首页 | 官方网站   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   1821篇
  免费   121篇
农业科学   1942篇
  2024年   4篇
  2023年   16篇
  2022年   22篇
  2021年   46篇
  2020年   50篇
  2019年   35篇
  2018年   87篇
  2017年   95篇
  2016年   85篇
  2015年   71篇
  2014年   88篇
  2013年   120篇
  2012年   171篇
  2011年   156篇
  2010年   88篇
  2009年   65篇
  2008年   123篇
  2007年   126篇
  2006年   98篇
  2005年   82篇
  2004年   76篇
  2003年   67篇
  2002年   47篇
  2001年   14篇
  2000年   11篇
  1999年   13篇
  1998年   4篇
  1997年   3篇
  1996年   3篇
  1994年   4篇
  1993年   2篇
  1991年   2篇
  1989年   6篇
  1988年   3篇
  1983年   2篇
  1977年   2篇
  1974年   4篇
  1971年   2篇
  1969年   2篇
  1966年   4篇
  1939年   2篇
  1937年   8篇
  1933年   2篇
  1926年   4篇
  1925年   2篇
  1924年   2篇
  1922年   4篇
  1914年   2篇
  1911年   2篇
  1898年   1篇
排序方式: 共有1942条查询结果,搜索用时 15 毫秒
61.
Pinus pinaster is of great ecological and economic importance and has traditionally been subjected to intensive uses such as wood and resin extraction. In the last decade, dendrochronological methods are increasingly being used to analyze the effects of climatic factors on the growth of the maritime pine, although tapped trees were generally avoided because it was thought that their growth was affected by resin extraction. In Spain, however, it is hard to find a long-lived forest of P. pinaster that has not been subjected to tapping for resin. In the present paper, we performed dendrochronological analyses of this species from wood cores and cross sections taken from both resin-tapped trees and resin-untapped trees killed by a fire in 2008 in the central Iberian region. On the one hand, we reconstructed the history of forest management by means of analysis of resin scars in the cross sections of resin-tapped trees. This facet of dendrochronological dating had not heretofore been developed, and little is therefore known about it. We dated 46 scars, which indicate a history of intensive resin extraction in the 1920–1950 period. Moreover, we attempted to answer the question: Have the old resin extractions in P. pinaster precluded the use of their growth rings for dendrochronological and dendroclimatic studies? We found that resin extraction did not alter general short-wavelength variability, and we developed a local chronology with all synchronized series, and the response function with respect to climate is similar to other oldest P. pinaster forests studied in Spain. The information we have recorded can be of use for providing tools to land managers for interpreting forest dynamics in resin-tapped regions.  相似文献   
62.
The potential of landscapes to supply multiple benefits to society beyond commodities production has received increasing research and policy attention. Linking the concept of multifunctionality with the ecosystem services (ES) approach offers a promising avenue for producing scientific evidence to inform landscape planning, e.g., about the relative utility of land-sharing and land-sparing. However, the value for decision-making of ES-based multifunctionality assessments has been constrained by a significant conceptual and methodological dispersion. To contribute towards a cohesive framework for landscape multifunctionality, we analyse case studies of joint ES supply regarding ten criteria designed to ultimately answer four aspects: (i) the multifunctionality of what (e.g., landscapes), (ii) the type of multifunctionality (e.g., based on ES synergies), (iii) the procedure of multifunctionality assessments, and (iv) the purpose of multifunctionality. We constructed a typology of methodological approaches based on scores for criteria describing the evaluation method and the level of stakeholder participation in assessments of joint ES supply. Surveyed studies and underlying types of methodological approaches (spatial, socio-spatial, functional, spatio-functional) differed in most criteria. We illustrate the influence of methodological divergence on planning recommendations by comparing two studies employing contrasting approaches (spatial and functional) to assess the joint supply of wildlife habitat and agricultural production in the Argentine Chaco. We distinguish between a pattern-based and process-based multifunctionality, where the latter can only be detected through approaches considering the ecological processes (e.g., ES complementarities) supporting the supply of multiple ES (functional and spatio-functional). Finally, we propose an integrated approach for assessing a socially-relevant process-based multifunctionality.  相似文献   
63.
Salivary chromogranin A (CgA) is considered to be a biomarker of activation of the sympatho-adrenomedullary system, and has recently been proposed as a useful indicator of the acute stress response in pigs. The aim of the present study was to determinate whether salivary CgA concentrations in healthy growing pigs exhibits any circadian pattern during the daytime, and to evaluate its stability under different storage conditions. A total of 80 pigs (40 in spring and another 40 in autumn) of two different ages and genders were used. To establish the circadian pattern, saliva samples were collected at 07.00, 11.00, 15.00 and 19.00 h on two consecutive days. Pooled samples were used for the stability study and were measured on the day of sampling and periodically for up to 360 days later. Samples were stored at 4 °C, ?20 °C or ?80 °C and the effect of repeated freezing and thawing was also evaluated.No circadian pattern was detected for salivary CgA in either season and there were no significant effects of gender or age. However, mean salivary CgA concentrations were significantly higher (P < 0.0001) in the pigs sampled in autumn, compared to those sampled in the spring. Short term storage at 4 °C is recommended for up to 2 days, whereas frozen samples can be stored for 1 year at ?20 °C or ?80 °C, without substantial reduction in CgA values. In addition, samples can be frozen and thawed up to seven times without significant loss of the biomarker.  相似文献   
64.
The objective of this study was to evaluate the effect of reduced-calorie avocado paste on lipid serum profile, insulin sensitivity, and hepatic steatosis in rats fed a hypercholesterolemic-high fructose diet. Thirty five male Wistar rats were randomly separated in five groups: Control group (ground commercial diet); hypercholesterolemic diet plus 60 % fructose solution (HHF group); hypercholesterolemic diet plus 60 % fructose solution supplemented with avocado pulp (HHF+A group); hypercholesterolemic diet plus 60 % fructose solution supplemented with reduced-calorie avocado paste (HHF+P group); and hypercholesterolemic diet plus 60 % fructose solution supplemented with a reduced-calorie avocado paste plus fiber (HHF+FP group). The A, P, and FP were supplemented at 2 g/kg/d. The study was carried out for seven weeks. Rats belonging to the HHF group exhibited significantly (P?≤?0.05) higher total cholesterol, triglycerides, and insulin levels in serum as well as lower insulin sensitivity than the control group. Supplementation with reduced-calorie avocado paste showed a significant (P?≤?0.05) decrease in total cholesterol (43.1 %), low-density lipoprotein (45.4 %), and triglycerides (32.8 %) in plasma as well as elevated insulin sensitivity compared to the HHF group. Additionally, the liver enzymes alanine aminotransferase and aspartate aminotransferase decreased significantly in the HHF-P group (39.8 and 35.1 %, respectively). These results are likely due to biocompounds present in the reduced-calorie avocado paste, such as polyphenols, carotenoids, chlorophylls, and dietary fibre, which are capable of reducing oxidative stress. Therefore, reduced-calorie avocado paste attenuates the effects of a hypercholesterolemic-high fructose diet in rats.  相似文献   
65.
Tanjungides A (1) (Z isomer) and B (2) (E isomer), two novel dibrominated indole enamides, have been isolated from the tunicate Diazona cf formosa. Their structures were determined by spectroscopic methods including HRMS, and extensive 1D and 2D NMR. The stereochemistry of the cyclised cystine present in both compounds was determined by Marfey’s analysis after chemical degradation and hydrolysis. We also report the first total synthesis of these compounds using methyl 1H-indole-3-carboxylate as starting material and a linear sequence of 11 chemical steps. Tanjungides A and B exhibit significant cytotoxicity against human tumor cell lines.  相似文献   
66.
Polyunsaturated aldehydes (PUAs) are organic compounds mainly produced by diatoms, after cell wounding. These compounds are increasingly reported as teratogenic for species of grazers and deleterious for phytoplanktonic species, but there is still scarce information regarding concentration ranges and the composition of PUAs in the open ocean. In this study, we analyzed the spatial distribution and the type of aldehydes produced by the large-sized (>10 μm) phytoplankton in the Atlantic Ocean surface. Analyses were conducted on PUAs released after mechanical disruption of the phytoplankton cells, referred to here as potential PUAs (pPUAs). Results show the ubiquitous presence of pPUA in the open ocean, including upwelling areas, as well as oligotrophic gyres. Total pPUA concentrations ranged from zero to 4.18 pmol from cells in 1 L. Identified PUAs were heptadienal, octadienal and decadienal, with heptadienal being the most common (79% of total stations). PUA amount and composition across the Atlantic Ocean was mainly related to the nitrogen:phosphorus ratio, suggesting nutrient-driven mechanisms of PUA production. Extending the range of trophic conditions considered by adding data reported for productive coastal waters, we found a pattern of PUA variation in relation to trophic status.  相似文献   
67.
Diverse actinomycetes produce a family of structurally and biosynthetically related non-ribosomal peptide compounds which belong to the chromodepsipeptide family. These compounds act as bisintercalators into the DNA helix. They give rise to antitumor, antiparasitic, antibacterial and antiviral bioactivities. These compounds show a high degree of conserved modularity (chromophores, number and type of amino acids). This modularity and their high sequence similarities at the genetic level imply a common biosynthetic origin for these pathways. Here, we describe insights about rules governing this modular biosynthesis, taking advantage of the fact that nowadays five of these gene clusters have been made public (thiocoraline, triostin, SW-163 and echinomycin/quinomycin). This modularity has potential application for designing and producing novel genetic engineered derivatives, as well as for developing new chemical synthesis strategies. These would facilitate their clinical development.  相似文献   
68.
We compared the abundance, species richness and diversity of saprobic filamentous microfungi in the forest and in coffee plantation systems (with different biophysical structures of the vegetation and agricultural management) and evaluated the degree of similarity in species composition among these sites. Soil washing was used to isolate the saprobic filamentous microfungi. Emerging colonies were quantified and transferred to tubes with culture medium and then mounted on semi-permanent slides for identification under the microscope. From 90 samples and 4,500 inoculated soils particles, 415 species were distinguished. The genera Trichoderma, Penicillium, Fusarium, Chaetomium and Humicola were the most frequent in all study sites. The transformation of the tropical montane cloud forest in coffee plantation had no significant effect on the abundance, species richness and diversity of the saprobic filamentous microfungi. Effects of the biophysical structures of the vegetation and the agricultural management of the sites were only detected at the level of dominant genera (Fusarium, Trichoderma and Penicillium) and on the evenness. The low degree of similarity among the six sites suggests the existence of a high exchange of species.  相似文献   
69.
  1. The protection of ecosystems with high diversity, such as coral reefs, is not an approach that guarantees their conservation. Thus, maintaining connectivity among coral populations over the long term is a strategy that should be adopted in order to protect diversity and ecological processes. Although coral reefs in Colombia are highly diverse, the population genetics baseline data of keystone species such as Orbicella faveolata are limited.
  2. To provide current information about the connectivity between populations of O. faveolata in the Colombian Caribbean, the genetic diversity and distribution of O. faveolata was evaluated with analysis of molecular variance (AMOVA), principal components, and migration estimations using 113 genets distributed in six populations. Also, a genetic structure analysis that included the available data for the Caribbean population was conducted, seeking to understand how the Colombian populations relate to the broader region.
  3. According to the global fixation index (FST) for the Colombian Caribbean, there was no genetic structure (FST = 0.002). Discriminant analysis of principal components (DAPC) showed that Corales del Rosario Archipelago (CR), Este de Isla Fuerte (EIF), San Bernardo Archipelago (SB), and Varadero Reef (VR) were grouped with Oeste de Isla Fuerte (OIF), with Bushnell (BS) representing the most divergent cluster. Migration analysis showed relatively high migration from VR to CR and SB, highlighting the importance of VR as a genetic reservoir for the region. Structure analysis showed that the Colombian population presented a specific genetic identity (FST = 0.254), suggesting that the Colombian Caribbean population could be a peripheral population that contributes significantly to genetic variation and is connected through a complex connectivity process.
  4. In conclusion, the estimated genetic connectivity reflects the influence of sea surface dynamics over the interpopulation exchange dynamics and the role of protected and nonprotected coral reef in the Colombian Caribbean. Thus, the Colombian Caribbean population of O. faveolata could be relevant to the conservation of genetic diversity on a larger scale.
  相似文献   
70.
  1. Freshwater environments host roughly half of the world’s fish diversity, much of which is concentrated in large, tropical river systems such as the Amazon. Fishes are critical to ecosystem functioning in the Amazon River basin but face increasing human threats. The basic biology of these species, and particularly migratory behaviour, remains poorly studied, in part owing to the difficulty associated with conducting tagging studies in remote tropical regions.
  2. Otolith microchemistry can circumvent logistical issues and is an increasingly important tool for studying fish life histories. However, this approach is still new in the Amazon, and its potential and limitations to inform fish conservation strategies remain unclear.
  3. Here, otolith microchemistry studies in the Amazon are reviewed, highlighting current possibilities, and several key factors that limit its use as a conservation tool in the Amazon are discussed. These include the dearth of spatiotemporal elemental data, poor understanding of environment–fish–otolith pathways, and insufficient funding, facilities, and equipment.
  4. A research initiative is proposed to harness the potential of this technique to support conservation in the Amazon. Key aspects of the proposal include recommendations for internal and external funding, which are critical to acquiring and maintaining technical staff, cutting-edge equipment, and facilities, as well as fostering regular scientific meetings and working groups. Meetings can facilitate a systematic approach to investigating environment–otolith pathways, broadening the chemical baseline for most Amazonian tributaries, and exploring potential valuable elements.
  5. These outcomes are urgently needed to conserve biodiversity and ecosystem functioning in the Amazon, especially given threats such as widespread hydroelectric damming. The initiative proposed here could make otolith microchemistry an important, cost-effective tool to inform and foster conservation in the Amazon, and act as a template for other imperilled tropical river basins, such as the Mekong and the Congo.
  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-23

京公网安备 11010802026262号