首页 | 本学科首页   官方微博 | 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   1186篇
  免费   57篇
  国内免费   2篇
林业   221篇
农学   41篇
基础科学   5篇
  342篇
综合类   82篇
农作物   36篇
水产渔业   76篇
畜牧兽医   351篇
园艺   17篇
植物保护   74篇
  2023年   5篇
  2022年   12篇
  2021年   24篇
  2020年   25篇
  2019年   25篇
  2018年   28篇
  2017年   28篇
  2016年   31篇
  2015年   28篇
  2014年   27篇
  2013年   96篇
  2012年   61篇
  2011年   55篇
  2010年   40篇
  2009年   51篇
  2008年   75篇
  2007年   56篇
  2006年   60篇
  2005年   48篇
  2004年   43篇
  2003年   36篇
  2002年   41篇
  2001年   24篇
  2000年   16篇
  1999年   9篇
  1998年   11篇
  1997年   7篇
  1995年   23篇
  1994年   13篇
  1992年   6篇
  1991年   5篇
  1990年   12篇
  1989年   8篇
  1987年   6篇
  1986年   5篇
  1985年   5篇
  1984年   6篇
  1983年   8篇
  1982年   9篇
  1981年   5篇
  1970年   7篇
  1968年   8篇
  1958年   6篇
  1941年   5篇
  1939年   5篇
  1938年   10篇
  1935年   5篇
  1931年   9篇
  1927年   5篇
  1899年   4篇
排序方式: 共有1245条查询结果,搜索用时 15 毫秒
91.
This paper examines the roles of physical capital, human capital, and social capital in China's economic growth during the reform period 1981–2010. Empirical estimation confirms that physical capital and human capital contribute to the economic growth, probably due to the capital accumulation and the improvement of labor productivity. The impact of social capital turns from being insignificant in the 1980s and the 1990s to be positive in the 2000s, suggesting its rising importance in recent decades. A declining role of physical capital in the economic growth in China from 1990s to 2000s is also found. The findings hold for several additional robustness checks, including focusing on longer term determinants of the economic growth, subregional analysis, and endogeneity. Furthermore, the foreign direct investment inflows and adjustment of economic ownership structure are also important for economic growth in China.  相似文献   
92.
Protoplast fusion can be used to produce somatic hybrids of species that cannot be obtained by sexual hybridization. The possibility to introgress genes from Solanum species into the cultivated tomato species Lycopersicon esculentum, and to obtain novel cytoplasm-nucleus combinations (cybrids) was considered as an important strategy to extend the genetic variation available for tomato breeding. Somatic hybrids between L. esculentum and other Lycopersicon species, as well as between L. esculentum and Solanum or Nicotiana species, have been produced. Specific mutants, genotypes with antibiotic resistances, and metabolic inhibition by iodoacetate or iodoacetamide and irradiation were used for the selection of hybrids. In addition, the improvement of protoplast culture techniques and the use of the favourable tissue culture traits derived from species such as L. peruvianum, which have been introduced into tomato by classical breeding, allowed the efficient recovery of somatic hybrids. However, the occurrence of somatic incongruity in fusion combinations of L. esculentum and Solanum and even more in L. esculentum and Nicotiana, did not allow the production of true cybrids and/or fertile hybrids, indicating the importance of both cytoplasm-nucleus and nucleus-nucleus interactions in somatic incongruity. Another problem with fusions between distantly related species is the strongly reduced fertility of the hybrids and the very limited homoeologous recombination between chromosomes of the parental species. Partial genome transfer from donor to recipient through microprotoplast (+) protoplast fusion, and the production of monosomic or disomic chromosome addition lines, light overcome some of these problems. In symmetric somatic hybrids between L. esculentum and S. tuberosum the occurrence of limited somatic and meiotic recombination was demonstrated. Fertile progeny plants could be obtained, though at a low frequency, when embryo rescue was performed on a large scale after backcrossing hexaploid somatic tomato (+) potato hybrids with a tetraploid potato genotype. The potential value of genomic in situ hybridization (GISH) and RFLPs for the analysis of the genome/chromosome composition of the hybrids has been demonstrated for intergeneric somatic hybrids between Lycopersicon and Solanum.Abbreviations cpDNA chloroplast DNA - mtDNA mitochondrial DNA  相似文献   
93.
94.
Acidification of surface waters and forest soils is severe in large parts of southern Sweden. The shallow groundwaters are also affected. Large scale liming of surface waters and streams is in operation, often combined with wetland liming to limit the effects of acid episodes, e.g. at snow melt. Acid episodes are perhaps the most severe problem in limed surface waters and in many as yet well buffered waters, because of temperature-layered acid inflow, often superficial. As a result of some investigations, a large scale forest liming programme covering 6.500–10.000 km2 was recently suggested. The main objectives of this forest liming programme are to retard cation depletion and to prevent nutrient imbalance and forest decline in acidified areas. This paper deals with the effects of forest soil liming on streams and surface waters. The response of water chemistry is very dependent on hydrological and soil properties. Although pH itself may be little affected by liming, the acidity (or negative ANC) decreases, inorganic Al-species decrease and the Al/BC-ratio increases in the runoff water. Especially interesting is that this is also true during acid episodes. This means that toxicity for acid sensitive biota decreases. These results indicate that large scale liming may have beneficial effects on surface water chemistry. Furthermore, as surface waters are expected to respond to smaller decreases in acid deposition than do forests soils, forest soil liming may allow less frequent liming of lakes. Consequently, forest soil liming in combination with the anticipated emission reductions may have very beneficial results on surface waters in certain areas of Sweden.  相似文献   
95.
Thirteen streams in the province of Jämtland in northern Sweden were monitored during spring in 1995 (December 94 – July 95) to study changes in water chemistry and metal concentrations during snow melt. The brooks are not treated with lime, with one exception, and can be approximately divided into three groups according to watershed characteristics; A) > 65% above tree line, B) > 65 % wetland, C) > 55% forested. During peak flow, pH dropped 0.5–2.5 units and alkalinity generally to zero. The brooks above tree line were lowest in base cations and reached the lowest pH-values (4.4–4.6) during peak flow, while sulphate levels were about the same as in the forested watersheds. During peak flow, organic anions showed the highest increase in the wetland and forested catchments. Compared to base flow, Al, Zn, Pb and to some extent Mn was enriched during peak flow. The results also illustrate the difficulties in generalising the reasons for alkalinity losses during spring flood in this kind of streams. In some of the brooks, the use of either base cations or silica, when calculating dilution effects, gave deviating results concerning the relative contribution of strong acids in the snow pack.  相似文献   
96.
Composition and Speciation of Soil Solution collected in a Heavy Metal polluted calcareous Soil Close to a brass foundry, which had emitted heavy metal containing dusts for over 80 years, soil water was collected in the topsoil (18 cm) and in the subsoil (40 cm) of a severely polluted Calcic Fluvisol by means of polyethylen suction cups over a period of 2 years. The total metal content of the topsoil (extracted with 2M HNO3 at 100 °C for 2 hours) was 38 nmol g?1, 24 μmol g?1, and 25 μmol g?1 for Cd, Cu, and Zn, respectively. The mean heavy metal concentrations of the soil solution were 0.5 mol L?1, 300 nmol L?1, and 200 nmol L?1 in the topsoil and 0.6 nmol L?1, 90 nmol L?1, and 30 nmol L?1 in the subsoil for Cd, Cu, and Zn, respectively. Solubility calculations showed that the soil solutions were undersaturated with respect to heavy metal carbonates as well as to hydroxides. It seems that the heavy metal concentration is determined by sorption processes rather than by precipitation. The composition of the soil solution has been shown to be governed by the presence, of calcite, by the soil temperature and by the partial pressure of CO2 in the soil air. The pCO2 in the soil air (in both depths) has been estimated at 2 mbar during the winter term and at 20 mbar during the summer term. A corresponding increase of the concentration of macroelements (Ca, Mg, Na) as well as of total dissolved carbonate and of dissolved organic matter (DOC) has been measured in the summer half year. No significant seasonal variations of the heavy metal concentrations were detected and no correlations with concentrations of other components could be found.  相似文献   
97.
The Waldrapp ibis, Geronticus eremita Linnaeus, 1758, is known historically from Ancient Egypt, the Alps in southern Europe, the Near and Middle East, Morocco, and Algeria. This paper summarises what is known about the species from archaeological findings and historical expeditions to these regions, and outlines the taxonomic confusion which characterised this bird for four centuries—the 16th to 19th.

The ancient and recent decline of populations of the Waldrapp is discussed. The birds are at present restricted to about twelve breeding sites, totalling only about 400 individuals, in Turkey and Morocco, with an additional 400-plus specimens in captivity in zoos of the world. A plea is made for the conservation of this endangered species.  相似文献   

98.
Modeling infectious diseases data is a relatively young research area in which clustering and stratification are key features. It is not unlikely for these data to have missing values. If values are missing completely at random, the analysis on the complete cases is valid. However, in practice this assumption is usually not fulfilled. This article shows the effect of ignoring missing data in modeling the force of infection of the bovine herpesvirus-1 in Belgian cattle and proposes the use of weighted generalized estimating equations with constrained fractional polynomials as a flexible modeling tool.  相似文献   
99.
Integrated monitoring of ecosystems (IM) is an international co-operative programme (ICP) to control effects of air pollution and climate change on water, soil and biological systems. It is a part of the Convention on Long-Range Transboundary Air Pollution (CLRTAP) of the United Nations Economic Commission of Europe (UN/ECE). The ICP-IM is undertaken on sites/catchments to investigate acidification, eutrophication and heavy metals with an integrated approach. In Sweden, long-term time series from forest ecosystems, with a long and stable continuity, will reveal trends and changes in processes and enable modelling to be undertaken. Investigations of acidity/alkalinity in relation to mineral and organic acids indicated the importance of atmospheric deposition. Recent results show very high inorganic nitrogen retention (99%), a net loss of sulphur originating mainly from organic horizons, and a high inorganic aluminium content in the illuvial soil horizons which could be detrimental to forests. Forest deficiency could also be caused by an observed ongoing translocation of Zn to deeper soil layers implying a movement towards increased release to surface waters.  相似文献   
100.
The emissions of SO2 in China correspond at present to 8–10 TgSyr?1. The rapid industrialization has caused a dramatic increase in the emissions in recent years and this increase is likely to continue. This paper describes studies of concentrations and effects of acidifying substances in parts of the Guizhou and the Sichuan provinces where the S-emissions are large. A small catchment about 10 km from Guiyang centre was equipped with instruments for studies of soils, soil water and streamwater chemistry. The molar ratio Al/(Ca+Mg) is > 0.8 in soil water in some places. Two small streams have median pH-values about 4.6 and 5.1. Laboratory studies with selected Chinese soils showed that the anion adsorption was low. These studies gave also important information on soil sensitivity. The studies confirm that acid deposition may affect soils in parts of south-western China, but the sensitivity varies dramatically and there is a strong need for more information.  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