全文获取类型
收费全文 | 2973篇 |
免费 | 171篇 |
国内免费 | 81篇 |
出版年
2024年 | 7篇 |
2023年 | 41篇 |
2022年 | 30篇 |
2021年 | 56篇 |
2020年 | 76篇 |
2019年 | 120篇 |
2018年 | 101篇 |
2017年 | 115篇 |
2016年 | 103篇 |
2015年 | 68篇 |
2014年 | 117篇 |
2013年 | 209篇 |
2012年 | 89篇 |
2011年 | 153篇 |
2010年 | 83篇 |
2009年 | 131篇 |
2008年 | 140篇 |
2007年 | 167篇 |
2006年 | 150篇 |
2005年 | 131篇 |
2004年 | 100篇 |
2003年 | 90篇 |
2002年 | 84篇 |
2001年 | 93篇 |
2000年 | 63篇 |
1999年 | 54篇 |
1998年 | 64篇 |
1997年 | 50篇 |
1996年 | 37篇 |
1995年 | 44篇 |
1994年 | 37篇 |
1993年 | 35篇 |
1992年 | 31篇 |
1991年 | 43篇 |
1990年 | 40篇 |
1989年 | 35篇 |
1988年 | 35篇 |
1987年 | 25篇 |
1986年 | 32篇 |
1985年 | 23篇 |
1984年 | 21篇 |
1983年 | 12篇 |
1982年 | 18篇 |
1981年 | 20篇 |
1980年 | 16篇 |
1979年 | 11篇 |
1978年 | 9篇 |
1977年 | 8篇 |
1976年 | 2篇 |
1974年 | 3篇 |
排序方式: 共有3225条查询结果,搜索用时 125 毫秒
21.
22.
Consumption of leaf detritus by a stream shredder: Influence of tree species and nutrient status 总被引:8,自引:3,他引:5
Four species of riparian vegetation (alder, birch, willow and poplar) were fertilized with nitrogen, phosphorus, nitrogen
+ phosphorus, or no fertilizer (control). The resulting leaf detritus (leached but not microbially colonized) was offered
to a stream shredder, Hydatophylax variabilis (Trichoptera: Limnephilidae). In one experiment, shredder consumption of leaf detritus from different nutrient treatments
(within tree species) was compared, and in a second experiment, consumption of different tree species (within nutrient treatments)
was compared. Larvae preferred leaf detritus from nitrogen + phosphorus treatments (except in poplar where nitrogen treatment
was preferred). Alder was preferred over other tree species for all treatments. Chemical and physical analyses of leaf litter
showed differences between tree species and nutrient treatments in nutrient content, tannins and leaf toughness. Leaf consumption
by larvae was positively associated with nitrogen content and negatively associated with condensed tannin content. Species
composition and nutrient status of riparian vegetation may strongly influence detrital food webs in streams. 相似文献
23.
Field enrichments with nitrate in two spring-fed drainage lines within the riparian zone of a small woodland stream near Toronto, Ontario showed an absence of nitrate depletion. Laboratory experiments with riparian substrates overlain with nitrate enriched solutions revealed a loss of only 5–8% of the nitrate during 48 h incubation at 12°C. However, 22–24% of the initial nitrate was depleted between 24 and 48 h when a second set of substrate cores was incubated at 20°C. Short-term (3 h) incubations of fresh substrates amended with acetylene were used to estimate in situ denitrification potentials which varied from 0.05–3.19 g N g–1 d–1 for organic and sandy sediments. Denitrification potentials were highly correlated with initial nitrate content of substrate samples implying that low nitrate levels in ground water and riparian substrates may be an important factor in controlling denitrification rates. The efficiency of nitrate removal in spring-fed drainage lines is also limited by short water residence times of < 1 h within the riparian zone. These data suggest that routes of ground water movement and substrate characteristics are important in determining nitrate depletion within stream riparian areas. 相似文献
24.
Joseph A. N. Zasadzinski 《生物化学与生物物理学报:生物膜》1988,946(2)
Mixtures of sn-1 (
) and sn-3 (
) enantiomers of fully hydrated dipalmitoylphosphatidylcholine (DPPC) were studied with differential scanning calorimetry and freeze-fracture microscopy. The pretransition temperature of racemic mixtures of DPPC was 1.8 C° below that of either pure sn-1 or sn-3 enantiomers, which had similar pretransition temperatures. The main transition temperature of racemic mixtures was also depressed, but to a lesser extent, 0.8 C°. Freeze-fracture images of liposomes of sn-1, sn-3, and racemic mixtures of DPPC frozen from the Pβ′ phase showed well-defined ripples of wavelength 13 nm. Lipid stereoconfiguration had no effect on ripple wavelength, configuration or amplitude, or on the number and nature of surface defects. 相似文献
25.
A survey of oligotrophic Lake Rara, the biggest lake in Nepal, was carried out from 1982 till 1984. Mean depth is 100 m, and
maximum depth is 167 m. The surface area covers 9.8 km2, and the lake contains 0.98 km3 volume of water.
Transparency was about 16 m, photoquantum yield decreased exponentially with depth below 5 m, and the extinction coefficient
was 8.3 × 10−2. The concentration of Chl.-a was in the range of 0.06–0.46 mg m−3, and total nitrogen was 18–30 μg 1−1. The whole water column was well oxygenated. Primary productivity was extremely low. It has more than 30 inflowing brooks
and one outlet. The water quality of the brooks changes drastically with their location. The pH, electrical conductivity,
and EDTA hardness in the waters from a landslide area were high. In the waters from a rich pine forest they were extremely
low.
The zooplankton consisted of two species of protozoa, five species of rotifers, two species of Cladocera, and two species
of Copepoda. The zooplankton density range was 6200–16200 individuals m−3. The minimum was on November 11th, 1983 and the maximum on August 19th, 1983. 相似文献
26.
M. A. Khan 《Hydrobiologia》1986,135(3):233-242
L. Naranbagh (alt. 1587 m) is a polymictic, shallow marl lake in the flood-plain valley of Kashmir, India. Macrofloral affinities resemble Potamogeton Type of Forsberg (1965) with alkaline waters, not rich in phosphorus. CaCO3 precipitation coupled with decline in Ca2+ and alkalinity values are characteristic of the lake. Fluctuations in Mg2+, Na+, K+, and Cl– were relatively conservative. The levels of PO
inf4
sup3–
-P and NO
inf3
sup–
-N indicate moderate fertility of the lake water.Persistence of a summer-autumn planktonic algal pulse is related to favourable irradiance, high water temperatures, and increased photosynthetic efficiency values. The most striking seasonality in photosynthetic rates (m–2 h–1) between winter minimum (3 mg Cassim) and summer maximum (75.4 mg Cassim) is determined by mainly climatic changes. Energy flow gave annual phytoplankton production of 51.95 × 102 KJ m–2 for the ecosystem.The nutrient levels and productivity rates suggest mesotrophic status of L. Naranbagh in classic oligoeutrophic classification of lake types. 相似文献
27.
Siegfried Scherer 《Journal of molecular evolution》1985,22(1):91-94
Summary A recently proposed model for the origin of prebiotic progenitors of life in particles suspended in a primitive, specially organized atmosphere is considered critically. It is concluded that the physical and chemical framework of the new hypothesis conflicts with the conditions necessary for the evolution of the progenitors of life in the atmosphere of the early Earth. Therefore this model seems not to be a reasonable alternative to the Oparin thesis. 相似文献
28.
John G. Raei 《Hydrobiologia》1985,126(3):275-285
For two years a community of larval chironomid midges was studied in a sandy-run portion of a fourth order natural stream in SE Ohio, U.S.A. in order to determine if the species partitioned the spatial resources. The habitat structure was simplified from ten habitat variables to three significant principal components. The three eigenvectors were easily interpreted as sediment size, sediment heterogeneity, and organic deposition. Species abundances were loaded on these axes and niche metrics examined. Strong differences in habitat preference were demonstrated for midge species on each component. Also, there were no differences in distributions for the intraspecific instars for each species tested at a given time, but for each of the instars tested, their distributions were found to change from time to time. Apparently, individuals of a species, regardless of instar, simultaneously choose the same habitat, however, the preferred habitat may shift temporally due to change in resource availability, or niche expansion or compression due to competition. 相似文献
29.
Peat and water chemistry at Big Run Bog,a peatland in the Appalachian mountains of West Virginia,USA 总被引:1,自引:1,他引:0
R. Kelman Wieder 《Biogeochemistry》1985,1(3):277-302
At Big Run Bog, aSphagnum-dominated peatland in the unglaciated Appalachian Plateau of West Virginia, significant spatial variation in the physical and chemical properties of the peat and in surface and subsurface (30 cm deep) water chemistry was characterized. The top 40 cm of organic peat at Big Run Bog had average values for bulk density of 0.09 g · cm–3, organic matter concentration of 77%, and volumetric water content of 88%. Changes in physical and chemical properties within the peat column as a function of depth contributed to different patterns of seasonal variation in the chemistry of surface and subsurface waters. Seasonal variation in water chemistry was related to temporal changes in plant uptake, organic matter decomposition and element mineralization, and to varying redox conditions associated with fluctuating water table levels. On the average, total Ca, Mg, and N concentrations in Big Run Bog peat were 33, 15, and 1050 mol · g–1, respectively; exchangeable Ca and Mg concentrations were 45 and 14 eq · g–1 , respectively. Surface water pH averaged 4.0 and Ca++ concentrations were less than 50 eq · L–1 . These chemical variables have all been used to distinguish bogs from fens. Physiographically, Big Run Bog is a minerotrophic fen because it receives inputs of water from the surrounding forested upland areas of its watershed. However, chemically, Big Run Bog is more similar to true ombrotrophic bogs than to minerotrophic fens. 相似文献
30.
Simple mathematical models are formulated to describe density independent and density dependent dispersal. These models clarify
hypotheses of density dependence and may be manipulated easily to suit particular applications. The models demonstrate that
the initial composition of a species aggregate must be controlled before valid conclusions can be drawn about the density
dependency of the aggregate's dispersal. Stochastic models of emigration are derived to assess the power of particular experimental
designs and statistical techniques to discriminate a known form of density dependent emigration.
Contribution No. 369, Great Lakes Research Division, University of Michigan
Contribution No. 369, Great Lakes Research Division, University of Michigan 相似文献