全文获取类型
收费全文 | 102577篇 |
免费 | 854篇 |
国内免费 | 890篇 |
专业分类
104321篇 |
出版年
2022年 | 47篇 |
2021年 | 84篇 |
2020年 | 51篇 |
2019年 | 62篇 |
2018年 | 11917篇 |
2017年 | 10746篇 |
2016年 | 7586篇 |
2015年 | 803篇 |
2014年 | 553篇 |
2013年 | 1191篇 |
2012年 | 4664篇 |
2011年 | 13314篇 |
2010年 | 12297篇 |
2009年 | 8521篇 |
2008年 | 10272篇 |
2007年 | 11889篇 |
2006年 | 841篇 |
2005年 | 1094篇 |
2004年 | 1529篇 |
2003年 | 1552篇 |
2002年 | 1357篇 |
2001年 | 405篇 |
2000年 | 302篇 |
1999年 | 186篇 |
1998年 | 174篇 |
1997年 | 149篇 |
1996年 | 132篇 |
1995年 | 119篇 |
1994年 | 105篇 |
1993年 | 169篇 |
1992年 | 147篇 |
1991年 | 129篇 |
1990年 | 96篇 |
1989年 | 100篇 |
1988年 | 92篇 |
1987年 | 95篇 |
1986年 | 71篇 |
1985年 | 74篇 |
1984年 | 83篇 |
1983年 | 78篇 |
1982年 | 89篇 |
1981年 | 69篇 |
1980年 | 76篇 |
1979年 | 50篇 |
1978年 | 49篇 |
1977年 | 39篇 |
1976年 | 41篇 |
1973年 | 38篇 |
1972年 | 277篇 |
1971年 | 288篇 |
排序方式: 共有10000条查询结果,搜索用时 12 毫秒
991.
Seasonal dynamics of zooplankton in a shallow eutrophic,man-made hyposaline lake in Delhi (India): role of environmental factors 总被引:1,自引:0,他引:1
Old Fort Lake, a small (1.6 ha), shallow, and recreational water body in Delhi (India) was studied through monthly surveys
in two consecutive years (January, 2000–December, 2001). Precipitation is the major source of water for this closed basin
lake. In addition, ground water is used for replenishing the lake regularly. This alkaline, hyposaline hard water lake contains
very high ionic concentration, especially of nitrates. Based on overall ionic composition, this lake can be categorized as
chloride–sulfate alkaline waters with the anion sequence dominated by SO4
2− > Cl− > HCO3
−, and the cations by Mg++ > Ca++. The overall seasonal variability in physicochemical profile was largely regulated by the annual cycle of evaporation and
precipitation, whereas the ground water largely influences its water quality. The lake exhibited phytoplankton-dominated turbid
state due to dominance of the blue green alga, Microcystis aeruginosa. The persistent cyanobacterial blooms and the elevated nutrient levels are indicative of the cultural eutrophication of the
lake. This study focuses on the relative importance of eutrophic vis-à-vis hyposaline conditions in determining the structure
and seasonal dynamics of zooplankton species assemblages. A total of 52 zooplankton species were recorded and rotifers dominated
the community structure qualitatively as well as quantitatively. The genus Brachionus comprised a significant component of zooplankton community with B. plicatilis as the most dominant species. The other common taxa were B. quadridentatus, B. angularis, Lecane grandis, L. thalera, L. punctata, Mesocyclops sp., and Alona rectangula. Multivariate data analysis techniques, Canonical Correspondence Analysis (CCA) along with Monte Carlo Permutation Tests
were used to determine the minimum number of environmental factors that could explain statistically significant (P < 0.05) proportions of variation in the species data. The significant variables selected by CCA were NH3–N followed by percent saturation of DO, COD, SS, BOD, NO2–N, rainfall, silicates, and PO4–P. The results indicate that the seasonal succession patterns of the zooplankton species were largely controlled by physicochemical
factors related directly or indirectly to the process of eutrophication, whereas hyposaline conditions in the lake determined
the characteristic species composition.
Guest Editors: J. John & B. Timms
Salt Lake Research: Biodiversity and Conservation—Selected papers from the 9th Conference of the International Society for
Salt Lake Research 相似文献
992.
Dogs demonstrate an age-related cognitive decline, which may be related to a decrease in the concentration of omega-3 polyunsaturated
fatty acids (n-3 PUFA) in the brain. Medium chain triglycerides (MCT) increase fatty acid oxidation, and it has been suggested that this
may raise brain n-3 PUFA levels by increasing mobilization of n-3 PUFA from adipose tissue to the brain. The goal of the present study was to determine whether dietary MCT would raise n-3 PUFA concentrations in the brains of aged dogs. Eight Beagle dogs were randomized to a control diet (n = 4) or an MCT (AC-1203) enriched diet (n = 4) for 2 months. The animals were then euthanized and the parietal cortex was removed for phospholipid, cholesterol and
fatty acid determinations by gas-chromatography. Dietary enrichment with MCT (AC-1203) resulted in a significant increase
in brain phospholipid and total lipid concentrations (P < 0.05). In particular, n-3 PUFA within the phospholipid, unesterified fatty acid, and total lipid fractions were elevated in AC-1203 treated subjects
as compared to controls (P < 0.05). Brain cholesterol concentrations did not differ significantly between the groups (P > 0.05). These results indicate that dietary enrichment with MCT, raises n-3 PUFA concentrations in the parietal cortex of aged dogs. 相似文献
993.
Understanding food web interactions in native or agricultural ecosystems is an important step towards establishing sustainable
pest management strategies. While the role of generalist predators as biological control agents is increasingly appreciated,
the study of trophic interactions between individual predator species and their prey provides practical difficulties. Recently,
different approaches have been suggested to determine prey items from predator guts using molecular methods. Macrolophus caliginosus is a generalist predator active in herbaceous agro-ecosystems. We developed a system to identify the DNA of its prey after
ingestion, using Myzus persicae as a model. Esterase (MpEST) and cytochrome oxidase I (MpCOI) genes were targeted in the aphid, while M. caliginosus COI gene was used as control for predator DNA. Real time PCR proved to be specific and sensitive enough to detect prey DNA upon
ingestion after feeding experiments. The system provided a linear amplification response with only 10 fg of prey genomic DNA
as template. The detection system of MpCOI gene was more sensitive than MpEST, while the detection period was similar for both genes. Possibilities for using the system in ecological and biosafety studies
with regard to sustainable pest management are discussed.
相似文献
Salvatore ArpaiaEmail: |
994.
Simultaneous electricity generation and selenium removal was evaluated in single-chamber microbial fuel cells (MFCs) with
acetate and glucose as carbon sources. Power output was not affected by selenite up to 125 mg l−1 with glucose as substrate. Coulombic efficiencies of MFCs with glucose increased from 25% to 38% at 150 mg Se l−1. About 99% of 50 and 200 mg Se l−1 selenite was removed in 48 and 72 h for MFCs fed with acetate and glucose, respectively, demonstrating the potential of using
MFC technology for Se remediation. 相似文献
995.
Consumption of aphids by spiders and the effect of additional prey: evidence from microcosm experiments 总被引:1,自引:0,他引:1
Spiders are common generalist predators in agroecosystems and have been suggested to lower herbivore abundance in crops. It
is not clear, however, if spiders can effectively suppress pest populations, and if so, by what mechanisms. In a microcosm
experiment, we examined the consumption of the bird cherry-oat aphid, Rhopalosiphum padi L. (Homoptera: Aphididae), a pest species in wheat fields, by three spider species that differ in their hunting methods.
We then tested the effect of additional prey type on the ability of erigonid spiders to reduce aphids. In a 48-h experiment
Mermessus denticulatus (Banks) (Araneae: Linyphiidae; Erigoninae) consumed more aphids than did Enoplognatha gemina Bosmans and Van Keer (Araneae: Theridiidae) and Bathyphantes cf. extricatus (O·P.-Cambridge) (Araneae: Linyphiidae; Linyphiinae). This difference may be due to the ability of erigonids to forage actively
on the vegetation in addition to using their webs to catch prey. In a 7-week experiment, we provided springtails (Collembola)
in high and low densities as additional prey to mated erigonids, prior to aphid introduction. Spiders in the low-density springtail
treatment built more webs on the vegetation, and caused a 50% reduction in aphid populations. There were significantly fewer
aphids in the low-density springtail treatment, but not in the high-density treatment, in comparison to the control (high-density
springtails without spiders). The results suggest that additional prey density affects predatory interactions between M. denticulatus and R. padi and that erigonids, which occur in high densities in wheat fields in the Negev desert, may be involved in aphid suppression
in these agroecosystems.
相似文献
Efrat Gavish-RegevEmail: |
996.
Basharat Ali Anjum Nasim Sabri Karin Ljung Shahida Hasnain 《World journal of microbiology & biotechnology》2009,25(3):519-526
Sixteen Bacillus strains isolated from rhizosphere, histoplane and phyllosphere of different plant species were identified by 16S rDNA gene
sequencing and evaluated for in vitro auxin production as well as growth stimulation of Vigna radiata (L.) Wilczek. Auxin production by Bacillus spp. in L-broth medium supplemented with 1,000 μg ml−1
L-tryptophan ranges from 0.60 to 3.0 μg IAA ml−1 as revealed by gas chromatography and mass spectrometric (GC–MS) analysis. Rhizospheric isolates exhibit relatively more
IAA synthesis than histoplane and phyllosphere isolates. Plant microbe interaction experiments conducted under gnotobiotic
conditions recorded 55.55, 46.46 and 46.20% increase in shoot length with Bacillus megaterium MiR-4, B. pumilus NpR-1 and B. subtilis TpP-1, respectively, over control. Bacillus inoculations also increased shoot fresh weight with B. megaterium MiR-4 (60.94%) and B. pumilus NpR-1 (37.76%). Highly significant positive correlation between auxin production analyzed by GC–MS and shoot length (r = 0.687**, P = 0.01) and shoot fresh weight (r = 0.703**, P = 0.01) was noted under gnotobiotic conditions. Similarly, significant correlation was also found between auxin production
by Bacillus spp. (GC–MS analysis) and different growth parameters such as shoot length (r = 0.495*, P = 0.05), number of pods (r = 0.498*, P = 0.05) and grain weight (r = 0.537*, P = 0.05) at full maturity under natural wire house conditions. Results showed that auxin production potential of plant associated
Bacillus spp. can be effectively exploited to enhance the growth and yield of V. radiata. 相似文献
997.
Ulrich Claussen Wanda Zevenboom Uwe Brockmann Dilek Topcu Peter Bot 《Hydrobiologia》2009,629(1):49-58
Eutrophication (nutrient enrichment and subsequent processes) and its adverse ecosystem effects have been discussed as main
issues over the last 20 years in international conferences and conventions for the protection of the marine environment such
as the North Sea Conferences and the 1992 OSPAR Convention (OSPAR; which combined and updated the 1972 Oslo Convention on
dumping waste at the sea and the 1974 Paris Convention on land-based sources of marine pollution). OSPAR committed itself
to reduce phosphorus and nitrogen inputs (in the order of 50% compared with 1985) into the marine areas and ‘to combat eutrophication
to achieve, by the year 2010, a healthy marine environment where eutrophication does not occur’. Within OSPAR, the Comprehensive
Procedure (COMPP) has been developed and used to assess the eutrophication status of the OSPAR maritime area in an harmonised
way. This is based on classification in terms of the following types of areas Non-Problem Areas (no effects), Potential Problem
Areas (not enough data to assess effects) and Problem Areas (effects due to elevated nutrients and/or due to transboundary
transport from adjacent areas). The COMPP consists of a set of harmonised assessment criteria with their area-specific assessment
levels and an integrated area classification approach. The criteria cover all aspects of nutrient enrichment (nutrient inputs,
concentrations and ratios) as well as possible direct effects (e.g. increased levels of nuisance and/or toxic phytoplankton
species, shifts and/or losses of submerged aquatic vegetation) and indirect effects (e.g. oxygen deficiency, changes and/or
death of benthos, death of fish, algal toxins). The COMPP also includes supporting environmental factors. It takes account
of synergies and harmonisation with the EC Water Framework Directive, and has formed a major basis for the EC eutrophication
guidance. Recently, additional components, such as total nitrogen, total phosphorus and transboundary transports have been
included in the assessment of, e.g. the German Bight. The second application of the COMPP resulting in an update of the eutrophication
status of the OSPAR maritime area will be finalised in 2008, and will include the agreed integrated set of Ecological Quality
Objectives (EcoQOs) with respect to eutrophication.
Guest editors: J. H. Andersen & D. J. Conley
Eutrophication in Coastal Ecosystems: Selected papers from the Second International Symposium on Research and Management of
Eutrophication in Coastal Ecosystems, 20–23 June 2006, Nyborg, Denmark 相似文献
998.
Xin Li Chun-Shan Quan Hui-Ying Yu Jian-Hua Wang Sheng-Di Fan 《World journal of microbiology & biotechnology》2009,25(1):151-154
A novel compound CF66I produced by Burkholeria cepacia was investigated for its antifungal effects against Fusarium solani by three different fluorescent dyes. Dual staining with propidium iodide (PI) and fluorescein diacetate (FDA) demonstrated
high doses of CF66I (120.0 μg ml−1) killed the fungi by acting primarily on the cell membrane. However, at fungistatic concentration (20.0 μg ml−1) of this compound, microscopic observations revealed swelling hyphae with abnormal chitin deposition, as determined by Calcofluor
white (CFW) staining, which was indicative of the alterations in cell wall structure. In addition, inhibition of intracellular
esterases activity was observed. These results led us to conclude that low doses of CF66I probably inhibited the fungal growth
by interfering with the cell metabolic pathways. 相似文献
999.
Placental (eutherian) mammals are currently classified into four superordinal clades (Afrotheria, Xenarthra, Laurasiatheria and Supraprimates) of which one, the Afrotheria (a unique lineage of African origin), is generally considered to be basal. Therefore, Afrotheria provide a pivotal evolutionary link for studying fundamental differences between the sex chromosomes of human/mouse (both representatives of Supraprimates and the index species for studies of sex chromosomes) and those of the distantly related marsupials. In this study, we use female fibroblasts to investigate classical features of X chromosome inactivation including replication timing of the X chromosomes and Barr body formation. We also examine LINE-1 accumulation on the X chromosomes of representative afrotherians and look for evidence of a pseudoautosomal region (PAR). Our results demonstrate that asynchronous replication of the X chromosomes is common to Afrotheria, as with other mammals, and Barr body formation is observed across all Placentalia, suggesting that mechanisms controlling this evolved before their radiation. Finally, we provide evidence of a PAR (which marsupials lack) and demonstrate that LINE1 is accumulated on the afrotherian and xenarthran X, although this is probably not due to transposition events in a common ancestor, but rather ongoing selection to retain recently inserted LINE1 on the X. 相似文献
1000.
Sugi (Cryptomeria japonica D. Don) is one of the most important evergreen coniferous plantation species in Japan. Much of the riparian forest that was
originally dominated by deciduous broadleaf trees has been converted into sugi plantations. The present study investigated
the seasonality of leaf-litter input and leaf dispersal to streams to assess the effects of converting riparian forest to
sugi plantations. The seasonality of leaf-litter input was assessed at three streams in Nagoya University Forest. At one stream
dominated by deciduous broadleaf trees, input was limited to autumn. At two streams in a sugi plantation, input was prolonged
from autumn to early spring, and was dominated by sugi needles from winter to early spring. These results suggest that sugi
plantations alter the seasonality of leaf-litter input from riparian forests and affect stream ecosystems. Leaf dispersal
was assessed by considering the relationship between leaf dispersal distance from three forest layers to the stream and leaf-litter
input into two streams. The maximum leaf dispersal distance was 26–28 m for deciduous broadleaf trees from mid-October to
November and 10–12 m for sugi needles from December to April. Leaf dispersal distance depended on the tree species. Four species
of deciduous broadleaf tree showed greater leaf dispersal than that of sugi. The mean weight of individual sugi needles was
higher than that of the broadleaf trees’ leaves, and dispersal depended on strong winds in winter and early spring. Although
the leaf dispersal distance from the understory was within 2–4 m, it could be a significant source of leaf-litter input to
streams. 相似文献