全文获取类型
收费全文 | 897篇 |
免费 | 105篇 |
国内免费 | 135篇 |
专业分类
1137篇 |
出版年
2024年 | 11篇 |
2023年 | 34篇 |
2022年 | 30篇 |
2021年 | 45篇 |
2020年 | 73篇 |
2019年 | 77篇 |
2018年 | 64篇 |
2017年 | 40篇 |
2016年 | 49篇 |
2015年 | 49篇 |
2014年 | 31篇 |
2013年 | 69篇 |
2012年 | 39篇 |
2011年 | 39篇 |
2010年 | 38篇 |
2009年 | 31篇 |
2008年 | 37篇 |
2007年 | 48篇 |
2006年 | 40篇 |
2005年 | 43篇 |
2004年 | 34篇 |
2003年 | 26篇 |
2002年 | 25篇 |
2001年 | 25篇 |
2000年 | 19篇 |
1999年 | 16篇 |
1998年 | 11篇 |
1997年 | 8篇 |
1996年 | 9篇 |
1995年 | 12篇 |
1994年 | 6篇 |
1993年 | 11篇 |
1992年 | 10篇 |
1991年 | 5篇 |
1990年 | 5篇 |
1989年 | 3篇 |
1988年 | 1篇 |
1987年 | 2篇 |
1986年 | 4篇 |
1985年 | 3篇 |
1984年 | 3篇 |
1982年 | 4篇 |
1981年 | 1篇 |
1979年 | 2篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1973年 | 1篇 |
1972年 | 2篇 |
排序方式: 共有1137条查询结果,搜索用时 0 毫秒
81.
82.
Pittaya Dankulchai Wiwatchai Sittiwong Wanwarang Teerasamit 《Reports of Practical Oncology and Radiotherapy》2022,27(2):260
BackgroundThe most common site of recurrence of prostate cancer after definite radiation therapy is the dominant intraprostatic lesion (DIL). This study aimed to investigate the feasibility and safety of definite volumetric modulated arc therapy (VMAT) with simultaneous integrated boost (SIB) to the DIL in patients with unfavorable intermediate to high-risk prostate cancer.Materials and methodsIn this prospective uncontrolled clinical trial, patients were delivered VMAT at a dose of 87.75 Gy in 39 fractions or 70 Gy in 20 fractions to the DIL in combination with androgen deprivation therapy. Genitourinary (GU) and rectal toxicity, International Prostate Symptom Score (IPSS) and IPSS quality of life (IPSS-QOL) score were collected.ResultsForty-five patients with a median follow-up of 20 months were analyzed. The cumulative incidence of acute grade ≥ 2 GU and rectal toxicity was 33.1% and 9.5%, respectively. Regarding late toxicity, the cumulative incidence of grade ≥ 2 GU and rectal toxicity was 12.6% and 2.8%, respectively. During treatment, the mean increase of IPSS was +7.4 ± 4.2 and the mean increase of IPSS-QOL was +1.7 ± 1.3. However, both IPSS and IPSS-QOL scores returned to their baseline levels by 3-months post-treatment. No significant correlation between baseline characteristics and grade ≥ 2 GU or rectal toxicity was found.ConclusionFocal SIB to the DIL of ≥ 90 Gy EQD2 in unfavorable intermediate to high-risk prostate cancer patients resulted in tolerable toxicity profiles. The mean IPSS and IPSS-QOL scores both worsened during treatment; however, both scores returned to baseline level by 3 months after treatment. 相似文献
83.
土壤-植物-大气连续体水热、CO2通量估算模型研究进展 总被引:3,自引:0,他引:3
土壤-植物-大气连续体(SPAC)水热、CO2通量的准确估算对理解陆地和大气的物质和能量交换过程有着重要意义.重点阐述了基于过程的土壤-植物-大气连续体水热、CO2通量模型,综述了统计模型、综合模型及基于遥感的模型的发展过程.其中水热通量统计模型包括基于温度和湿度以及基于温度和辐射的方法;CO2通量统计模型包括基于气候因子或蒸散因子以及基于光能利用率的方法.水热通量过程模型包括大叶、双源、多源和多层的水热传输物理模型;CO2通量过程模型包括叶片尺度及由大叶、双叶和多层方法扩展到冠层尺度的生理生态模型以及光合-蒸腾耦合模型.综合模型包括生物物理模型、生物化学模型和生物地理模型.统计模型形式简单,资料易得,对大范围的水热通量模拟具有指导意义;过程模型准确的揭示了水热和CO2通量传输的物理和生理过程,是大尺度综合模型的基础.未来生态系统水热、CO2通量估算模型将集成各种技术手段进行多尺度网络观测和大尺度机理模拟. 相似文献
84.
Matthew B. Dugas Susannah R. Halbrook Allison M. Killius Jillian F. del Sol Corinne L. Richards‐Zawacki 《Ethology : formerly Zeitschrift fur Tierpsychologie》2015,121(8):813-822
The phenomenon of aposematism, or the pairing of antipredator defence with conspicuous or distinctive signals, serves as an excellent example of how traits act in concert to shape fitness. Not only does this complex phenotype require the integration of multiple traits, it alters the fitness pay‐offs of yet others. The protection offered by aposematism may, for example, reduce the costs associated with foraging or sexual display. Thus, well‐protected aposematic lineages should be bolder, more active and less likely to respond to perceived threats of predation than more cryptic lineages. Comparisons of differently coloured morphs of the polytypic strawberry poison frog (Oophaga pumilio) have supported the predicted behavioural correlates of aposematism, with the exception of those regarding responsiveness to simulated predators. We tested the key prediction that aposematic coloration will be associated with reduced sensitivity to predators in two polymorphic O. pumilio populations. The novel approach of studying polymorphic populations allowed us to assess the effect of colour in the absence of potentially confounding habitat differences. We found that colour was associated with the probability that a frog would attempt escape and the distance at which it fled, but only in one population, and not in the predicted direction. An overall comparison of the two populations superficially supported our predictions, but this pattern actually arose because frogs occupying higher perches were less likely to respond, a pattern that may reflect the value of high perches and the costs associated with returning to them after attempted escape. These results highlight the complexity of the relationship between predators and prey, the challenges associated with understanding how and why traits are correlated, and the intimate ties between behaviour and morphological evolution. 相似文献
85.
Gjalt Huppes Arjan de Koning Sangwon Suh Reinout Heijungs Lauran van Oers Per Nielsen Jeroen B. Guine 《Journal of Industrial Ecology》2006,10(3):129-146
For developing product policy, insight into the environmental effects of products is required. But available life-cycle assessment studies (LCAs) are hardly comparable between different products and do not cover total consumption. Input-output analysis with environmental extensions (EEIOA) of full consumption is not available for the European Union. Available country studies have a low sector resolution and a limited number of environmental extensions. This study fills the gap between detailed LCA and low-resolution EEIOA, specifying the environmental effects of household consumption in the European Union, discerning nearly 500 sectors, while specifying a large number of environmental extensions. Added to the production sectors are a number of consumption activities with direct emissions, such as automobile driving, cooking and heating, and a number of postconsumer waste management sectors. The data for Europe have been constructed by using the sparse available and coarse economic and environmental data on European countries and adding technological detail mainly based on data from the United States.
A small number of products score high on environmental impact per Euro and also have a substantial share of overall consumer expenditure. Several meat and dairy products, household heating, and car driving thus have a large share of the total environmental impact. Due to their sales volume, however, products with a medium or low environmental score per Euro may also have a substantial impact. This is the case with bars and restaurants, clothing, residential construction, and even a service such as telecommunications. The limitations in real European data made heroic assumptions necessary to operationalize the model. One conclusion, therefore, is that provision of data in Europe urgently needs to be improved, at least to the level of sector detail currently available for the United States and Japan. 相似文献
A small number of products score high on environmental impact per Euro and also have a substantial share of overall consumer expenditure. Several meat and dairy products, household heating, and car driving thus have a large share of the total environmental impact. Due to their sales volume, however, products with a medium or low environmental score per Euro may also have a substantial impact. This is the case with bars and restaurants, clothing, residential construction, and even a service such as telecommunications. The limitations in real European data made heroic assumptions necessary to operationalize the model. One conclusion, therefore, is that provision of data in Europe urgently needs to be improved, at least to the level of sector detail currently available for the United States and Japan. 相似文献
86.
M. F. Moura M. C. Picanço R. N. C. Guedes E. C. Barros M. Chediak E. G. F. Morais 《Journal of Applied Entomology》2007,131(3):215-220
Abstract: A conventional sampling plan for the leafhopper Empoasca kraemeri Ross & Moore (Auchenorrhyncha: Cicadellidae) in common beans (Phaseolus vulgaris L.) is presented based on samples from 51 commercial fields. The research encompassed three phases. The first phase had the objective of determining the most suitable leaf to be used as a precise and representative sampling unit. In the second phase of the project, the available sampling methods for E. kraemeri were compared, including the approach identified in the first phase. In the third phase, the theoretical frequency distribution of the sampling data was assessed and the number of samples necessary for the sampling plan was established. The best leaves to sample adult leafhoppers were the fourth and the fifth from the apex, while the best leaf for sampling nymphs was the fifth from the apex. The best sampling technique for nymphs and adults was the beating of the apical leaves against a plastic tray. The sampling data obtained with this technique were fit to a negative binomial distribution with common aggregation parameter for adults ( K common = 1.04) and nymphs ( K common = 0.47). This sampling plan required 16.8 and 31.2 min to be carried out at an expense of US$ 0.31 and US$ 0.54 for sampling adults and nymphs in a field, respectively. 相似文献
87.
Chlorantraniliprole seed treatments in rice provide effective suppression of rice water weevil populations in the United States; however, heavy reliance on prophylactic insecticide treatments as a sole strategy could destabilize management programs for this insect. The present research evaluated the compatibility of seed treatments with two other potential management tactics—plant resistance and shallow flooding–by conducting two split‐plot experiments in 2009 and 2011. In both experiments, no substantial antagonism was found among the 3 different tactics. Statistical interactions in these experiments arose from the strong and persistent effects of chlorantraniliprole on larval densities rather than incompatibility of tactics. In 2009, weevil densities differed among varieties and were significantly lower on the cultivar “Jefferson.” In 2011, weevil densities were reduced significantly in shallow‐flooded plots compared to deep‐flooded plots. Significant reductions in weevil numbers by chlorantraniliprole seed treatments, even at application rates 5 fold lower than commercially recommended rates, demonstrated the potential to reduce application rates of this highly potent larvicide. These latter results suggest that future studies on the relationship between chlorantraniliprole seed treatment rate and weevil fitness are warranted. 相似文献
88.
Michael J. Furlong 《Insect Science》2015,22(1):6-19
The importance of natural enemies as the foundation of integrated pest management (IPM) is widely accepted, but few studies conduct the manipulative field experiments necessary to directly quantify their impact on pest populations in this context. This is particularly true for predators. Studying arthropod predator–prey interactions is inherently difficult: prey items are often completely consumed, individual predator–prey interactions are ephemeral (rendering their detection difficult) and the typically fluid or soft‐bodied meals cannot be easily identified visually within predator guts. Serological techniques have long been used in arthropod predator gut‐contents analysis, and current enzyme linked immunosorbent assays (ELISA) are highly specific and sensitive. Recently, polymerase chain reaction (PCR) methods for gut‐contents analysis have developed rapidly and they now dominate the diagnostic methods used for gut‐contents analysis in field‐based research. This work has identified trophic linkages within food webs, determined predator diet breadth and preference, demonstrated the importance of cannibalism and intraguild predation within and between certain taxa, and confirmed the benefits (predator persistence) and potential disadvantages (reduced feeding on pest species) of the availability of alternative nonpest prey. Despite considerable efforts to calibrate gut‐contents assays, these methods remain qualitative. Available techniques for predator gut‐contents analysis can provide rapid, accurate, cost‐effective identification of predation events. As such, they perfectly compliment the ecological methods developed to directly assess predator impacts on prey populations but which are imperfect at identifying the key predators. These diagnostic methods for gut‐contents analysis are underexploited in agricultural research and they are almost never applied in unison with the critical field experiments to measure predator impact. This paper stresses the need for a combined approach and suggests a framework that would make this possible, so that appropriate natural enemies can be targeted in conservation biological control. 相似文献
89.
Effects of grass species and grass growth on atmospheric nitrogen deposition to a bog ecosystem surrounded by intensive agricultural land use 下载免费PDF全文
Miriam Hurkuck Christian Brümmer Karsten Mohr Oliver Spott Reinhard Well Heinz Flessa Werner L. Kutsch 《Ecology and evolution》2015,5(13):2556-2571
We applied a 15N dilution technique called “Integrated Total Nitrogen Input” (ITNI) to quantify annual atmospheric N input into a peatland surrounded by intensive agricultural practices over a 2-year period. Grass species and grass growth effects on atmospheric N deposition were investigated using Lolium multiflorum and Eriophorum vaginatum and different levels of added N resulting in increased biomass production. Plant biomass production was positively correlated with atmospheric N uptake (up to 102.7 mg N pot−1) when using Lolium multiflorum. In contrast, atmospheric N deposition to Eriophorum vaginatum did not show a clear dependency to produced biomass and ranged from 81.9 to 138.2 mg N pot−1. Both species revealed a relationship between atmospheric N input and total biomass N contents. Airborne N deposition varied from about 24 to 55 kg N ha−1 yr−1. Partitioning of airborne N within the monitor system differed such that most of the deposited N was found in roots of Eriophorum vaginatum while the highest share was allocated in aboveground biomass of Lolium multiflorum. Compared to other approaches determining atmospheric N deposition, ITNI showed highest airborne N input and an up to fivefold exceedance of the ecosystem-specific critical load of 5–10 kg N ha−1 yr−1. 相似文献
90.
Drivers of wildlife population dynamics are generally numerous and interacting. Some of these drivers may impact demographic processes that are difficult to estimate, such as immigration into the focal population. Populations may furthermore be small and subject to demographic stochasticity. All of these factors contribute to blur the causal relationship between past management action and current population trends. The urban Peregrine Falcon Falco peregrinus population in Cape Town, South Africa, increased from three pairs in 1997 to 18 pairs in 2010. Nestboxes were installed over this period to manage the interface between new urban pairs of Falcons and the human users of colonized buildings, and incidentally to improve breeding success. We used integrated population models (IPMs) formally to combine information from a capture–mark–recapture study, monitoring of reproductive success and counts of population size. As all local demographic processes were directly observed, the IPM approach also allowed us to estimate immigration by difference. The provision of nestboxes, as a possible stimulant of population growth, improved breeding success and accounted for an estimated 3–26% of the population increase. The most important driver of growth, however, was immigration. Despite low sample sizes, the IPM approach allowed us to obtain relatively precise estimates of the population‐level impact of nestbox deployment. The goal of conservation interventions is often to increase population size, so the effectiveness of such interventions should ideally be assessed at the population level. IPMs are powerful tools in this context for combining demographic information that may be limited due to small population size or practical constraints on monitoring. Our study quantitatively documented both the immigration process that led to growth of a small population and the effect of a management action that helped the process. 相似文献