全文获取类型
收费全文 | 6355篇 |
免费 | 521篇 |
国内免费 | 32篇 |
专业分类
林业 | 330篇 |
农学 | 437篇 |
基础科学 | 54篇 |
973篇 | |
综合类 | 695篇 |
农作物 | 275篇 |
水产渔业 | 387篇 |
畜牧兽医 | 3019篇 |
园艺 | 93篇 |
植物保护 | 645篇 |
出版年
2020年 | 62篇 |
2019年 | 113篇 |
2018年 | 210篇 |
2017年 | 218篇 |
2016年 | 173篇 |
2015年 | 193篇 |
2014年 | 190篇 |
2013年 | 347篇 |
2012年 | 260篇 |
2011年 | 313篇 |
2010年 | 245篇 |
2009年 | 207篇 |
2008年 | 301篇 |
2007年 | 266篇 |
2006年 | 232篇 |
2005年 | 198篇 |
2004年 | 192篇 |
2003年 | 194篇 |
2002年 | 144篇 |
2001年 | 234篇 |
2000年 | 179篇 |
1999年 | 169篇 |
1998年 | 69篇 |
1997年 | 74篇 |
1996年 | 53篇 |
1995年 | 59篇 |
1994年 | 44篇 |
1993年 | 51篇 |
1992年 | 103篇 |
1991年 | 105篇 |
1990年 | 108篇 |
1989年 | 106篇 |
1988年 | 85篇 |
1987年 | 80篇 |
1986年 | 74篇 |
1985年 | 58篇 |
1984年 | 64篇 |
1983年 | 52篇 |
1981年 | 44篇 |
1980年 | 48篇 |
1979年 | 72篇 |
1978年 | 52篇 |
1977年 | 52篇 |
1976年 | 59篇 |
1975年 | 52篇 |
1974年 | 61篇 |
1973年 | 75篇 |
1972年 | 44篇 |
1971年 | 45篇 |
1969年 | 43篇 |
排序方式: 共有6908条查询结果,搜索用时 15 毫秒
31.
Seven types of fungal melanins were compared with humic compounds from chernozem and podzol soils and with commercial humic acid. The results indicate some similarities but also distinct differences between the melanins and humic substances in spectral characteristics in the UV and visible regions and in their resistance to thermal degradation. 相似文献
32.
33.
In well-aerated culture solutions Ca-montmorillonite at 0.25% concentration markedly accelerated and increased growth, glucose consumption and CO2 evolution by various Streptomyces, Micromonospora and Nocardia species. The montmorillonite was a little more active than Ca-humate and was usually still but somewhat less effective when confined to dialysis tubing. Ca-exchange resin, Na2SiO3 and finely powdered CaCO3 exerted very little or no effect. In many cultures the relation of glucose consumption to biomass formation indicated a more efficient use of the glucose C for cell synthesis in the presence of clay. In other cultures the greater biomass formation was associated with a more rapid and complete utilization of the glucose present. 相似文献
34.
The profile distribution of Be is given for a toposequence of podzols, brown earths, stagnogley, forest bog and “Ockererde” (slope gley with infiltrated Al, Mn and Fe). Analysed were the parent rock, fine earth, four fractions of mineral soil (fine skeleton, sand, silt, clay) and the needles of spruce trees.Beryllium is involved in selective weathering processes of elements. Accordingly, the Be-content increases from the fine skeleton to sand, rock, silt and clay, progressively. Through pedogenetic enrichment, Be accumulates in the mica-rich silt fraction.Uptake of Be by spruce is very low even from these acid soils. The accumulation in the needles becomes greater with increasing age.The pedochemical behaviour of Be is very similar to that of Al.Be is incorporated very little in the organic matter cycle, and is therefore not enriched in the soil humus.The brown earth has lost Be, especially in the Ah-horizon.In the Ae-horizon of the podzol, loss of Be is even greater; the loss is partly offset by Be-enrichment in the B-horizon.In the profiles of depressions, the Be-content is high and is ascribed to lateral flow from the slopes. Be-enrichment is especially high in horizons with a pH of about 5.The Be-balance of the watershed indicates an export of about 10% of the original amount. 相似文献
35.
Dried sclerotia of Sclerotium delphinii rotted in moist soil whereas those of Sclerotium cepivorum. Botrytis cinerea and Botrytis tulipae did not. A number of fungi invaded dried sclerotia of S. delphinii in soil, the principal coloniser found in the first sampling being Trichodermu hamatum. Leakage of 14C compounds from dried labelled sclerotia placed in water was rapid and was little affected by variation in leakage temperature from 1 to 25°C or by prolonging the drying period beyond a day. Leakage from dried sclerotia which were allowed to imbibe water through a small part of their surface was much reduced. Sclerotia which were redried after leakage leaked again when returned to water but with all four fungi the first of three leakage cycles gave the highest 14C levels. Loss in dry weight in the first leakage cycle was greater with S. delphinii than with the other three fungi and this may be related to the poor survival of dried sclerotia of S. delphinii in moist soil. Substances lost during leakage appear to originate from within sclerotial hyphae rather than from the hyphal free space. 相似文献
36.
ANDRIJ Z. HORODYSKY DAVID W. KERSTETTER ROBERT J. LATOUR JOHN E. GRAVES 《Fisheries Oceanography》2007,16(3):240-256
Researchers have applied numerous techniques to improve billfish stock assessments, including habitat‐based models that incorporate behavioral and oceanographic parameters to standardize historical catch‐per‐unit‐effort time‐series data. These methods have allowed researchers to account for significant changes in the depths of pelagic longline (PLL) gear deployments over time. This study presents habitat‐use data recovered from high‐resolution 5‐ and 10‐day pop‐up satellite archival tags (PSATs) attached to 47 surviving white marlin released from commercial and recreational fishing gears offshore of the U.S. East Coast, the northern Caribbean, and Venezuela between 2002 and 2004. Data recovered from transmitting tags indicated that white marlin spent nearly half of their time associated with warm, near‐surface waters (<10 m). All fish displayed frequent short duration (mean: 39.8 min) vertical excursions from surface waters to depths averaging 51 m. Qualitative and multivariate classifications of data from completely transmitted movements of surviving white marlin revealed two major types of descents: one pattern was characterized by deep ‘V’‐shaped excursions of relatively short duration (mean: 23.4 min) while the other featured descents that were more broadly ‘U’‐shaped and confined to a specific depth range for an extended period of time (mean: 75.8 min). Based on the frequency, persistence, and patterns of these vertical movements, white marlin appear to direct a considerable proportion of foraging effort well below surface waters, a behavior that may account for relatively high catch rates of white marlin on some deep‐set PLL deployments. 相似文献
37.
38.
39.
40.
Factors involved in the establishment of Taraxacum officinale (dandelion) seedlings in grasslands and plant mortality in the first growing season were studied during 2008–2011. The study was conducted at 11 grassland sites that differed in the quantity of seed rain and the intensity of seedling predation by slugs. In April, of each study year, 10 plots open to the sedimentation of naturally dispersed seeds were established at each site, five plots of which were also sown with 250 T. officinale seeds each. The amount of natural seed rain, slug feeding activity and number of plants in each plot were then recorded at monthly intervals. Seed rain and seedling predation accounted for c. 50% of the variation in seedling establishment. Populations of established seedlings then declined through October, at which point they were at 25% of the original abundance on average. The percentage of plants that survived varied among sites and years. The temporal and spatial variation in T. officinale establishment and survival of young plants was enormous, even within a small area. Predation of seedlings by slugs can effectively prevent the establishment of T. officinale in grassland. This only occurred, however, at slug abundances not acceptable under conditions of public access. 相似文献