全文获取类型
收费全文 | 1937篇 |
免费 | 152篇 |
国内免费 | 171篇 |
出版年
2023年 | 44篇 |
2022年 | 48篇 |
2021年 | 57篇 |
2020年 | 62篇 |
2019年 | 101篇 |
2018年 | 77篇 |
2017年 | 67篇 |
2016年 | 83篇 |
2015年 | 58篇 |
2014年 | 93篇 |
2013年 | 107篇 |
2012年 | 68篇 |
2011年 | 67篇 |
2010年 | 61篇 |
2009年 | 105篇 |
2008年 | 109篇 |
2007年 | 116篇 |
2006年 | 100篇 |
2005年 | 70篇 |
2004年 | 70篇 |
2003年 | 77篇 |
2002年 | 77篇 |
2001年 | 53篇 |
2000年 | 42篇 |
1999年 | 40篇 |
1998年 | 32篇 |
1997年 | 23篇 |
1996年 | 30篇 |
1995年 | 28篇 |
1994年 | 22篇 |
1993年 | 35篇 |
1992年 | 20篇 |
1991年 | 32篇 |
1990年 | 20篇 |
1989年 | 18篇 |
1988年 | 18篇 |
1987年 | 13篇 |
1986年 | 16篇 |
1985年 | 13篇 |
1984年 | 11篇 |
1983年 | 5篇 |
1982年 | 9篇 |
1981年 | 7篇 |
1980年 | 10篇 |
1979年 | 12篇 |
1978年 | 12篇 |
1977年 | 4篇 |
1976年 | 8篇 |
1975年 | 2篇 |
1974年 | 3篇 |
排序方式: 共有2260条查询结果,搜索用时 31 毫秒
351.
352.
JAMES T. RANDERSON FORREST M. HOFFMAN† PETER E. THORNTON‡ § NATALIE M. MAHOWALD¶ KEITH LINDSAY‡ YEN-HUEI LEE‡ CYNTHIA D. NEVISON SCOTT C. DONEY GORDON BONAN‡ RETO STÖCKLI†† ‡‡ CURTIS COVEY§§ STEVEN W. RUNNING¶¶ INEZ Y. FUNG 《Global Change Biology》2009,15(10):2462-2484
With representation of the global carbon cycle becoming increasingly complex in climate models, it is important to develop ways to quantitatively evaluate model performance against in situ and remote sensing observations. Here we present a systematic framework, the Carbon‐LAnd Model Intercomparison Project (C‐LAMP), for assessing terrestrial biogeochemistry models coupled to climate models using observations that span a wide range of temporal and spatial scales. As an example of the value of such comparisons, we used this framework to evaluate two biogeochemistry models that are integrated within the Community Climate System Model (CCSM) – Carnegie‐Ames‐Stanford Approach′ (CASA′) and carbon–nitrogen (CN). Both models underestimated the magnitude of net carbon uptake during the growing season in temperate and boreal forest ecosystems, based on comparison with atmospheric CO2 measurements and eddy covariance measurements of net ecosystem exchange. Comparison with MODerate Resolution Imaging Spectroradiometer (MODIS) measurements show that this low bias in model fluxes was caused, at least in part, by 1–3 month delays in the timing of maximum leaf area. In the tropics, the models overestimated carbon storage in woody biomass based on comparison with datasets from the Amazon. Reducing this model bias will probably weaken the sensitivity of terrestrial carbon fluxes to both atmospheric CO2 and climate. Global carbon sinks during the 1990s differed by a factor of two (2.4 Pg C yr?1 for CASA′ vs. 1.2 Pg C yr?1 for CN), with fluxes from both models compatible with the atmospheric budget given uncertainties in other terms. The models captured some of the timing of interannual global terrestrial carbon exchange during 1988–2004 based on comparison with atmospheric inversion results from TRANSCOM (r=0.66 for CASA′ and r=0.73 for CN). Adding (CASA′) or improving (CN) the representation of deforestation fires may further increase agreement with the atmospheric record. Information from C‐LAMP has enhanced model performance within CCSM and serves as a benchmark for future development. We propose that an open source, community‐wide platform for model‐data intercomparison is needed to speed model development and to strengthen ties between modeling and measurement communities. Important next steps include the design and analysis of land use change simulations (in both uncoupled and coupled modes), and the entrainment of additional ecological and earth system observations. Model results from C‐LAMP are publicly available on the Earth System Grid. 相似文献
353.
ANITA ANTONINKA JULIE E. WOLF MATTHEW BOWKER AIMÉE T. CLASSEN NANCY COLLINS JOHNSON 《Global Change Biology》2009,15(4):914-929
Cryptic belowground organisms are difficult to observe and their responses to global changes are not well understood. Nevertheless, there is reason to believe that interactions among above- and belowground communities may mediate ecosystem responses to global change. We used grassland mesocosms to manipulate the abundance of one important group of soil organisms, arbuscular mycorrhizal (AM) fungi, and to study community and ecosystem responses to CO2 and N enrichment. Responses of plants, AM fungi, phospholipid fatty acids and community-level physiological profiles were measured after two growing seasons. Ecosystem responses were examined by measuring net primary production (NPP), evapotranspiration, total soil organic matter (SOM), and extractable mineral N. Structural equation modeling was used to examine the causal relationships among treatments and response variables. We found that while CO2 and N tended to directly impact ecosystem functions (evapotranspiration and NPP, respectively), AM fungi indirectly impacted ecosystem functions by influencing the community composition of plants and other root fungi, soil fungi and soil bacteria. We found that the mycotrophic status of the dominant plant species in the mesocosms determined whether the presence of AM fungi increased or decreased NPP. Mycotrophic grasses dominated the mesocosm communities during the first growing season, and the mycorrhizal treatments had the highest NPP. In contrast, nonmycotrophic forbs were dominant during the second growing season and the mycorrhizal treatments had the lowest NPP. The composition of the plant community strongly influenced soil N, and the community composition of soil organisms strongly influenced SOM accumulation in the mesocosms. These results show how linkages between above- and belowground communities can determine ecosystem responses to global change. 相似文献
354.
This study consisted of sampling benthic algae at 32 sites in the Gangqu River, an important upstream tributary of the Yangtze
River. Our aims were to characterize the benthic algae communities and relationships with environmental variables. Among the
162 taxa observed, Achnanthes linearis and Achnanthes lanceolata var. elliptica were the dominant species (17.10% and 14.30% of the total relative abundance, respectively). Major gradients and principal
patterns of variation within the environmental variables were detected by principal component analysis (PCA). Then non-metric
multidimensional scaling (NMS) divided all the sites into three groups, which were validated by multi-response permutation
procedures (MRPP). Canonical correspondence analysis (CCA) indicated that three environmental variables (TN, TDS, and TP)
significantly affected the distribution of benthic algae. Weighted averaging regression and cross-calibration produced strong
models for predicting TN and TDS concentration, which enabled selection of algae taxa as potentially sensitive indicators
of certain TN and TDS levels: for TN, Achnanthes lanceolata, Achnanthes lanceolata var. elliptica, and Cymbella ventricosa var. semicircularis; for TDS, Cocconeis placentula, Cymbella alpina var. minuta, and Fragilaria virescens. The present study represents an early step in establishing baseline conditions. Further monitoring is suggested to gain
a better understanding of this region. 相似文献
355.
A. Heenan S. D. Simpson M. G. Meekan S. D. Healy V. A. Braithwaite 《Journal of fish biology》2009,75(7):1857-1867
To determine whether enhancing the survival of new recruits is a sensible target for the restorative management of depleted coral‐reef fish populations, settlement‐stage ambon damsel fish Pomacentrus amboinensis were captured, tagged and then either released immediately onto small artificial reefs or held in aquaria for 1 week prior to release. Holding conditions were varied to determine whether they affected survival of fish: half the fish were held in bare tanks (non‐enriched) and the other half in tanks containing coral and sand (enriched). Holding fish for this short period had a significantly positive effect on survivorship relative to the settlement‐stage treatment group that were released immediately. The enrichment of holding conditions made no appreciable difference on the survival of fish once released onto the reef. It did, however, have a positive effect on the survival of fish while in captivity, thus supporting the case for the provision of simple environmental enrichment in fish husbandry. Collecting and holding settlement‐stage fish for at least a week before release appear to increase the short‐term survival of released fish; whether it is an effective method for longer‐term enhancement of locally depleted coral‐reef fish populations will require further study. 相似文献
356.
Dohnal AM Graffi S Witt V Eichstill C Wagner D Ul-Haq S Wimmer D Felzmann T 《Journal of cellular and molecular medicine》2009,13(1):125-135
Manufacturing procedures for cellular therapies are continuously improved with particular emphasis on product safety. We previously developed a dendritic cell (DC) cancer vaccine technology platform that uses clinical grade lipopolysaccharide (LPS) and interferon (IFN)-y for the maturation of monocyte derived DCs. DCs are frozen after 6 hrs exposure at a semi-mature stage (smDCs) retaining the capacity to secret interleukin (IL)-12 and thus support cytolytic T-cell responses, which is lost at full maturation. We compared closed systems for monocyte enrichment from leucocyte apheresis products from healthy individuals using plastic adherence, CD14 selection, or CD2/19 depletion with magnetic beads, or counter flow centrifugation (elutriation) using a clinical grade in comparison to a research grade culture medium for the following DC generation. We found that elutriation was superior compared to the other methods showing 36 ± 4% recovery, which was approximately 5-fold higher as the most frequently used adherence protocol (8 ± 1%), and a very good purity (92 ± 5%) of smDCs. Immune phenotype and IL-12 secretion (adherence: 1.4 ± 0.4; selection: 20 ± 0.6; depletion: 1 ±0.5; elutriation: 3.6 ± 1.5 ng/ml) as well as the potency of all DCs to stimulate T cells in an allogeneic mixed leucocyte reaction did not show statistically significant differences. Research grade and clinical grade DC culture media were equally potent and freezing did not impair the functions of smDCs. Finally, we assessed the functional capacity of DC cancer vaccines manufactured for three patients using this optimized procedure thereby demonstrating the feasibility of manufacturing DC cancer vaccines that secret IL-12 (9.4 ± 6.4 ng/ml). We conclude that significant steps were taken here towards clinical grade DC cancer vaccine manufacturing. 相似文献
357.
Wenling Zhang ;Zhaoyang Zeng ;Yanhong Zhou ;Wei Xiong ;Songqing Fan ;Lan Xiao ;Donghai Huang ;Zheng Li ;Dan Li ;Minghua Wu ;Xiaoling Li ;Shourong Shen ;Rong Wang ;Li Cao ;Ke Tang ;Guiyuan Li 《Acta biochimica et biophysica Sinica》2009,(5):414-428
Previous studies have revealed that Epstein-Barr virus (EBV) was closely associated with nasopharyngeal carcinoma (NPC). This study aimed to characterize the global pathways affected in the EBV-associated NPC. Combined with microdissection, gene expression profries in 22 NPCs and 10 non-tumor nasopharyngeal epithelial (NPE) tissue samples were analyzed. All NPC specimens served in the microarray analysis were positive for EBV, as judged by identification of the expression of EBV nuclear antigen 1 (EBNA1). Through gene set enrichment analysis (GSEA), we found that cell cycle pathway was the most disregulated pathway in NPC (P = 0.000, false discovery rate q-value = 0.007), which included some aberrant expressed components. We first found that overexpression of CDK4, cyclin D1, and Rb proteins, and loss of expression of proteins p16, p27, and p19 were statistically significant in NPC tissues compared with non-cancerous NPE (P〈 0.05) by real-time RT- PCR and tissue microarray. EBV-encoded small RNA-1 (EBER-1) hybridization signals in the NPC showed significant associations with the overexpression of Rb (P = 0.000), cyclin D1 (P = 0.000), CDK4 (P = 0.000), and the loss of expression of p16 proteins (P = 0.039). In the final logistic regression analysis model, EBER-1 and abnormal expression of p16, Rb, cyclin D1, and E2F6 were inde- pendent contributions to nasopharyngeal carcinogenesis. Through survival analysis, only cyclin D1 could predict the prognosis of NPC patients. These results suggested that cell cycle pathway was the most disregulated pathway in the EBV-associated NPC, and EBER-1 was closely associated with p16, CDK4, cyclin D1, and Rb. cyclin D1 could be the prognosis biomarker for NPC. 相似文献
358.
We report on the isolation and evaluation of 11 microsatellites from a widespread eastern North American wetland sedge, Carex scoparia. Loci exhibit 3-9 alleles over five populations and significant F(IS) (0.204-0.717) in most populations. All primers cross-amplify in at least two other species, and 10 cross-amplify in the more distantly related C. stipata. These markers will be used to examine population genetics and patterns of chromosomal diversification in this ecologically important sedge species and its relatives. 相似文献
359.
Xin Zhan Hai-Yan Hu Cai-Huan Ke Song-Nian Hu De-Xiang Wang Fei Chen 《Conservation Genetics》2009,10(4):1185-1187
Eleven novel microsatellite markers were isolated from small abalone, Haliotis diversicolor Reeve. These loci were tested on 22 individuals from two different geographic populations. We identified a total of 162 alleles
from the 11 microsatellite loci. All of the loci were highly polymorphic. Polymorphism information content (PIC) is ranging
from 0.7276 to 0.9163. Observed and expected heterozygosities ranged from 0.2727 to 1.0000 and from 0.7738 to 0.9429, respectively.
Three loci deviated significantly from Hardy–Weinberg equilibrium. No pairs of loci displayed linkage disequilibrium. These
polymorphic markers will be used to analyze population structure, genetic diversity and construct a genetic linkage map.
Xin Zhan and Hai-Yan Hu contribute equally to this study. 相似文献
360.