全文获取类型
| 收费全文 | 1641篇 |
| 免费 | 91篇 |
| 国内免费 | 2篇 |
专业分类
| 林业 | 122篇 |
| 农学 | 31篇 |
| 基础科学 | 4篇 |
| 256篇 | |
| 综合类 | 186篇 |
| 农作物 | 83篇 |
| 水产渔业 | 95篇 |
| 畜牧兽医 | 809篇 |
| 园艺 | 34篇 |
| 植物保护 | 114篇 |
出版年
| 2023年 | 18篇 |
| 2022年 | 8篇 |
| 2021年 | 28篇 |
| 2020年 | 42篇 |
| 2019年 | 50篇 |
| 2018年 | 41篇 |
| 2017年 | 41篇 |
| 2016年 | 46篇 |
| 2015年 | 43篇 |
| 2014年 | 41篇 |
| 2013年 | 83篇 |
| 2012年 | 104篇 |
| 2011年 | 146篇 |
| 2010年 | 41篇 |
| 2009年 | 70篇 |
| 2008年 | 108篇 |
| 2007年 | 107篇 |
| 2006年 | 90篇 |
| 2005年 | 97篇 |
| 2004年 | 82篇 |
| 2003年 | 85篇 |
| 2002年 | 63篇 |
| 2001年 | 19篇 |
| 2000年 | 20篇 |
| 1999年 | 23篇 |
| 1998年 | 16篇 |
| 1997年 | 17篇 |
| 1996年 | 14篇 |
| 1995年 | 8篇 |
| 1994年 | 10篇 |
| 1993年 | 8篇 |
| 1992年 | 13篇 |
| 1991年 | 8篇 |
| 1990年 | 9篇 |
| 1989年 | 7篇 |
| 1988年 | 4篇 |
| 1987年 | 5篇 |
| 1986年 | 4篇 |
| 1985年 | 7篇 |
| 1984年 | 7篇 |
| 1983年 | 11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 | 5篇 |
排序方式： 共有1734条查询结果，搜索用时 194 毫秒
61.
Temporal changes in inorganic and organic sulfur compounds (sulfate, glutathione, cysteine, methionine) were analyzed in xylem sap of 40-year-old Norway spruce (Picea abies (L.) Karst.) trees growing on acidic soils at a healthy and a declining stand in the Fichtelgebirge (North Bavaria, Germany). Studies were carried out (1) to quantify glutathione (GSH) transport in the xylem of spruce, (2) to study the significance of reduced sulfur versus sulfate (SO(4) (2-)) transport in the xylem, and (3) to compare total sulfur (S) transport in the xylem with the amount of foliar uptake of SO(2) in an air-polluted environment. Glutathione was the main reduced S compound in the xylem ranging in concentration from 0.5 to 5 &mgr;mol l(-1). Concentrations of inorganic SO(4) (2-) in the xylem sap were up to 50 times higher than those of GSH ranging from 60 to 230 &mgr;mol l(-1). During the growing season, concentrations of all S compounds in the xylem were highest in May (up to 246 &mgr;mol l(-1)) and decreased during summer and fall (up to 21 &mgr;mol l(-1)). On average, SO(4) (2-) concentrations in xylem sap were 30% higher at the declining site compared with the healthy site. Diurnal changes in organic S compounds were significant for GSH and cysteine with high concentrations during the night and low concentrations during the day. Diurnal changes in inorganic concentrations were not significant. Xylem sap concentrations of SO(4) (2-) and cysteine were twice as high and GSH concentrations were tenfold higher in surface roots than in branches. At both sites, transport of organic S was low (up to 3% of total S) compared to transport of SO(4) (2-). Annual transport of total S in the xylem (SO(4) (2-) was the main component) ranged from 60 to 197 mmol tree(-1) year(-1) at the healthy site and from 123 to 239 mmol tree(-1) year(-1) at the declining site. Although gaseous uptake of SO(2) was estimated to be similar at both sites (38 mmol tree(-1) year(-1); Horn et al. 1989), the ratio between annual gaseous uptake of SO(2) and transport of S in the xylem was 1:4 and 1:5 at the healthy and declining sites, respectively. 相似文献
62.
Zusammenfassung Ziel des Projekts ist die Erarbeitung spektraler Rückstrahlungscharakteristiken von Einzelb?umen und Bestandesoberfl?chen
zur Erfassung und kontinuierlichen Beobachtung von Vitalit?tsunterschieden bzw. Sch?digungen an Waldb?umen, insbesondere an
Fichten. (Ammer et al. 1983;Hildebrandt 1976). Die für die Spektroradiometermessungen wichtigen objektunabh?ngigen Einflu?parameter wie Sonnenstand, Me?richtung,
Neigungswinkel, ?ffnungswinkel und Beleuchtungsverh?ltnisse sind durch eine Vielzahl von Voruntersuchungen bestimmt worden.
Erste Me?ergebnisse an Testfl?chen im H?glwald und im Erbersberger Forst lassen deutliche Reaktionen im Spektralverhalten
erkennen, die auf Vitalit?ts- und ph?nologische Unterschiede zurückgeführt werden k?nnen.
Beitrag Nr. 16 der H?glwald-Serie 1986 相似文献
Judging vigor of spruces on the basis of spectral reflection measurements
Summary Objective of the research project is the evaluation of characteristic spectral signatures for single trees and stands to assess and continuously monitor differences in vigor, or damages of trees, respectively, especially for spruce. The object-independent parameters which are important for the spectro-radiometer measurements, such as angle and elevation of the sun, inclination, measuring direction, angle of view, and illumination have been determined by numerous previous investigations. First results from the testsites “H?glwald” and “Ebersberger Forst” show distinct differences in the spectral curves which can be related to different degrees of vigor and phenology.
Beitrag Nr. 16 der H?glwald-Serie 1986 相似文献
63.
Barbara Moser Christoph Bachofen Jonathan D. Müller Marek Metslaid Thomas Wohlgemuth 《Annals of Forest Science》2016,73(4):959-970
Key message
Pinus sylvestris seedlings quickly expand their roots to deeper soil layers while Pseudotsuga menziesii concentrates its root system in the topsoil, thereby running the risk of desiccation during long dry spells, as indicated by lower survival after simulated summer drought.Context
Pseudotsuga menziesii (Douglas-fir) is regarded as a promising species to maintain the productivity of Central European lowland forests given the projected increase of long dry spells.Aims
Will the species be able to regenerate from seed and spread outside plantations in a drier temperate Europe?Methods
We measured the relative growth rate, biomass allocation, root architecture, and phenotypic plasticity of Pseudotsuga menziesii seedlings sown in a common garden and grown under current precipitation and prolonged drought, respectively. The species’ competitive ability with respect to Pinus sylvestris L., the most drought-tolerant native conifer in Central Europe, was assessed during three growing seasons.Results
Pinus sylvestris seedlings had higher relative growth rates than did Pseudotsuga menziesii seedlings, first in terms of aboveground biomass and later in terms of shoot height. This resulted in heavier and taller seedlings after three growing seasons under both moist and dry conditions. Shorter vertical roots corresponded with lower survival of Pseudotsuga menziesii seedlings under dry conditions.Conclusion
Fast root proliferation allows Pinus sylvestris seedlings to reach deeper water pools that are less rapidly depleted during transient drought. By contrast, the shallow root system might put Pseudotsuga menziesii seedlings at the risk of desiccation during prolonged dry spells.64.
Jennifer D. Knoepp Barbara C. Reynolds D.A. Crossley Wayne T. Swank 《Forest Ecology and Management》2005,220(1-3):300-312
Soil nutrient concentrations decreased in an aggrading southern Appalachian forest over a 20-year period. Construction of nutrient budgets showed significant nutrient sequestration aboveground including increased forest floor mass. We hypothesized that the changes in forest floor mass resulted from decreased litter decomposition rates because of decreased litter quality. In 1992 and 1993, we repeated a litter decomposition experiment conducted in 1969 and 1970 to test this hypothesis. In addition, we examined microarthropod populations and functional groups as litter decomposed. For four of the five species tested, first-year decomposition rates were about the same in both experiments. Initial litter nutrient concentrations of P were lower in all tree species in the most recent sampling. N, Ca, and Mg concentrations also declined in some species. These declines often resulted in decreased nutrient release rates during decomposition. Microarthropod populations differed significantly among litter species, as well as between years (probably resulting from differences in growing-season rainfall). For some litter species we found significant relationships between microarthropod populations and nutrient concentration (primarily C and N); however, most r2-values were low. Data suggest that changes in forest floor mass probably resulted from decreased litter quality and that those changes may have an effect on microarthropod populations. 相似文献
65.
66.
67.
Surface Electromyography of the Vastus Lateralis,Biceps Femoris,and Gluteus Medius Muscle in Sound Dogs During Walking and Specific Physiotherapeutic Exercises 
下载免费PDF全文
下载免费PDF全文 68.
69.
70.