全文获取类型
收费全文 | 92047篇 |
免费 | 23032篇 |
国内免费 | 31470篇 |
专业分类
测绘学 | 16977篇 |
大气科学 | 12864篇 |
地球物理 | 17277篇 |
地质学 | 58874篇 |
海洋学 | 16867篇 |
天文学 | 1347篇 |
综合类 | 8213篇 |
自然地理 | 14130篇 |
出版年
2024年 | 591篇 |
2023年 | 2015篇 |
2022年 | 5549篇 |
2021年 | 6752篇 |
2020年 | 5386篇 |
2019年 | 6000篇 |
2018年 | 5777篇 |
2017年 | 5490篇 |
2016年 | 5398篇 |
2015年 | 6701篇 |
2014年 | 6388篇 |
2013年 | 7934篇 |
2012年 | 8555篇 |
2011年 | 8646篇 |
2010年 | 8337篇 |
2009年 | 7796篇 |
2008年 | 7807篇 |
2007年 | 7422篇 |
2006年 | 7046篇 |
2005年 | 5941篇 |
2004年 | 4448篇 |
2003年 | 3225篇 |
2002年 | 3326篇 |
2001年 | 2959篇 |
2000年 | 2448篇 |
1999年 | 1225篇 |
1998年 | 587篇 |
1997年 | 486篇 |
1996年 | 369篇 |
1995年 | 277篇 |
1994年 | 269篇 |
1993年 | 196篇 |
1992年 | 190篇 |
1991年 | 117篇 |
1990年 | 143篇 |
1989年 | 78篇 |
1988年 | 70篇 |
1987年 | 77篇 |
1986年 | 68篇 |
1985年 | 66篇 |
1984年 | 52篇 |
1983年 | 42篇 |
1982年 | 35篇 |
1981年 | 26篇 |
1980年 | 14篇 |
1979年 | 48篇 |
1978年 | 9篇 |
1958年 | 15篇 |
1957年 | 33篇 |
1954年 | 58篇 |
排序方式: 共有10000条查询结果,搜索用时 140 毫秒
31.
ZHANG Guangliang BAI Junhong JIA Jia WANG Xin WANG Wei ZHAO Qingqing ZHANG Shuai 《中国地理科学(英文版)》2018,(3)
Plant invasion alters the fundamental structure and function of native ecosystems by affecting the biogeochemical pools and fluxes of materials and energy. Native(Suaeda salsa) and invasive(Spartina alterniflora) salt marshes were selected to study the effects of Spartina alterniflora invasion on soil organic carbon(SOC) contents and stocks in the Yellow River Delta. Results showed that the SOC contents(g/kg) and stocks(kg/m~2) were significantly increased(P 0.05) after Spartina alterniflora invasion of seven years, especially for the surface soil layer(0–20 cm). The SOC contents exhibited an even distribution along the soil profiles in native salt marshes, while the SOC contents were gradually decreased with depth after Spartina alterniflora invasion of seven years. The natural ln response ratios(Ln RR) were applied to identify the effects of short-term Spartina alterniflora invasion on the SOC stocks. We also found that Spartina alterniflora invasion might cause soil organic carbon losses in a short-term phase(2–4 years in this study) due to the negative Ln RR values, especially for 20–60 cm depth. And the SOCD in surface layer(0–20 cm) do not increase linearly with the invasive age. Spearman correlation analysis revealed that silt + clay content was exponentially related with SOC in surface layer(Adjusted R~2 = 0.43, P 0.001), suggesting that soil texture could play a key role in SOC sequestration of coastal salt marshes. 相似文献
32.
参对光照变化非常敏感,研究刺参对光照的分子响应非常重要。本研究应用RNA测序获取了刺参暴露于强光(“强光”)、正常光照(“对照”)和完全黑暗(“黑暗”)环境下体壁的基因表达谱情况,通过“对照”与“黑暗”,“对照”与“强光”和“黑暗”与“强光”的比较,在|log2 ratio|≥1和FDR≤0.001的标准下分别发现了1161、113和1705个差异表达基因(DEGs)。基因本体分析表明,“cellular process”和“binding”在“生物过程”和“分子功能”类别中的DEGs富集最多,而“cell”和“cell part”在“细胞组分”类别中的DEGs富集最多。将DEGs与Kyoto Encyclopedia基因和基因组数据库上的于214、41和229条通路进行比对,发现了51、2和57条通路分别显著富集。本研究发现的光特异性DEGs可作为深入研究刺参对光照变化的生化适应机制的重要目标基因。 相似文献
33.
Due to the Tibetan Plateau's unique high altitude and low temperature climate conditions,the region's alpine steppe ecosystem is highly fragile and is suffering from severe degradation under the stress of increasing population,overgrazing,and climate change.The soil stoichiometry,a crucial part of ecological stoichiometry,provides a fundamental approach for understanding ecosystem processes by examining the relative proportions and balance of the three elements.Understanding the impact of degradation on the soil stoichiometry is vital for conservation and management in the alpine steppe on the Tibetan Plateau.This study aims to examine the response of soil stoichiometry to degradation and explore the underlying biotic and abiotic mechanisms in the alpine steppe.We conducted a field survey in a sequent degraded alpine steppe with seven levels inNorthern Tibet.The plant species,aboveground biomass,and physical and chemical soil properties such as the moisture content,temperature,pH,compactness,total carbon(C),total nitrogen(N),and total phosphorus(P)were measured and recorded.The results showed that the contents of soil C/N,C/P,and N/P consistently decreased along intensifying degradation gradients.Using regression analysis and a structural equation model(SEM),we found that the C/N,C/P,and N/P ratios were positively affected by the soil compactness,soil moisture content and species richness of graminoids but negatively affected by soil pH and the proportion of aboveground biomass of forbs.The soil temperature had a negative effect on the C/N ratio but showed positive effect on the C/P and N/P ratios.The current study shows that degradation-induced changes in abiotic and biotic conditions such as soil warming and drying,which accelerated the soil organic carbon mineralization,as well as the increase in the proportion of forbs,whichwere difficult to decompose and input less organic carbon into soil,resulted in the decreases in soil C/N,C/P,and N/P contents to a great extent.Our results provide a sound basis for sustainable conservation and management of the alpine steppe. 相似文献
34.
随着化肥、农膜等在农业生产中的过量投入,耕地面源污染的程度随之加重。文章选取塔里木河流域上游和田地区为研究区域,依据P-S-R框架理论,构建和田地区耕地面源污染生态风险评价指标体系,加入土壤理化数据,使用生态风险评价模型对和田地区1980 年及2016 年耕地面源污染状况进行生态风险评价,运用耕地生态风险模型、生态风险转移矩阵、Arcgis分析和田地区耕地面源污染时空分异状况。研究结论如下:和田地区1980 年耕地生态风险等级均为II级或III级,呈“中间高,两侧低”分布;2016 年耕地生态风险等级上升至IV级或V级,呈“倒W型”分布,各县耕地面源污染程度较1980 年均有较大幅度的上升,其中墨玉县和于田县在2016 年耕地生态风险等级达到最高的V级,而民丰县因自身生态环境的强脆弱性,同样需要提高关注。根据面源污染“从源头治理”的原则,应切实推进和田地区耕地生态环境保护与治理,提高政府重视程度,增强技术指导,开展试点工作,改善和田地区耕地面源污染现状。 相似文献
35.
海洋环境保护标准化对我国海洋环境保护事业的发展具有重要的技术支撑作用。文章从海洋环境保护标准体系、标准发布和制修订以及标准类别特征等方面,概述我国海洋环境保护标准化的发展现状;分析海洋环境保护标准化的需求,主要包括标准的适用性和有效性有待评估和提升、标准制修订的进程有待加快、监测方法标准的体例和结构有待规范和统一、海洋高新技术标准体系有待建立以及与多学科和其他行业标准有待统筹5个方面。基于此,提出促进和保障海洋环境保护标准化工作发展的4点建议,即建立海洋环境保护标准化工作专家库、健全海洋环境保护标准体系、加强多部门合作和积极参与海洋国际标准化工作。 相似文献
36.
弧形海岸裂流的数值模拟研究 总被引:3,自引:0,他引:3
弧形海岸波浪产生的裂流严重危害人类活动,但是目前对其特征缺乏充分认识。本文对Haller物理模型实验和三亚大东海的数值模拟表明FUNWAVE模式具有较好的裂流模拟能力。基于该模式进行了多种弧形海岸条件的裂流数值模拟,给出裂流的一些特征:(1)海岸弯曲度增大,裂流增强;(2)海岸坡度对裂流有比较大的影响,太陡或太平缓的海岸不利于形成裂流;(3)海岸尺寸减小,裂流减弱;(4)波高和波周期增大,裂流增强,但是对于某些海岸而言,0.4m波高可能就存在危害比较大的裂流。 相似文献
37.
文章分析了2013年南海南部4个季节航次的叶绿素a (Chl a)调查数据, 结果显示: 150m以浅水柱Chl a质量浓度均值分别为早春0.14mg•m-3、初夏0.12mg•m-3、初秋0.18mg•m-3、初冬0.16mg•m-3。早春和初夏偏低的原因与早春风速小, 初夏水温高, 不利于水体的垂直混合, 限制了深层海水中丰富的营养盐向上层水体补充有关。4个季节中海水次表层Chl a质量浓度最大值层(SCML)均出现在50m和75m, 这两个水层的Chl a质量浓度差异小, 季节变化不大, 平均值变化范围分别为0.24~0.26mg•m-3和0.22~0.26mg•m-3。受混合层深度和温跃层上界深度的共同影响, 50m水层Chl a质量浓度主要受制于深层富营养盐海水的向上补充, 75m水层Chl a质量浓度受水温的影响明显。 相似文献
38.
Zhong-bao Liu Fang-xiao Zhou Zhen-tao Qin Xue-gang Luo Jing Zhang 《Astrophysics and Space Science》2018,363(7):140
Support Vector Machine (SVM) is a popular data mining technique, and it has been widely applied in astronomical tasks, especially in stellar spectra classification. Since SVM doesn’t take the data distribution into consideration, and therefore, its classification efficiencies can’t be greatly improved. Meanwhile, SVM ignores the internal information of the training dataset, such as the within-class structure and between-class structure. In view of this, we propose a new classification algorithm-SVM based on Within-Class Scatter and Between-Class Scatter (WBS-SVM) in this paper. WBS-SVM tries to find an optimal hyperplane to separate two classes. The difference is that it incorporates minimum within-class scatter and maximum between-class scatter in Linear Discriminant Analysis (LDA) into SVM. These two scatters represent the distributions of the training dataset, and the optimization of WBS-SVM ensures the samples in the same class are as close as possible and the samples in different classes are as far as possible. Experiments on the K-, F-, G-type stellar spectra from Sloan Digital Sky Survey (SDSS), Data Release 8 show that our proposed WBS-SVM can greatly improve the classification accuracies. 相似文献
39.
拥有强大的渗透压调节能力对广盐性鱼类的生存至关重要。目前,关于鱼类渗透压调节机制已有不少研究,但均存在较大的局限性。本文从广盐性鱼类渗透压信号转导机制、渗透胁迫的细胞调控机制、渗透调控的内分泌调节机制和无机离子通道和转运蛋白介导的渗透调控等方面对广盐性鱼类的渗透压适应性和生理可塑性机制进行分析,以期从分子、细胞、通路和生理等层次初步探索广盐性鱼类盐度胁迫后的可塑性表型变化和不同盐度条件下的应答机制,为广盐性鱼类的渗透压调节机制的深入研究奠定基础。对广盐性鱼类渗透压适应性与生理可塑性机制研究,有助于研究其环境适应机理,促进野生鱼种的人工化养殖以及新品种的育种从而提高经济效益、社会效益和生态效益,对促进水产养殖学进步以及养殖业的发展具有重要意义,同时,为研究广盐性鱼类的渗透压调节机制开辟新的研究方向。 相似文献
40.
基于ROMS和4DVAR的沿轨与网格化SSH数据同化效果评价 总被引:1,自引:1,他引:0
Remote sensing products are significant in the data assimilation of an ocean model. Considering the resolution and space coverage of different remote sensing data, two types of sea surface height(SSH) product are employed in the assimilation, including the gridded products from AVISO and the original along-track observations used in the generation. To explore their impact on the assimilation results, an experiment focus on the South China Sea(SCS) is conducted based on the Regional Ocean Modeling System(ROMS) and the four-dimensional variational data assimilation(4 DVAR) technology. The comparison with EN4 data set and Argo profile indicates that, the along-track SSH assimilation result presents to be more accurate than the gridded SSH assimilation, because some noises may have been introduced in the merging process. Moreover, the mesoscale eddy detection capability of the assimilation results is analyzed by a vector geometry–based algorithm. It is verified that, the assimilation of the gridded SSH shows superiority in describing the eddy's characteristics, since the complete structure of the ocean surface has been reconstructed by the original data merging. 相似文献