首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   818篇
  免费   126篇
  国内免费   127篇
  2024年   1篇
  2023年   27篇
  2022年   27篇
  2021年   43篇
  2020年   31篇
  2019年   59篇
  2018年   42篇
  2017年   30篇
  2016年   49篇
  2015年   45篇
  2014年   37篇
  2013年   49篇
  2012年   34篇
  2011年   33篇
  2010年   27篇
  2009年   47篇
  2008年   38篇
  2007年   49篇
  2006年   51篇
  2005年   43篇
  2004年   33篇
  2003年   31篇
  2002年   30篇
  2001年   26篇
  2000年   15篇
  1999年   25篇
  1998年   17篇
  1997年   16篇
  1996年   13篇
  1995年   14篇
  1994年   13篇
  1993年   8篇
  1992年   9篇
  1991年   6篇
  1990年   6篇
  1989年   7篇
  1988年   5篇
  1987年   4篇
  1986年   5篇
  1985年   4篇
  1984年   4篇
  1983年   2篇
  1982年   6篇
  1981年   7篇
  1979年   1篇
  1978年   2篇
排序方式: 共有1071条查询结果,搜索用时 15 毫秒
1.
大气颗粒物污染对土地覆盖变化的响应   总被引:4,自引:0,他引:4  
土地利用-覆盖变化(LUCC)直接或间接影响颗粒物污染。了解颗粒物污染对LUCC的响应,对维护和改善生态环境具有重要的意义。基于卫星遥感技术,从广域的空间尺度分析颗粒物污染对LUCC的响应。使用MODIS数据分别提取与颗粒物污染相关性较高的城市用地、林地等土地利用类型,确定土地利用类型的变化趋势,利用长时间序列MODIS气溶胶光学厚度(AOD,Aerosol optical depth)产品分析颗粒物污染与土地利用类型的变化的相关性。以山东省青岛市、淄博市、济南市3个典型城市为例,研究了AOD随土地利用类型的变化趋势。同时,考虑并分析了颗粒物污染对土地利用变化响应的敏感性,以及城市区域变化对环境的影响。研究结果表明,不同的城市类型,由于决定环境变化主导因素的差异,颗粒物污染对LUCC的响应具有明显的差异。青岛市地区,由于受海洋影响显著,大气颗粒物污染与LUCC的相关性较低,如中度污染天气与林地的相关系数为-0.451;而淄博市和济南市的相关系数分别为-0.473、-0.507。  相似文献   
2.
Ultrasound scanning traits have been adapted in selection programs in many countries to improve carcass traits for lean meat production. As the genetic parameters of the traits interested are important for breeding programs, the estimation of these parameters was aimed at the present investigation. The estimated parameters were direct and maternal heritability as well as genetic correlations between the studied traits. The traits were backfat thickness (BFT), skin+backfat thickness (SBFT), eye muscle depth (MD) and live weights at the day of scanning (LW). The breed investigated was Kivircik, which has a high quality of meat. Six different multi-trait animal models were fitted to determine the most suitable model for the data using Bayesian approach. Based on deviance information criterion, a model that includes direct additive genetic effects, maternal additive genetic effects, direct maternal genetic covariance and maternal permanent environmental effects revealed to be the most appropriate for the data, and therefore, inferences were built on the results of that model. The direct heritability estimates for BFT, SBFT, MD and LW were 0.26, 0.26, 0.23 and 0.09, whereas the maternal heritability estimates were 0.27, 0.27, 0.24 and 0.20, respectively. Negative genetic correlations were obtained between direct and maternal effects for BFT, SBFT and MD. Both direct and maternal genetic correlations between traits were favorable, whereas BFT–MD and SBFT–MD had negligible direct genetic correlation. The highest direct and maternal genetic correlations were between BFT and SBFT (0.39) and between MD and LW (0.48), respectively. Our results, in general, indicated that maternal effects should be accounted for in estimation of genetic parameters of ultrasound scanning traits in Kivircik lambs, and SBFT can be used as a selection criterion to improve BFT.  相似文献   
3.
Monitoring ecosystem functions in forests is a priority in a climate change scenario, as climate‐induced events may initially alter the functions more than slow‐changing attributes, such as biomass. The ecosystem functional properties (EFPs) are quantities that characterize key ecosystem processes. They can be derived by point observations of gas and energy exchanges between the ecosystems and the atmosphere that are collected globally at FLUXNET flux tower sites and upscaled at ecosystem level. The properties here considered describe the ability of ecosystems to optimize the use of resources for carbon uptake. They represent functional forest information, are dependent on environmental drivers, linked to leaf traits and forest structure, and influenced by climate change effects. The ability of vegetation optical depth (VOD) to provide forest functional information is investigated using 2011–2014 satellite data collected by the Soil Moisture and Ocean Salinity mission and using the EFPs as reference dataset. Tropical forests in Africa and South America were analyzed, also according to ecological homogeneous units. VOD jointly with water deficit information explained 93% and 87% of the yearly variability in both flux upscaled maximum gross primary productivity and light use efficiency functional properties, in Africa and South America forests respectively. Maps of the retrieved properties evidenced changes in forest functional responses linked to anomalous climate‐induced events during the study period. The findings indicate that VOD can support the flux upscaling process in the tropical range, affected by high uncertainty, and the detection of forest anomalous functional responses. Preliminary temporal analysis of VOD and EFP signals showed fine‐grained variability in periodicity, in signal dephasing, and in the strength of the relationships. In selected drier forest types, these satellite data could also support the monitoring of functional dynamics.  相似文献   
4.
Sodium and oxygen are prevalent impurities in kesterite solar cells. Both elements are known to strongly impact performance of the kesterite devices and can be connected to efficiency improvements seen after heat treatments. The sodium distribution in the kesterite absorber is commonly reported, whereas the oxygen distribution has received less attention. Here, a direct relationship between sodium and oxygen in kesterite absorbers is established using secondary ion mass spectrometry and explained by defect analyses within the density functional theory. The calculations reveal a binding energy of 0.76 eV between the substitutional defects NaCu and OS in the nearest neighbor configuration, indicating an abundance of Na? O complexes in kesterite absorbers at relevant temperatures. Oxygen incorporation is studied by introducing isotopic 18O at different stages of the Cu2ZnSnS4/Mo/soda‐lime glass baseline processing. It is observed that oxygen from the Mo back contact and contaminations during the sulfurization are primary contributors to the oxygen distribution. Indeed, unintentional oxygen incorporation leads to immobilization of sodium. This results in a strong correlation between sodium and oxygen, in excellent agreement with the theoretical calculations. Consequently, oxygen availability should be monitored to optimize postdeposition heat treatments to control impurities in kesterite absorbers and ultimately, the solar cell efficiency.  相似文献   
5.
目的:探讨早期食管癌的浸润深度的临床病理因素。方法:对术前胃镜发现、术后病理证实的55例早期食管癌进行统计,分析早期食管癌内镜下表现、肿瘤分化程度、淋巴转移与浸润深度之间的关系。结果:表面扩散生长型、腔内生长型高度<5mm、双向生长型高度<2mm及壁内生长型深度<0.5mm提示黏膜内癌。腔内生长型高度≥5mm、壁内生长型高度≥0.5mm、双向生长型高度≥2mm及混合生长型提示黏膜下癌。肿瘤分化程度与浸润深度呈负相关。4例淋巴结转移及2例淋巴管癌栓,其中1例为黏膜癌。5例为黏膜下层癌,原位癌均未发生转移。结论:内镜下表现与浸润深度有密切关系,肿瘤恶性程度越高则越易向深层浸润,浸润越深淋巴结转移率越高。  相似文献   
6.
吴鹏飞  曾宗永  王跃招  朱波 《生态学杂志》2005,24(10):1241-1244
在2002年9月,分别利用洞穴深度法、标志重捕法和洞口计数法对分布在若尔盖草原荒漠中的青海沙蜥(Phrynocephalus vlangalii)的种群密度进行调查,所得的结果分别是190.4、76.8和250.7只.1000 m-2。通过对3种结果的比较与分析,证明洞穴深度法有较高的可靠性,这种方法的理论依据是:沙蜥(Phrynocephalus)是一种变温动物,不能长时间处在温度低于致死低温(-2.5℃)以下的环境中,为了能够成功地度过漫长而寒冷的冬季,它必须居住在深度达到最大冻土层之下的洞穴中。这是沙蜥躲避低温伤害的一种行为机制。该方法可以适用于分布在中国的沙蜥属的其它物种密度调查。  相似文献   
7.
Between May 2000 and February 2001, a major bloom of the toxic dinoflagellate Gymnodinium catenatum (a causative organism of Paralytic Shellfish Poisoning, PSP) affected over 1500 km of coastline of New Zealand’s North Island. As this was the first record of this species in New Zealand, we aimed to resolve whether this represented a recent introduction/spreading event or perhaps an indigenous cryptic species stimulated by environmental/climatic change. To answer this question, we analysed for G. catenatum resting cysts in 210Pb dated sediment cores (18–34 cm long; sedimentation rates 0.34–0.69 cm per year) collected by SCUBA divers from Manukau Harbour, where the species was first detected, and from Hokianga Harbour, where the highest shellfish toxicity was recorded, while using Wellington Harbour as a well-monitored control site. The results of this study conclusively demonstrate that abundant G. catenatum has been in northern New Zealand at least since the early 1980s, increasing up to 1200 cysts/g around the year 2000, but with low cyst concentrations possibly present since at least 1937. In contrast, Wellington Harbour cores contained only very sparse G. catenatum cysts (8 cysts/g), present only to a depth of 7 cm (surface mixed layer depth), reflecting an apparent recent range expansion of this dinoflagellate in New Zealand, possibly stimulated by unusual climatic conditions associated with the 2000 La Nina event. The significant increases since the early 1980s also of Protoperidinium cysts at Hokianga Harbour and of Gonyaulax, Protoperidinium and Protoceratium cysts at Manukau Harbour suggest a broad scale environmental change has occurred in Northland, New Zealand.  相似文献   
8.
Wavefront shaping can compensate the wavefront distortions in deep tissue focusing, leading to an improved penetration depth. However, when using the backscattered signals as the feedback, unexpected compensation bias may be introduced, resulting in focusing position deviations or even no focus in the illumination focal plane. Here we investigated the reliability of wavefront shaping based on coherent optical adaptive technique in deep tissue focusing by measuring the position deviations between the foci in the illumination focal plane and the epi‐detection plane. The experimental results show that when the penetration depth reaches 150 μm in mouse brain tissue (with scattering coefficient ~22.42 mm?1) using a 488 nm laser and an objective lens with 0.75 numerical aperture, the center of the real focus will deviate out of one radius range of the Airy disk while the optimized focus in the epi‐detection plane maintained basically at the center. With the penetration depth increases, the peak to background ratio of the focus in the illumination focal plane decreases faster than that in the epi‐detection plane. The results indicate that when the penetration depth reaches 150 μm, feedback based on backscattered signals will make wavefront shaping lose its reliability, which may provide a guidance for applications of non‐invasive precise optogenetics or deep tissue optical stimulation using wavefront shaping methods. A, Intensity distribution in the epi‐detection plane and the illumination focal plane before and after correction, corresponding to brain sections with 250 and 300 μm thickness, respectively. Scale bar is 2 μm. B, Averaged focusing deviations in the epi‐detection plane (optimized) and the illumination focal plane (monitored) after compensation. The unit of the ordinate is one Airy disk diameter. Black dashed line represents one Airy disk radius. Bars represent the SE of each measurement set.   相似文献   
9.
Abstract.  1. It has long been known that foraging bumblebee workers vary greatly in size, within species, and within single nests. This phenomenon has not been adequately explained. Workers of their relatives within the Apidae exhibit much less size variation.
2. For the bumblebee Bombus terrestris size, as measured by thorax width, was found to correspond closely with tongue length, so that larger bees are equipped to feed from deeper flowers.
3. The mean size of worker bees attracted to flowers was found to differ between plant species, and larger bees with longer tongues tended to visit deeper flowers.
4. Finally, handling time depended on the match between corolla depth and tongue length: large bees were slower than small bees when handling shallow flowers, but quicker than small bees when handling deep flowers.
5. Size variation within bumblebees may be adaptive, since it enables the colony as a whole to efficiently exploit a range of different flowers. Possible explanations for the marked differences in size variation exhibited by bumblebees compared with Apis species and stingless bees (Meliponinae) are discussed.  相似文献   
10.
The relative importance of seed availability, waterdepth, and soil phosphorus (P) concentrations oncattail (Typha domingensis pers.) earlyestablishment in an Everglades wetland area wasexamined using seed bank analysis and controlledexperiments. The experiments measured seed germinationand seedling growth in tanks with cattail seedaddition subjected to two P concentrations(un-enriched vs. enriched) and water depth (saturatedvs. flooded soils). A limited seed bank (223 ± 69m2) of cattail was found in the surface soil ofthe area studied. The germination of added seeds wasinhibited under flooded conditions, and only 0.6% ofthe germination was found. In contrast,under-saturated soil conditions, a maximum of 6% and15% germination was observed in P-un-enriched andP-enriched treatments, respectively. High mortality ofseedlings occurred regardless of P treatments followinga cold spell. However, P enrichment resulted inincreased seedling growth and asexual propagation.These results suggested the importance of theconcurrence of appropriate hydrologic regimes, Penrichment, and air temperature on the recruitment ofplant species.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号