrient conditions, and the two invaders did not differ in performance under other growth conditions. Thus, only considering competition may we find out the difference between highly invasive species and their closely related, less invasive subspecies. Management of M. invisa should focus on habitats with high soil nutrient availability, in which M. invisa is more likely to dominate. 相似文献
4.
X. Y. Wang C. W. Zhang W. T. Huang J. Yue J. J. Dou L. Y. Wang Q. Wang Y. Q. Cheng 《Plant pathology》2020,69(1):149-158
Efforts to control viral diseases of grapevine include the production of certified material and development of virus-resistant transgenic grapevines. However, effective antiviral agents, once the viruses have infected the plants, are still lacking. This study shows that a crude garlic extract has significant antiviral activity against grapevine viruses. Replication of grapevine leafroll-associated virus 2 (GLRaV-2) was obviously inhibited in grapevine cv. Cabernet Sauvignon calli treated with diluted (1:100) garlic extract. The relative RNA levels of GLRaV-2 and grapevine fleck virus (GFkV) in cv. Summer Black grapevine in in vitro-grown plantlets 10 days after treatment with diluted (1:100) garlic extract were about 22% and 20%, respectively, of that in controls. The viral RNA accumulation of GLRaV-2, GFkV, grapevine virus A (GVA), grapevine fanleaf virus (GFLV) and grapevine rupestris stem pitting-associated virus (GRSPaV) in field-grown grapevine cv. Centennial Seedless plants sprayed with diluted (1:100) garlic extract were about 31–40%, 26–38%, 18–31%, 17–42% and 15–18%, respectively, of that in controls. Moreover, the garlic extract treatment led to a significant decrease in viral RNA accumulation of GLRaV-3, GLRaV-2, GVA, GFkV, GFLV, GRSPaV and grapevine Pinot Gris virus in pot-grown grapevine cv. Shine Muscat plants, and viral disease symptoms in these plants were obviously attenuated. In addition, this extract significantly induced expression of pathogenesis-related protein genes and stimulated activity of antioxidant enzymes in grapevines. Taken together, these results indicate that the crude garlic extract acts as a significant inhibitor against a broad range of grapevine viruses. 相似文献
5.
Jeannette S. Bayuelo-Jiménez Takashi Muraoka Eulogio de la Cruz-Torres Esperanza Quintero-Ponce Lydia C. Paredes-Gutiérrez Mohammad Zaman 《Soil Use and Management》2020,36(2):240-249
Land-use change from forest to agriculture in the volcanic ash-derived soils of Mexico has increased over recent decades. It is likely that land uses and management practices, particularly fertilizer use have affected phosphorus (P) distribution and availability. The objective of this study was to evaluate the effects of land-use types (native forest and maize mono-cropping), and the related P addition, on the forms and distribution of soil P and their isotopic exchangeability. An Andisol, sampled from a cropping site, along with the contiguous area under native forest was treated with 32P-labelled potassium phosphate (KH232PO4). The soil samples were extracted after incubation times of 7, 21, 35 and 49 days. Phosphorus content and 32P recovery in fractions sequentially extracted were assessed for each incubation time. Total soil P was dominated by inorganic fractions (79 to 86%) in both land-use types. Resin-Pi, bicarbonate extractable inorganic P (Bic-Pi) and sodium hydroxide extractable inorganic P (NaOH0.1-Pi) were all raised with P addition. However, the proportion of organic P fraction was reduced under cropped soil. The recovery of 32P in soils with P addition indicates that resin-Pi, Bic-Pi and NaOH0.1-Pi comprised nearly all the exchangeable P. In native soils with no P addition, more than 19% of the 32P was recovered in Bic-Po and NaOH0.1-Po forms. This finding indicates that organic P cycling is crucial when soil Pi reserves are presented in an inadequate amount. Ecologically based management has to be designed for replenishment and succeeding maintenance of soil organic P compounds to increase sustainable agricultural production. 相似文献
6.
Morantes César Fernández Yarza Florencia Montes María L. Mercader Roberto C. Curutchet Gustavo Torres Sánchez Rosa M. 《Water, air, and soil pollution》2020,231(1):1-13
Water, Air, & Soil Pollution - The treatment efficiency of Chlorella sorokiniana and Scenedesmus species, immobilized in sodium alginate, was evaluated for removing nitrate from groundwater.... 相似文献
7.
da Silva Isadora Gonçalves Castoldi Renata de Oliveira Charlo Hamilton César de Souza Miranda Mateus Nunes Thaíssa Dias Cardoso Costa Luciene Lacerda Lemes Ernane Miranda 《Journal of Crop Science and Biotechnology》2020,23(2):191-196
Journal of Crop Science and Biotechnology - The cultivation of sweet corn is expanding in Brazil, but there are serious constraints about the availability of commercial cultivars. The selection of... 相似文献
8.
Mattana Efisio Peguero Brígido Di Sacco Alice Agramonte Wilvin Encarnación Castillo Wilkin Rafael Jiménez Francisco Clase Teodoro Pritchard Hugh W. Gómez-Barreiro Pablo Castillo-Lorenzo Elena Terrero Encarnación Marianny Way Michael J. García Ricardo Ulian Tiziana 《New Forests》2020,51(4):705-721
New Forests - Native trees from the Caribbean were tested for seed desiccation responses, by adapting the “100-seed test” protocol. Ninety-seven seed lots of 91 species were collected... 相似文献
9.
Thistles from the genus Onopordum (Asteraceae) are of Mediterranean and Eurasian origin. They are very common in the south of Syria, in particular in Damascus and Al Suwayda, and are found in fallow fields, pastures, roadsides and neglected areas. In 2015–2017, several sites with large populations of Onopordum spp. were identified. Weekly surveys of some of the sites, mainly in the regions of Damascus and Al Suwayda, revealed a significant diversity of phytophagous insect species, some of which had a very high population density. A total of 29 insect species were detected, belonging to four orders and 21 families. The most abundant species were from the order Coleoptera. These were observed to be feeding on different plant parts of Onopordum spp. Purely endophagous species comprised 38% of the species observed on Onopordum spp., ectophagous species comprised 45% with 17% of species mixed ectophagous and endophagous. Species that only fed on Onopordum spp. represented 34% of all the species observed. 相似文献
10.
Boulanger Yan Arseneault Dominique Boucher Yan Gauthier Sylvie Cyr Dominic Taylor Anthony R. Price David T. Dupuis Sébastien 《Landscape Ecology》2019,34(1):159-174
Landscape Ecology - Forest landscapes at the boreal–temperate ecotone have been extensively altered. Reducing the gap between current and presettlement forest conditions through... 相似文献