全文获取类型
收费全文 | 4769篇 |
免费 | 339篇 |
出版年
2023年 | 26篇 |
2022年 | 38篇 |
2021年 | 80篇 |
2020年 | 60篇 |
2019年 | 61篇 |
2018年 | 71篇 |
2017年 | 79篇 |
2016年 | 111篇 |
2015年 | 242篇 |
2014年 | 266篇 |
2013年 | 310篇 |
2012年 | 426篇 |
2011年 | 377篇 |
2010年 | 231篇 |
2009年 | 200篇 |
2008年 | 303篇 |
2007年 | 313篇 |
2006年 | 304篇 |
2005年 | 232篇 |
2004年 | 227篇 |
2003年 | 222篇 |
2002年 | 230篇 |
2001年 | 41篇 |
2000年 | 41篇 |
1999年 | 54篇 |
1998年 | 50篇 |
1997年 | 31篇 |
1996年 | 36篇 |
1995年 | 50篇 |
1994年 | 21篇 |
1993年 | 36篇 |
1992年 | 21篇 |
1991年 | 21篇 |
1990年 | 26篇 |
1989年 | 13篇 |
1988年 | 15篇 |
1987年 | 18篇 |
1986年 | 19篇 |
1985年 | 19篇 |
1984年 | 20篇 |
1983年 | 19篇 |
1982年 | 24篇 |
1981年 | 11篇 |
1979年 | 10篇 |
1978年 | 14篇 |
1977年 | 19篇 |
1976年 | 10篇 |
1975年 | 10篇 |
1974年 | 12篇 |
1971年 | 6篇 |
排序方式: 共有5108条查询结果,搜索用时 31 毫秒
981.
There is a prohibition on the use of modern external inputs, such as chemical fertilizers, in Baduy swidden farming. Ordinarily, the consequence of this in a situation of acute forest pressure would be a decrease in fallow times, and an inevitable depletion of soil fertility. The Baduy (particularly Outer Baduy) have several ways of alleviating this problem, one of which is the introduction of Paraserianthes (Albizia) falcataria. By alternating this commercially valuable perennial leguminous tree with rice, soil fertility is maintained and the socio-economic position of the Baduy improved. As a result, swidden farming, which is considered by the Baduy to be central to their cultural identity, continues to be viable, despite increasing population density and the continuing depletion of mature forest. 相似文献
982.
Jari Niemelä Johan Kotze Allan Ashworth Pietro Brandmayr Konjev Desender Tim New Lyubomir Penev Michael Samways John Spence 《Journal of Insect Conservation》2000,4(1):3-9
We introduce an initiative to assess and compare landscape changes related to human activities on a global scale, using a single group of invertebrates. The GLOBENET programme uses common field methodology (pitfall trapping), to appraise assemblages of ground beetles (Coleoptera, Carabidae) in visually-similar land-mosaics (urban-rural gradients). Carabids were selected as the focal taxon as they are sufficiently varied (both taxonomically and ecologically), abundant and sensitive to the environment. However, work on other taxa is comparable with the GLOBENET framework. The continuum of decreasing human pressure from city centres into the surrounding countryside was selected to represent human-caused disturbance for this initial stage of GLOBENET because these gradients can be found virtually all over the world. Through the broad-scale assessment envisioned in the GLOBENET programme, we seek to separate general, repeated effects on biodiversity from those that depend on local environments or particular biotic assemblages. Based on this understanding we aim to develop simple tools and protocols for assessing ecological effects of human-caused landscape changes, which could help to sustainably manage landscapes for biodiversity and for human requirements. For instance, the response of different functional groups of carabids to these landscape changes may help guide management practices. Further GLOBENET developments and information are available at our website: http://www.helsinki.fi/science/globenet/ 相似文献
983.
Costs of sporophyte production in the moss,Dicranum polysetum 总被引:2,自引:0,他引:2
We investigated the cost of sporophyte production in the moss Dicranum polysetum both by examining patterns of growth and reproduction in unmanipulated shoots and by experimentally manipulating sexual reproduction.The estimated proportion of total carbon investment allocated to sexual reproduction in sporophyte-producing shoots over the study period was 74.8%. Unmanipulated shoots that aborted all sporophytes had a significantly higher growth in the top shoots than shoots that produced sporophytes. In sporophyte-producing shoots, total apical growth decreased proportionately with the number of sporophytes.Experimental prevention of sporophyte development resulted in significantly higher total apical growth of the gametophytes. Shoots where current perichaetia were lacking when marked had a mass increase in the top shoots similar to manipulated sporophytic shoots whereas sporophytic control shoots grew significantly less than these two categories.The difference between control shoots and manipulated shoots in the mass of vegetative apical growth was mainly because of different length increments whereas mass per unit length was similar between groups. The probability to reproduce sexually in the year after the manipulation, and the biomass allocated to this reproduction, were not affected by the experimental treatment. 相似文献
984.
Peggion Cristina Piazza Claudia Formaggio Fernando Crisma Marco Toniolo Claudio Kaptein Bernard Broxterman Quirinus B. Kamphuis Johan 《International journal of peptide research and therapeutics》2000,7(1):9-16
Summary We synthesized by solution-phase methods the naturally occurring, 10-amino acid residue lipopeptaibol antibiotics trikoningins
KBI and KBII, and the [l-Iva1] KB analogue, in which the amino acid in position 1 is different, with the aim at investigating the effect of hydrophobicity
and chirality in that position. A solution conformational analysis, using FT-IR absorption and CD techniques, indicated that
all of the three decapeptides are predominantly helical in a membrane-mimetic environment. Permeability measurements showed
an increase of the activity from the [Aib1] peptide to the more hydrophobic [Iva1] peptides. Conversely, the effect of a change in chirality, obtained by replacingd-Iva1 withl-Iva, turned out to be of minor significance. 相似文献
985.
Bioaugmentation of Activated Sludge by an Indigenous 3-Chloroaniline-Degrading Comamonas testosteroni Strain, I2gfp 总被引:3,自引:0,他引:3 下载免费PDF全文
Nico Boon Johan Goris Paul De Vos Willy Verstraete Eva M. Top 《Applied microbiology》2000,66(7):2906-2913
A strain identified as Comamonas testosteroni I2 was isolated from activated sludge and found to be able to mineralize 3-chloroaniline (3-CA). During the mineralization, a yellow intermediate accumulated temporarily, due to the distal meta-cleavage of chlorocatechol. This strain was tested for its ability to clean wastewater containing 3-CA upon inoculation into activated sludge. To monitor its survival, the strain was chromosomally marked with the gfp gene and designated I2gfp. After inoculation into a lab-scale semicontinuous activated-sludge (SCAS) system, the inoculated strain maintained itself in the sludge for at least 45 days and was present in the sludge flocs. After an initial adaptation period of 6 days, complete degradation of 3-CA was obtained during 2 weeks, while no degradation at all occurred in the noninoculated control reactor. Upon further operation of the SCAS system, only 50% 3-CA removal was observed. Denaturing gradient gel electrophoresis (DGGE) of 16S rRNA genes revealed a dynamic change in the microbial community structure of the activated sludge. The DGGE patterns of the noninoculated and the inoculated reactors evolved after 7 days to different clusters, which suggests an effect of strain inoculation on the microbial community structure. The results indicate that bioaugmentation, even with a strain originating from that ecosystem and able to effectively grow on a selective substrate, is not permanent and will probably require regular resupplementation. 相似文献
986.
Effect of Dissemination of 2,4-Dichlorophenoxyacetic Acid (2,4-D) Degradation Plasmids on 2,4-D Degradation and on Bacterial Community Structure in Two Different Soil Horizons 总被引:9,自引:4,他引:5 下载免费PDF全文
Winnie Dejonghe Johan Goris Saïd El Fantroussi Monica Hfte Paul De Vos Willy Verstraete Eva M. Top 《Applied microbiology》2000,66(8):3297-3304
Transfer of the 2,4-dichlorophenoxyacetic acid (2,4-D) degradation plasmids pEMT1 and pJP4 from an introduced donor strain, Pseudomonas putida UWC3, to the indigenous bacteria of two different horizons (A horizon, depth of 0 to 30 cm; B horizon, depth of 30 to 60 cm) of a 2,4-D-contaminated soil was investigated as a means of bioaugmentation. When the soil was amended with nutrients, plasmid transfer and enhanced degradation of 2,4-D were observed. These findings were most striking in the B horizon, where the indigenous bacteria were unable to degrade any of the 2,4-D (100 mg/kg of soil) during at least 22 days but where inoculation with either of the two plasmid donors resulted in complete 2,4-D degradation within 14 days. In contrast, in soils not amended with nutrients, inoculation of donors in the A horizon and subsequent formation of transconjugants (105 CFU/g of soil) could not increase the 2,4-D degradation rate compared to that of the noninoculated soil. However, donor inoculation in the nonamended B-horizon soil resulted in complete degradation of 2,4-D within 19 days, while no degradation at all was observed in noninoculated soil during 89 days. With plasmid pEMT1, this enhanced degradation seemed to be due only to transconjugants (105 CFU/g of soil), since the donor was already undetectable when degradation started. Denaturing gradient gel electrophoresis (DGGE) of 16S rRNA genes showed that inoculation of the donors was followed by a shift in the microbial community structure of the nonamended B-horizon soils. The new 16S rRNA gene fragments in the DGGE profile corresponded with the 16S rRNA genes of 2,4-D-degrading transconjugant colonies isolated on agar plates. This result indicates that the observed change in the community was due to proliferation of transconjugants formed in soil. Overall, this work clearly demonstrates that bioaugmentation can constitute an effective strategy for cleanup of soils which are poor in nutrients and microbial activity, such as those of the B horizon. 相似文献
987.
Sex and size related differences in the dry season feeding patterns of elephants in Chobe National Park, Botswana 总被引:4,自引:0,他引:4
Differences in feeding patterns of the African elephant were examined by sex and age during the dry season in a dystrophic savanna-woodland ecosystem in northern Botswana. Adult males had the least diverse diet in terms of woody plant species, but they consumed more plant parts than family units. The diameter of stems of food plants broken or bitten off was also greater for adult males than for females and subadult males. Adult males spent more time foraging on each woody plant than did females. The number of woody plant species and individuals present were higher at feeding sites of family units than at feeding sites of adult males, indicating that family units positioned themselves at feeding sites with higher species diversity than those of males. We argue that the most likely explanation for these differences is related to the pronounced sexual size dimorphism exhibited by elephants, resulting in sex differences in browsing patterns due to the allometric relationships that govern the tolerance of herbivores for variation in diet quality. From our results this Body Size Hypothesis is accepted rather than the alternative Scramble Competition Hypothesis, which predicts that adult male elephants consume lower quality browse because they are displaced from preferred browse as an outcome of scramble competition with adult females and their offspring. If the feeding patterns of adult male elephants were affected by intersexual scramble competition, we would expect adult males to browse at a higher level in the canopy than the smaller-bodied females and their offspring. No evidence was found for this, although adult females were found to browse at a higher level in the canopy when feeding in close proximity to subadults and juveniles than when feeding alone. Sex differences in elephant browsing patterns are, we propose, of relevance to understanding and managing elephant impacts on African woodlands. 相似文献
988.
Johan U. Grobbelaar 《Journal of applied phycology》2000,12(3-5):201-206
The successful coupling between physiology andtechnology is central to the success of algalbiotechnology. Imperative is a proper understandingof the variables and their impacts on biomass and/orbiocompound production. The crux lies inphotosynthesis and the capturing of light energy atthe optimal rate for eventual maximal photochemistry(biosynthesis). It is in the hands of algalbiotechnologists to understand the dynamics andregulatory mechanisms of especially PSII (photosystemII) activity in order to advance this technologyfurther. Biophysical and technological optimisationand design aimed at maximising photon flux capture aresome of the avenues that needs be pursued. This needsto be augmented by molecular, biochemical andphysiological inputs. Unfortunately detailedsystematic analyses of the variables, theirinteraction and possible synergism have rarely beendone. The debate regarding the merits andproductivity in closed, either plate or tubular,vertical or horizontal, and open pond reactors need tobe resolved. Exciting developments regarding onlinemeasurements and feedback control for optimalproductivities are part of the solutions andapproaches that need to be followed. Multistagesystems that not only utilise autotrophic growth andstress components, but also combinedautotrophic/heterotrophic systems could providesolutions to specific production requirements. Theseand other important issues are addressed in theoverview. The challenges facing algalbiotechnologists and future research needs are also discussed. 相似文献
989.
990.