全文获取类型
收费全文 | 12235篇 |
免费 | 832篇 |
国内免费 | 111篇 |
专业分类
林业 | 1154篇 |
农学 | 506篇 |
基础科学 | 103篇 |
3140篇 | |
综合类 | 525篇 |
农作物 | 702篇 |
水产渔业 | 1312篇 |
畜牧兽医 | 4429篇 |
园艺 | 223篇 |
植物保护 | 1084篇 |
出版年
2024年 | 19篇 |
2023年 | 94篇 |
2022年 | 170篇 |
2021年 | 313篇 |
2020年 | 328篇 |
2019年 | 316篇 |
2018年 | 550篇 |
2017年 | 566篇 |
2016年 | 561篇 |
2015年 | 433篇 |
2014年 | 512篇 |
2013年 | 779篇 |
2012年 | 977篇 |
2011年 | 900篇 |
2010年 | 575篇 |
2009年 | 478篇 |
2008年 | 689篇 |
2007年 | 674篇 |
2006年 | 565篇 |
2005年 | 547篇 |
2004年 | 463篇 |
2003年 | 428篇 |
2002年 | 352篇 |
2001年 | 243篇 |
2000年 | 251篇 |
1999年 | 207篇 |
1998年 | 62篇 |
1997年 | 64篇 |
1996年 | 43篇 |
1995年 | 42篇 |
1994年 | 29篇 |
1993年 | 24篇 |
1992年 | 66篇 |
1991年 | 47篇 |
1990年 | 51篇 |
1989年 | 58篇 |
1988年 | 44篇 |
1987年 | 44篇 |
1986年 | 51篇 |
1985年 | 55篇 |
1984年 | 42篇 |
1983年 | 37篇 |
1982年 | 20篇 |
1979年 | 34篇 |
1978年 | 32篇 |
1977年 | 31篇 |
1974年 | 30篇 |
1973年 | 45篇 |
1972年 | 34篇 |
1971年 | 31篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
901.
To evaluate differences in growth and adaptability of maritime pine (Pinus pinaster Ait.), we studied growth, polycyclism, needle tissue carbon isotope composition (delta(13)C) as an estimate of water-use efficiency (WUE) and survival of seven populations at 10 years of age growing in a performance trial at a provenance test site in Escaroupim, Portugal. Six populations were from relatively high rainfall sites in Portugal and southwestern France (Atlantic group), and one population was from a more arid Mediterranean site in Spain. There were significant differences between some populations in total height, diameter at breast height, delta(13)C of bulk needle tissue, polycyclism and survival. A population from central Portugal (Leiria, on the Atlantic coast) was the tallest and had the lowest delta(13)C. Overall, the variation in delta(13)C was better explained by the mean minimum temperatures of the coldest month than by annual precipitation at the place of origin. Analyses of the relationships between delta(13)C and growth or survival revealed a distinct pattern for the Mediterranean population, with low delta(13)C (and WUE) associated with the lowest growth potential and reduced survival. There were significant negative correlations between delta(13)C and height or survival in the Atlantic group. Variation in polycyclism was correlated with annual precipitation at the place of origin. Some Atlantic populations maintained a high growth potential while experiencing moderate water stress. A detailed knowledge of the relationships between growth, survival and delta(13)C in contrasting environments will enhance our ability to select populations for forestry or conservation. 相似文献
902.
Characteristics and decomposition rates of pruning residues from a shaded coffee system in Southeastern Brazil 总被引:1,自引:0,他引:1
In the Zona da Mata Mineira of Southeastern Brazil the development of sustainable land requires the integration of crops with trees. The objectives of this study then were to (i) characterize prunings from the main tree species in an agroforestry system; (ii) determine the effects of the physical and chemical characteristics of the prunings on their decomposition patterns in the laboratory; (iii) assess the effect of mixing leaves of different species on decomposition rates; and (iv) propose a decomposition index for the residues studied. The study was carried out with pruning residues from Cajanus cajan, Solanum variable, Cassia ferruginea, Piptadenia gonoacantha, Croton urucurana, and Melinis multiflora. The materials were characterized for total C, N, P, Ca, Mg and K contents; lignin, cellulose, hemicellulose and soluble polyphenols contents. The pruning residues had high polyphenols and lignin contents, high C:N and C:P ratios, and low contents of Ca, Mg, and K. The low decomposition rates of the prunings were related to the P, K, hemicellulose and polyphenol contents. The rates of N mineralization from most of the residues indicate that there is a potential to supply the needs of a crop of maize. The residues of some species, if decomposed alone, would not supply sufficient nutrients, and need to be mixed with leaves of other species.This revised version was published online in November 2005 with corrections to the Cover Date. 相似文献
903.
Analysis of the ethyl acetate extract of the aerials parts of Podocalyx loranthoides led to the isolation of I 7, II 4'-dimethylamentoflavone (1) and II 4'-methylamentoflavone (2). Compound (1) gave a moderate effect against Leishmania mexicana promastigotes. 相似文献
904.
The potential of the entomopathogenic nematode Steinernema feltiae (Filipjev) (Rhabditida: Steinernematidae) Rioja strain was assessed under laboratory conditions. Last instar larvae of Leptinotarsa decemlineata Say (Coleptera: Chrysomelidae), Spodoptera littoralis Boisdouval (Lepidoptera: Noctuidae) and Trichoplusia ni Hübner (Lepidoptera: Noctuidae) were exposed to infective juveniles (IJs) under laboratory conditions. Larval mortality,
days to larval death, infection cycle length, and reproductive potential were recorded. Efficacy was assessed performing dose–response
experiments. The results indicate that control of L. decemlineata with S. feltiae Rioja strain is not economically profitable (LC50 = 99.61 IJs/cm2), whereas results obtained for T. ni (LC50 = 0.27 IJs/cm2) are promising. Due to the life cycle of this insect, the efficacy needs to be investigated in foliar application studies.
The effects on S. littoralis (LC50 = 0.69 IJs/cm2) was considered the most suitable for development of the Rioja strain as a biocontrol agent for soil application. 相似文献
905.
Benigno González-Rivas Mulualem Tigabu Guillermo Castro-Marín Per Christer Odén 《林业研究》2009,20(2):99-104
A study was conducted to examine the germination requirements of Cedrela odorata, Guaiacum sanctum and Calycophyllum candidissimum seeds, and the effect of light intensity on survival and growth of C. Odorata and G. Sanctum seedlings planted on open, partially-open and beneath the canopy of a dry forest in Nicaragua. The results show that germination of C. Candidissimum seeds was significantly higher in light than darkness at constant temperatures ranging from 20–35oC. Both C. Odorata and G. Sanctum seeds germinated in light and darkness to a varying extent, and the optimal germination temperature was around 20-25oC. Alternating temperature regimes did not improve germination. Survival of G. Sanctum seedlings was 70% beneath the canopy and 80% in the open, and its relative growth rate in collar diameter was 1.88 mm/month in the open and 1.42 mm/month in the understory. Survival was generally poor for C. Odorata, particularly in the understory. We concluded that light is an absolute requirement for the germination of C. Candidissimum seeds while germination of C. Odorata and G. Sanctum seeds are more sensitive to temperature. Owing to its high survival rate on open site, G. Sanctum could serve as a candidate species for reforestation of degraded sites. 相似文献
906.
Juan A. Oliet María Tejada K. Francis Salifu Aleyda Collazos Douglass F. Jacobs 《European Journal of Forest Research》2009,128(3):253-263
Holm oak (Quercus ilex L.) seedlings were exponentially (E) nutrient loaded using incremental increases in fertilizer addition or conventionally
(C) fertilized using a constant fertilizer rate during nursery culture. The fertility treatments (mg N plant−1) were control (0), 25E, 100E, and 100C. Subsequently, 1-year-old plants were transplanted under simulated soil fertility
gradients in a greenhouse to evaluate effects of nutrient loading and post-transplant fertility on seedling performance. Post-transplant
fertility consisted of fertilizing plants at two rates (0 vs. 200 mg N plant−1). A water-soluble fertilizer 20-20-20 was supplied in both nursery and post-transplant experiments. Nutrient loading increased
plant N content by 73% in 100E and by 75% in 100C relative to controls, although no significant differences were detected
between constant and exponential fertilization regimes at the 100 mg N plant−1 rate. When transplanted, nutrient loading promoted post-transplant root growth relative to shoot, implicating potential to
confer competitive advantage to loaded holm oak seedlings after trans-planting. In contrast, post-transplant fertility increased
new shoot dry mass by 140% as well as N, P and K content relative to controls. Results suggest that holm oak seedlings can
be successfully nutrient loaded in the nursery at higher fertility rates, improving its potential to extend new roots, but
alternative fertilization regimes and schedules that better fit nutrient availability to the growth rhythm and conservative
strategy of this species must be tested. 相似文献
907.
Ciro Abbud Righi Paulo Maurício Lima de Alencastro Graa Carlos Clemente Cerri Brigitte Josefine Feigl Philip Martin Fearnside 《Forest Ecology and Management》2009,258(11):2535-2546
Estimates of greenhouse-gas emissions from deforestation are highly uncertain because of high variability in key parameters and because of the limited number of studies providing field measurements of these parameters. One such parameter is burning efficiency, which determines how much of the original forest's aboveground carbon stock will be released in the burn, as well as how much will later be released by decay and how much will remain as charcoal. In this paper we examined the fate of biomass from a semideciduous tropical forest in the “arc of deforestation,” where clearing activity is concentrated along the southern edge of the Amazon forest. We estimated carbon content, charcoal formation and burning efficiency by direct measurements (cutting and weighing) and by line-intersect sampling (LIS) done along the axis of each plot before and after burning of felled vegetation. The total aboveground dry biomass found here (219.3 Mg ha−1) is lower than the values found in studies that have been done in other parts of the Amazon region. Values for burning efficiency (65%) and charcoal formation (6.0%, or 5.98 Mg C ha−1) were much higher than those found in past studies in tropical areas. The percentage of trunk biomass lost in burning (49%) was substantially higher than has been found in previous studies. This difference may be explained by the concentration of more stems in the smaller diameter classes and the low humidity of the fuel (the dry season was unusually long in 2007, the year of the burn). This study provides the first measurements of forest burning parameters for a group of forest types that is now undergoing rapid deforestation. The burning parameters estimated here indicate substantially higher burning efficiency than has been found in other Amazonian forest types. Quantification of burning efficiency is critical to estimates of trace-gas emissions from deforestation. 相似文献
908.
Empirical models for predicting the production of wild mushrooms in Scots pine (Pinus sylvestris L.) forests in the Central Pyrenees 总被引:1,自引:0,他引:1
José Antonio Bonet Timo Pukkala Christine R. Fischer Marc Palahí Juan Martínez de Aragón Carlos Colinas 《Annals of Forest Science》2008,65(2):206-206
Mushroom picking has become a widespread autumn recreational activity in the Central Pyrenees and other regions of Spain. Predictive models that relate mushroom production or fungal species richness with forest stand and site characteristics are not available. This study used mushroom production data from 24 Scots pine plots over 3 years to develop a predictive model that could facilitate forest management decisions when comparing silvicultural options in terms of mushroom production. Mixed modelling was used to model the dependence of mushroom production on stand and site factors. The results showed that productions were greatest when stand basal area was approximately 20 m2 ha?1. Increasing elevation and northern aspect increased total mushroom production as well as the production of edible and marketed mushrooms. Increasing slope decreased productions. Marketed Lactarius spp., the most important group collected in the region, showed similar relationships. The annual variation in mushroom production correlated with autumn rainfall. Mushroom species richness was highest when the total production was highest. 相似文献
909.
Miren del Río Rafael Calama Isabel Cañellas Sonia Roig Gregorio Montero 《Annals of Forest Science》2008,65(3):308-308
The effect of different thinning intensities on growth and yield was studied in Pinus sylvestris L. stands at the south-western limit of its distribution area (Central Spain), using five long-term thinning trials. Data were analysed collectively considering several factors (trial, block, plot and period) as random effects. Total volume and volume increment decreased with thinning intensity, this loss being more significant in the case of moderate and heavy thinning. No difference was found among treatments for total basal area or the increment in basal area. The results revealed an optimum basal area (Assmann’s definition) between 85 and 100% of the basal area in unthinned plots. Volume growth loss associated with heavy thinnings (reduction of 18% in volume increment) was smaller than that reported in Central and Northern European regions (greater than 25%). Height increment was not influenced by thinning, whereas dominant and quadratic mean diameter increments increased with the thinning intensity. The response of diameter growth to thinning was greater at younger ages (less than 50 y) and in medium-sized trees. 相似文献
910.
Jaime Puértolas Marta Pardos María Dolores Jiménez Ismael Aranda José Alberto Pardos 《Annals of Forest Science》2008,65(6):611-611