全文获取类型
收费全文 | 8819篇 |
免费 | 2044篇 |
国内免费 | 58篇 |
专业分类
林业 | 1839篇 |
农学 | 412篇 |
基础科学 | 414篇 |
1569篇 | |
综合类 | 3985篇 |
农作物 | 562篇 |
水产渔业 | 526篇 |
畜牧兽医 | 628篇 |
园艺 | 209篇 |
植物保护 | 777篇 |
出版年
2025年 | 125篇 |
2024年 | 283篇 |
2023年 | 293篇 |
2022年 | 346篇 |
2021年 | 389篇 |
2020年 | 353篇 |
2019年 | 421篇 |
2018年 | 229篇 |
2017年 | 317篇 |
2016年 | 379篇 |
2015年 | 342篇 |
2014年 | 499篇 |
2013年 | 554篇 |
2012年 | 713篇 |
2011年 | 783篇 |
2010年 | 615篇 |
2009年 | 562篇 |
2008年 | 525篇 |
2007年 | 511篇 |
2006年 | 408篇 |
2005年 | 364篇 |
2004年 | 239篇 |
2003年 | 254篇 |
2002年 | 217篇 |
2001年 | 171篇 |
2000年 | 180篇 |
1999年 | 164篇 |
1998年 | 112篇 |
1997年 | 97篇 |
1996年 | 82篇 |
1995年 | 60篇 |
1994年 | 55篇 |
1993年 | 45篇 |
1992年 | 58篇 |
1991年 | 49篇 |
1990年 | 40篇 |
1989年 | 28篇 |
1988年 | 24篇 |
1987年 | 18篇 |
1986年 | 5篇 |
1985年 | 4篇 |
1984年 | 4篇 |
1983年 | 1篇 |
1980年 | 2篇 |
1956年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 22 毫秒
1.
The Thai river sprat, Clupeichthys aesarnensis Wongratana, is a clupeid with a short life span, and supports artisanal fisheries in a number of reservoirs in the Mekong Basin. The growth parameters, mortality rates and the status of the Thai river sprat in Sirinthorn Reservoir (28 800 ha), NE Thailand (15°N; 105°E), are presented. The fishery is based on lured lift‐nets, operated 7–14 days in the new moon period, September to April each year. It was shown that the von Bertalanffy growth function (VBGF) model was Lt (mm) = 78.43[1 ? exp{?0.211[t ? (?0.7996)]}] and its growth conformed to an isometric pattern. Natural mortality rate (month?1) was 0.13 month?1. Total mortality rates ranged from 0.69 to 1.53 month?1 depending on the weather and the fishing season. Recruitment was continuous throughout the year but peaked in June and July. The yield per recruit model indicated that the exploitation rate of this fishery is probably too high. 相似文献
2.
3.
D. Moret-Fernández Y. PueyoC.G. Bueno C.L. Alados 《Agricultural Water Management》2011,98(12):1822-1827
Pasture productivity depends on soil hydro-physical properties, which in turn are deeply affected by livestock grazing. However, the comparative response of different soil types, and particularly gypseous soil types, to grazing has hardly been studied before. This paper compares the effect of grazing on the soil hydro-physical properties of silty gypseous (Gy) and non-gypseous (NGy) soils located in a semi-arid region (Middle Ebro Valley, NE, Spain). Two different soil managements were selected: ungrazed natural shrubland (N) and grazed shrubland (GR) soils. The gypsum, CaCO3 and organic matter content (OM), soil texture, soil bulk density (ρb), penetration resistance (PR), saturated sorptivity (S), hydraulic conductivity (K), and the water retention curve (WRC) for undisturbed soil samples from 1 to 10 cm depth soil layer were measured. The ρb and PR in NGy soils were significantly higher than those observed in the Gy ones. Soil compaction due to grazing treatment tended to increase ρb and decrease the K and S values. While no differences in PR were observed in the Gy soils between grazing treatments, the PR measured in the NGy soils under GR was significantly higher than the corresponding values observed under N. Differences in K and S between GR and N treatments were only significant (p < 0.05) in NGy soils, where K and S values under the N treatment were almost four times greater than the corresponding values measured under GR. Overall, no differences in the WRCs were observed between soil types and grazing treatments. While the WRCs of NGy soils were not significantly affected by the grazing treatment, Gy soils under N treatment present a significantly higher level of soil macropores than under GR treatment. The hydro-physical features of Gy soils tended to be less affected by grazing than those of the NGy soils. These results suggest that livestock grazing, in both Gy and NGy soils, has a negative effect on the physical soil properties, which should be taken into account by land managers of these semi-arid regions where silty gypseous and non-gypseous areas coexist. 相似文献
4.
Steven J. Cooke Abigail J. Lynch John J. Piccolo Julian D. Olden Andrea J. Reid Steve J. Ormerod 《水产资源保护:海洋与淡水生态系统》2021,31(6):1499-1511
- In 1949, Aldo Leopold formalized the concept of the ‘land ethic’, in what emerged as a foundational and transformational way of thinking about natural resource management, biodiversity conservation, and stewardship in terrestrial systems. Yet, the land ethic has inherent linkages to aquatic ecosystems; Leopold himself conducted research on rivers and lakes, and freshwater ecosystems figured widely in his writing.
- We reflect on the land ethic and other aspects of Leopold's scholarship to identify key messages that provide insight into the stewardship and management of freshwater ecosystems around the globe. We also frame what we call the ‘freshwater ethic’ around Leopold's legacy. Although Leopold could not have envisaged the stressors affecting modern aquatic ecosystems, his core principles remain salient. These apply not only to ecosystem protection, but also to the ethics of modern conservation economics, sustainability, and the protection of natural capital, in which lakes, rivers, and wetlands now figure prominently.
- We identify key ‘Aldo-inspired’ recommendations for protecting and restoring freshwater ecosystems in the Anthropocene that emanate directly from his writings (e.g. adopt an ecosystem approach, identify win–win–win scenarios, recognize the irreplaceability of wild waters, and strive for freshwater optimism).
- In an epoch where links between people and nature are becoming more explicit in environmental management, policy, and governance, we suggest that Aldo Leopold's work illustrates how inspirational, seminal thinkers have offered leadership in this domain. We contend that today there is still much that can be learned from Leopold, especially by the next generation of environmental practitioners, to ensure the effective stewardship of our aquatic ecosystems.
- We submit that the adoption of a freshwater ethic in parallel with Leopold's land ethic will enhance the stewardship of the world's increasingly threatened fresh waters by raising the profile of the plight of fresh waters and identifying enduring actions that, if embraced, will help conserve and restore biodiversity.
5.
长距离天然气管道干空气干燥技术及应用 总被引:1,自引:0,他引:1
阐述了长距离天然气管道干燥的必要性,介绍了目前国外天然气管道常用的干燥方法,建立了干空气干燥数学模型,分析了影响管道干燥效果的几个主要因素。针对忠武输气管道,详细介绍了干空气干燥技术及应用。 相似文献
6.
The screw withdrawal resistance and the mechanical behavior of agglomerated panels reinforced with coir and pejibaye fibers were evaluated using through and non-through screw withdrawal tests. Widespread commercially materials, such as Pinus solid wood, Medium Density Fiberboard (MDF) and plywood, were also tested in order to compare the behavior and results obtained with the lignocellulosic-reinforced agglomerated panels. Among the conventional materials tested, the highest maximum withdrawal load/thickness ratio was measured in solid wood. Panels reinforced with natural fibers showed lower withdrawal resistance than conventional materials. However, panels reinforced with coir fibers reached about 74% of the maximum strength achieved by MDF samples. Panels reinforced with pejibaye showed the worst mechanical performance. Hybridization between pejibaye and coir fibers resulted, however, in a performance improvement of approximately 50% in the maximum withdrawal load comparing with panels reinforced only with pejibaye fibers. The behavior along screw withdrawal was analyzed and could be split into four different steps. Each of these steps was correlated to the screw withdrawal rate, and also with the interactions between the material and the screw. The mechanisms involved in each step are presented and discussed. 相似文献
7.
W.A. Houston 《Mangroves and Salt Marshes》1999,3(1):29-40
A hailstorm in October 1994 was found to have moderately or severely impacted on 5.3% of the mangrove forests in Port Curtis. All mangrove species showed evidence of hail damage, including the three most common species (Rhizophora stylosa, Ceriops tagal and Avicennia marina). Physical effects of hail damage included stripping of leaves from plants, holes punched through leaves, bruising to bark, divots removed from bark, branch and plant death. Speciesspecific differences in vulnerability to the effects of hail were observed with C.tagal experiencing relatively higher mortality rates than the other two common mangrove species. A delayed pattern of mortality was observed in two species _ C.tagal and A.marina. Alterations to mangrove community structure included: (1) reductions in stem density, stem diameter and basal area, (2) reductions in canopy cover (based on a photographic index of foliage projective cover) and (3) changes in relative abundance of species in mangrove zones. Recovery was observed in some stands but others had not recovered to prehail levels of canopy cover two years after the hailstorm. Recovery had occurred by regeneration of fresh leaves but no recruitment of young plants had been observed during the study. The forests in the impact area were dominated by either Ceriops tagal or Rhizophora stylosa with Avicennia marinaas a subdominant in places. C.tagal dominated forests within the impact area were relatively more severely affected (41.8% in the severe category) than R.stylosa dominated communities (only 17.4% in the severe category). This indicated that C.tagal dominated forests were more vulnerable to the effects of hail damage than R.stylosa dominated forests. In addition, hailimpacted C.tagal dominated forests represented a relatively high percentage of the area of C.tagal dominated forests in Port Curtis (44.3%). This percentage was much higher than hailimpacted R.stylosa dominated forests in Port Curtis (2.7%). These two factors – relatively severe impact on C.tagal communities and a relatively higher percentage affected within the Port Curtis area – illustrate that hailstorms, as a form of natural disturbance, are an important influence on the forest ecology of mangrove ecosystems in this region. 相似文献
8.
为了进一步改善履带式拖拉机的传动性能,利用有限元分析技术展开设计.根据履带式拖拉机的变速箱体内部各轴系的位置关系与承受载荷程度,结合履带式拖拉机田间作业环境特点,充分将组成部件离散化,利用ANSYS进行分析,得到变速箱体压力分布云图,达到预期优化目标并进行性能试验.试验表明:变速箱内部组合结构局部优化后,固有频率试验值... 相似文献
9.
Gyungsoo Cha 《Journal of Forest Research》1997,2(3):147-152
Change in potential natural vegetation (PNV) distribution associated with climate change due to the doubling atmospheric carbon dioxide (2×CO2) was estimated with a global natural vegetation mapping system based on the modified Kira scheme to the globe and the continents. With an input of widely-distributed global climate data, the system interpolates data onto a 1° latitude by 1° longitude grid over the globe, generates estimates of vegetation type, and produces a composite PNV map. The input climate data corresponding to the 1×CO2 and 2×CO2 consists of observations prior to AD 1958 at 2,001 weather stations worldwide and the 2×CO2 simulation output from the Japan Meteorological Research Institue's General Circulation Model, respectively. As a result of the simulated global warming, the vegetation zones expanded mostly from the tropics toward the poles. PNV area changed by 6.98 billion (G) ha of the total land area (15.04 Gha) and potential forest area corresponding to the closed forest and open forest (woodland) reached 9.74 Gha with the increase of 1.29 Gha. The potential forest area in Europe had obvious advantages to the climate change accompanied with the increase of actual forest area. Although the actual forest area has decreased in North America and Asia, the potential forest area in these continents also benefitted from the climate change. In the end, the remaining continents tended to bear the brunt of the climate change. 相似文献
10.
B. K. Epperson 《New Forests》1992,6(1-4):257-278
The spatial pattern and structure of genetic variation are important aspects of the population genetics of forest stands. Combined with limits to seed and pollen dispersal, spatial structure affects the level of inbreeding and the action of natural selection. The genetic constitution of stand regeneration, following different forestry practices, is also influenced by spatial structure. For example, natural regeneration with seed trees involves sampling seed trees from a stand that may be genetically nonhomogeneous. This paper reviews theoretical and empirical results on spatial patterns of genetic variation, produced under limited gene flow and selection, in terms of recently developed spatial statistics (e.g., spatial autocorrelation). Genetic correlations in samples from spatially structured populations are also described, as well as how spatial samples can be used to characterize the structure of genetic variation, and how inferences can be made about (spatially distributed) components of fitness and yield. 相似文献