全文获取类型
收费全文 | 1597篇 |
免费 | 87篇 |
国内免费 | 2篇 |
专业分类
林业 | 109篇 |
农学 | 27篇 |
基础科学 | 4篇 |
251篇 | |
综合类 | 175篇 |
农作物 | 76篇 |
水产渔业 | 85篇 |
畜牧兽医 | 815篇 |
园艺 | 33篇 |
植物保护 | 111篇 |
出版年
2023年 | 17篇 |
2022年 | 9篇 |
2021年 | 26篇 |
2020年 | 39篇 |
2019年 | 46篇 |
2018年 | 38篇 |
2017年 | 40篇 |
2016年 | 44篇 |
2015年 | 43篇 |
2014年 | 42篇 |
2013年 | 82篇 |
2012年 | 102篇 |
2011年 | 138篇 |
2010年 | 40篇 |
2009年 | 68篇 |
2008年 | 104篇 |
2007年 | 107篇 |
2006年 | 88篇 |
2005年 | 95篇 |
2004年 | 82篇 |
2003年 | 84篇 |
2002年 | 62篇 |
2001年 | 17篇 |
2000年 | 20篇 |
1999年 | 21篇 |
1998年 | 15篇 |
1997年 | 17篇 |
1996年 | 13篇 |
1995年 | 7篇 |
1994年 | 9篇 |
1993年 | 8篇 |
1992年 | 12篇 |
1991年 | 9篇 |
1990年 | 12篇 |
1989年 | 7篇 |
1988年 | 6篇 |
1987年 | 5篇 |
1986年 | 4篇 |
1985年 | 6篇 |
1984年 | 7篇 |
1983年 | 10篇 |
1982年 | 6篇 |
1980年 | 4篇 |
1979年 | 9篇 |
1978年 | 5篇 |
1977年 | 5篇 |
1976年 | 7篇 |
1975年 | 18篇 |
1974年 | 4篇 |
1970年 | 4篇 |
排序方式: 共有1686条查询结果,搜索用时 406 毫秒
981.
The paper gives an overview of ecological theories and hypotheses that have been raised in order to predict diversity‐function relationships. In particular, those reasons are discussed that may explain the discrepancy between the theoretical expectation for widespread effects of diversity on functioning and the ambiguous empirical evidence for such effects. Structural differences in the ecology of plants, invertebrates, and micro‐organisms are considered which lead to differences in diversity‐function relationships among these groups of organisms. Four criteria are derived that determine diversity‐function relationships: (1) motility of the organisms under consideration, (2) decoupling of population persistence and functional activity in these organisms, (3) species richness of the organisms' community, and (4) equilibrium stability of the considered ecological process. From these criteria the authors predict that measurable effects of diversity on functioning are (a) likely to be found in plants and in micro‐organisms while they are (b) unlikely to be found in the soil fauna. They predict that diversity is (c) likely to affect primary production, soil energy turnover, and nutrient losses from the system, while it is (d) unlikely to durably influence litter decomposition rate. It is shown that these predictions are largely corroborated by empirical evidence compiled from the literature. The issue of spatial and temporal scale is briefly discussed. 相似文献
982.
Belowground interactions for water between trees and grasses in a temperate semiarid agroforestry system 总被引:1,自引:0,他引:1
María Elena Fernández Javier Gyenge Julian Licata Tomás Schlichter Barbara J. Bond 《Agroforestry Systems》2008,74(2):185-197
A fundamental hypothesis of agroforestry is the complementary use of soil resources. However, productivity of many agroforestry
systems has been lower than expected due to net competition for water, highlighting the need for a mechanistic understanding
of belowground interactions. The goal of this study was to examine root–root interactions for water in a temperate semiarid
agroforestry system, based on ponderosa pines and a Patagonian grass. The hypotheses were: (a) A greater proportion of water
uptake by pines is from deeper soil layers when they are growing with grasses than when they are growing alone; (b) Growth
of grasses is improved by the use of water hydraulically lifted by pines. We used stable isotopes of O to analyze water sources
of plants, and we measured sapflow direction in pine roots and continuous soil water content with a very sensitive system.
We also installed barriers to isolate the roots of a set of grasses from pine roots, in which we measured water status, relative
growth and water sources, comparing to control plants. The results indicated that pines and grasses show some complementary
in the use of soil water, and that pines in agroforestry systems use less shallow water than pines in monoculture. We found
evidence of hydraulic lift, but contradicting results were obtained comparing growth and isotope results of the root isolation
experiment. Therefore, we could not reject nor accept that grasses use water that is hydraulically lifted by the pines, or
that this results in a positive effect on grass growth. This information may contribute to understand the complex and variable
belowground interactions in temperate agroforestry. 相似文献
983.
Julian M. Norghauer Jay R. Malcolm Barbara L. Zimmerman Jeanine M. Felfili 《Forest Ecology and Management》2008
Research into the mechanisms underlying edaphic associations reported for many tropical forest tree species, including those in the Meliaceae, has paid little attention to the earliest post-dispersal life stages. We conducted a reciprocal sowing experiment to examine germination and establishment rates of big-leaf mahogany (Swietenia macrophylla King) during the first seedling growing season in two local edaphic habitats in southeastern Amazonia. In this region, adult populations are restricted primarily to low-ground soils associated with seasonal streams, but rarely are found on high-ground soils. Soil analyses revealed that low-ground soils at the study site were silty loams whereas high-ground soils were clays. High-ground soils were less acidic and more fertile than low-ground soils, especially with respect to total nitrogen, Mg, and sum of basic cations. 相似文献
984.
Age-related trends in red spruce foliar plasticity in relation to declining productivity 总被引:2,自引:0,他引:2
Phenotypic plasticity in needle morphology with increasing tree size and age was investigated by comparing four age classes of red spruce (Picea rubens Sarg.) ranging from juvenile (3-12 years old) to mature (over 100 years old). With increase in tree age there were significant increases in leaf mass per unit area (LMA), mesophyll and vascular bundle area as a percentage of total needle cross-sectional area, and stomatal density. Within the vascular bundle, both xylem cross-sectional area and tracheid lumen area increased significantly, whereas air space as a percentage of total cross-sectional area decreased. These morphological changes were associated with a significant decrease in photosynthetic capacity and stomatal conductance, and an increase in (13)C enrichment. Although both photosynthetic capacity and whole-tree conductance decreased significantly between age classes 3 and 12 years, they did not differ between age classes 53 and 127 years, even though needle (13)C enrichment was significantly greater in the 127-year age class. Thus there appear to be compensatory mechanisms that maintain photosynthetic capacity as trees increase in size and vascular complexity, which in red spruce and other species, may affect leaf hydraulic conductance. Although increased LMA may contribute to reduced photosynthetic capacity in red spruce, similar relationships are not seen in other conifers. 相似文献
985.
A distinct set of homoeologous cellulose synthase catalytic subunit (CesA) genes are coordinately up-regulated with the onset of secondary wall formation in cotton fiber as shown by quantitative-RTPCR. 相似文献
986.
Four full-sib families of interior spruce (Picea glauca (Moench) Voss) x Picea engelmanii Parry ex Engelm.) with contrasting growth rates (two fast-growing and two slow-growing families) were grown aeroponically with either a 2% relative nitrogen addition rate or free access to nitrogen. Fast-growing families showed greater plasticity in allocating biomass to shoots at high nitrogen supply and to roots at low nitrogen supply than slow-growing families. Compared with the slow-growing families, short-term net ammonium uptake rate measured with an ion selective electrode was significantly greater in fast-growing families at high ammonium supply, but not at low supply. Net nitrate uptake showed the same trend, but differences among families were not significant. Results indicate that differences in seedling growth rate are partly a result of physiological differences in net nitrogen uptake efficiency and nitrogen productivity. 相似文献
987.
Melanie Rupp Paola Pilo Barbara Müller Ralph Knüsel Beat von Siebenthal Joachim Frey Paul‐Daniel Sindilariu Heike Schmidt‐Posthaus 《Journal of fish diseases》2019,42(5):685-691
In non‐salmonid fish, Aeromonas salmonicidacan cause local infections with severe skin ulcerations, known as atypical furunculosis. In this study, we present a systemic infection by a virulent A. salmonicidain European perch (Perca fluviatilis).This infection was diagnosed in a Swiss warm water recirculation aquaculture system. The isolate of A. salmonicida encodes a type three secretion system (TTSS) most likely located on a plasmid similar to pAsa5/pASvirA, which is known to specify one of the main virulence attributes of the species A. salmonicida. However, the genes specifying the TTSS of the perch isolate show a higher temperature tolerance than strains isolated from cold‐water fish. The function of the TTSS in virulence was verified in a cytotoxicity test using bluegill fry and epithelioma papulosum cyprinid cells. 相似文献
988.
1. The likelihood that seabirds will be hooked and drowned in longline fisheries increases when baited hooks sink slowly. Fishermen target different fishing depths by setting the mainline through a line shooter, which controls the tension (or slackness) in the line. An experiment was conducted in Australia's pelagic longline fishery to test the hypothesis of no difference in sink rates of baited hooks attached to mainline set under varying degrees of tension. 2. Mainline was set in three configurations typically used in the fishery: (a) surface set tight with no slackness astern; (b) surface set loose with 2 s of slack astern; and (c) deep set loose with 7 s of slack astern. 3. Tension on the mainline had a powerful effect on sink rates. Baited hooks on branch lines attached to tight mainlines reached 2 m depth nearly twice as fast as those on the two loose mainline tensions, averaging 5.8 s (0.35 m s?1) compared with 9.9 s (0.20 m s?1) and 11.0 s (0.18 m s?1) for surface set loose and deep set loose tensions, respectively. 4. The likely reason for the difference is propeller turbulence. Tight mainline entered the water aft of the area affected by turbulence whereas the two loose mainlines and the clip ends of branch lines were set directly into it about 1 m astern of the vessel. The turbulence presumably slowed the sink rates of baited hooks at the other end of the branch lines. 5. The results suggest that mainline deployed with a line shooter (as in deep setting) into propeller turbulence at the vessel stern slows the sink rates of baited hooks, potentially increasing their availability to seabirds. Unless mainline can be set to avoid propeller turbulence the use of line shooters for deep setting should not be promoted as an effective deterrent to seabirds. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
989.
Bäuerlein R Sandmeier B Villmann C Hammon A Gareis M Becker CM Pischetsrieder M 《Journal of agricultural and food chemistry》2008,56(1):44-49
As a potential transmitter of bovine spongiform encephalopathy (BSE), tissue from bovine central nervous system (CNS) is not accepted in meat and meat products. Western blot analysis of the CNS marker myelin proteolipid protein (PLP) detects CNS contamination selectively and sensitively. In this study, a rapid dot blot assay using an anti-PLP antibody was developed to screen CNS contamination of meat and contact surfaces. The detection limit was 0.01% bovine brain in minced bovine muscle. When applied to a swab test, down to 0.5 mg of CNS tissue on meat or other surfaces was detectable. Other offal tissues or peripheral nerves did not interfere with the assay. The test allows a differentiation between mammalian and avian CNS but not among mammalian species. The swab test was applied immediately after slaughtering at several areas of the bovine head. CNS was not detectable at any region which may enter the food chain. 相似文献
990.
Christopher M. Free Sean C. Anderson Elizabeth A. Hellmers Barbara A. Muhling Michael O. Navarro Kate Richerson Lauren A. Rogers William H. Satterthwaite Andrew R. Thompson Jenn M. Burt Steven D. Gaines Kristin N. Marshall J. Wilson White Lyall F. Bellquist 《Fish and Fisheries》2023,24(4):652-674
Marine heatwaves are increasingly affecting marine ecosystems, with cascading impacts on coastal economies, communities, and food systems. Studies of heatwaves provide crucial insights into potential ecosystem shifts under future climate change and put fisheries social-ecological systems through “stress tests” that expose both vulnerabilities and resilience. The 2014–16 Northeast Pacific heatwave was the strongest and longest marine heatwave on record and resulted in profound ecological changes that impacted fisheries, fisheries management, and human livelihoods. Here, we synthesize the impacts of the 2014–2016 marine heatwave on US and Canada West Coast fisheries and extract key lessons for preparing global fisheries science, management, and industries for the future. We set the stage with a brief review of the impacts of the heatwave on marine ecosystems and the first systematic analysis of the economic impacts of these changes on commercial and recreational fisheries. We then examine ten key case studies that provide instructive examples of the complex and surprising challenges that heatwaves pose to fisheries social-ecological systems. These reveal important insights into improving the resilience of monitoring and management and increasing adaptive capacity to future stressors. Key recommendations include: (1) expanding monitoring to enhance mechanistic understanding, provide early warning signals, and improve predictions of impacts; (2) increasing the flexibility, adaptiveness, and inclusiveness of management where possible; (3) using simulation testing to help guide management decisions; and (4) enhancing the adaptive capacity of fishing communities by promoting engagement, flexibility, experimentation, and failsafes. These advancements are important as global fisheries prepare for a changing ocean. 相似文献