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   1858篇
  免费   406篇
  国内免费   276篇
测绘学   48篇
大气科学   29篇
地球物理   455篇
地质学   254篇
海洋学   957篇
综合类   119篇
自然地理   678篇
  2024年   7篇
  2023年   52篇
  2022年   111篇
  2021年   137篇
  2020年   126篇
  2019年   97篇
  2018年   93篇
  2017年   112篇
  2016年   95篇
  2015年   101篇
  2014年   126篇
  2013年   169篇
  2012年   117篇
  2011年   130篇
  2010年   99篇
  2009年   89篇
  2008年   102篇
  2007年   100篇
  2006年   93篇
  2005年   89篇
  2004年   66篇
  2003年   55篇
  2002年   38篇
  2001年   36篇
  2000年   37篇
  1999年   42篇
  1998年   32篇
  1997年   32篇
  1996年   30篇
  1995年   21篇
  1994年   18篇
  1993年   18篇
  1992年   11篇
  1991年   5篇
  1990年   7篇
  1989年   6篇
  1988年   5篇
  1987年   4篇
  1986年   1篇
  1985年   5篇
  1984年   7篇
  1983年   5篇
  1982年   8篇
  1981年   4篇
  1980年   2篇
排序方式: 共有2540条查询结果,搜索用时 15 毫秒
1.
Arsenic contamination from roxarsone in livestock manure is common, and livestock manure continuously accumulates in the open environment. Evaluations of the environmental processes of As mobilization and transformation are critical for predicting the fate of As compounds after roxarsone degradation. In this study, spatiotemporal variations in As species and microbial community structure were characterized using laboratory column experiments with background soil collected from Yanggu County (northern Shandong Plain, China), a region of intense poultry production. Organic and inorganic arsenic were detected by high-performance liquid chromatography (HPLC) and HPLC with hydride generation atomic fluorescence spectrometry (HPLC-HG-AFS), respectively. High-throughput sequencing technology was used to describe microbial diversity. Results showed that roxarsone was transformed completely within 7 days, and As(Ⅲ) and As(Ⅴ) were the major degradation products. The concentration of As(Ⅲ) was much lower than that of As(Ⅴ). The As(Ⅲ) concentration increased significantly after Day 14, whereas the As(Ⅴ) concentration increased significantly after Day 84, indicating that As(Ⅲ) was initially produced. The microbial community structure changed significantly as roxarsone transformed into various As compounds. A critical and dominant bacterial strain, norank_f__Family_XVⅢ, was found to be related to the degradation of roxarsone into As(Ⅲ). This study improves our understanding of the fate of As species released from poultry litter to soil and groundwater, which is a threat to human health and environment.  相似文献   
2.
Macrophyte community diversity and composition respond to ecosystem conservation and local environmental factors. In this study, we developed a multidimensional diversity framework for macrophyte communities, including the taxonomic and functional alpha and beta diversity. We used the framework to explore the relationships among water level regimes and these diversity parameters in a case study of China's Baiyangdian Lake. Analysis of indicators of hydrologic alteration divided the water level from 1959 to 2019 into four regimes (dry, <6.42 m; low, 6.42–7.23 m; medium, 7.23–8.19 m; high, >8.19 m). Alpha and beta diversity were significantly higher in the medium regime than in the low and high regimes. Redundancy analysis indicated that the maximum water depth significantly affected taxonomic alpha diversity, and total nitrogen (TN) and chemical oxygen demand (COD) concentration significantly affected functional alpha diversity, respectively. Mantel tests showed that TN, Secchi depth (SD), and water depth in the high water level regime significantly increased the total beta diversity and turnover components. TN was the main factor that increased total taxonomic beta diversity. Water level regime mainly influenced interspecific relationships by changing the TN and COD concentration. The water level should be maintained between the medium and high water level regimes to promote restoration of the macrophyte community and improve ecosystem stability. The biodiversity evaluation framework would provide a deeper insight into the hydrological process management for restoration of aquatic macrophyte communities in shallow lakes.  相似文献   
3.
Seamounts are vulnerable ecosystems in the deep sea and can be heavily impacted by human activities, such as bottom fishing and deep-sea mining. The species composition and distribution patterns of benthic fauna is key information for the designation of marine protected areas and environmental management plans. Three contracts for cobalt-rich crust exploration have been granted to China, Japan and Korea in the Northwest Pacific Ocean by the International Seabed Authority. However, our knowledge of benthic biodiversity in this area is extremely insufficient. During 2013–2020, eight Chinese Ocean Mineral Resources R&D Association (COMRA) cruises were conducted to investigate the benthic assemblages of nine seamounts in this region. In this study, 191 ophiuroids collected from seamounts in the Northwest Pacific were identified into 29 species in 11 families. Ophiacanthidae and Euryalidae were the two most dominant families with 12 and 6 species, respectively. Ophiotomidae and Ophiopyrgidae were represented by two species each, while seven families were represented by only one species. Four species were widely distributed among 4–5 seamounts, and 17 species were found only at a single site. An integrated regional taxonomic dataset of Ophiuroidea was generated and analyzed. A total of 23 and 14 species were obtained from the Magellan Seamount Chain (MSC) and the Marcus-Wake seamounts (MWS), respectively, with 8 species shared between the two seamount groups. The individual-based rarefaction curves did not reach an asymptote, suggesting that the sampling effort was inadequate for either the entire region or each single seamount. Most species distributed in a narrow depth range, and the species composition was different between water depths above and below 2000 m. Our results greatly improve the understanding of megafaunal biodiversity from seamounts in the Northwest Pacific Ocean, and highlight the necessity of further surveys to provide more robust information for environmental protection and management in this region.  相似文献   
4.
Quantifying geomorphic conditions that impact riverine ecosystems is critical in river management due to degraded riverine habitat, changing flow and thermal conditions, and increasing anthropogenic pressure. Geomorphic complexity at different scales directly impacts habitat heterogeneity and affects aquatic biodiversity resilience. Here we showed that the combination of continuous spatial survey at high resolution, topobathymetric light detection and ranging (LiDAR), and continuous wavelet analysis can help identify and characterize that complexity. We used a continuous wavelet analysis on 1-m resolution topobathymetry in three rivers in the Salmon River Basin, Idaho (USA), to identify different scales of topographic variability and the potential effects of this variability on salmonid redd site selection. On each river, wavelet scales characterized the topographic variability by portraying repeating patterns in the longitudinal profile. We found three major representative spatial wavelet scales of topographic variability in each river: a small wavelet scale associated with local morphology such as pools and riffles, a mid-wavelet scale that identified larger channel unit features, and a large wavelet scale related to valley-scale controls. The small wavelet scale was used to identify pools and riffles along the entire lengths of each river as well as areas with differing riffle-pool development. Areas along the rivers with high local topographic variability (high wavelet power) at all wavelet scales contained the largest features (i.e., deepest or longest pools) in the systems. By comparing the wavelet power for each wavelet scale to Chinook salmon redd locations, we found that higher small-scale wavelet power, which is related to pool-riffle topography, is important for redd site selection. The continuous wavelet methodology objectively identified scales of topographic variability present in these rivers, performed efficient channel-unit identification, and provided geomorphic assessment without laborious field surveys.  相似文献   
5.
Bacterioplankton play critical roles in biogeochemical cycling. Although spatial and temporal variations in bacterioplankton community compositions (BCCs) within individual habitat have been reported, knowledge gaps remain for studies conducted within different habitats. In this work, we examined the seasonal and spatial variability of BCCs in Nanfei River and Lake Chaohu which had significant environmental heterogeneity using a high-throughput sequencing technique of 16S rRNA gene amplicons. The results showed that spatial variation has a more obvious impact on the BCCs than seasonal changes. The microbial diversity gradually decreased and BCCs changed obviously along water flow direction from Nanfei River to the western and estern parts of Lake Chaohu over all seasons. LEfSe analysis showed that Nanfei River had higer abundance of species belonging to the orders Rhodocyclales, Methylococcales, Campylobacterales and Flavobacteriales, samples from eastern part of Lake Chaohu were abundant in taxonomies including the order Rickettsiales, while the western part had high abundance of taxonomies belonging to the order Chroococcales. The redundancy analysis (RDA) indicated that BCCs in Nanfei River were associated with high nutrient (TP, PO4-P, TN, NH3-N, NO2-N and NO3-N) concentrations and electrical conductivity. Variance partitioning RDA analysis indicated that the combined effects of all variables may be most important affecting BCCs. This study may provide a framework for modeling the change in bacterioplankton communities through different habitats from a river to lake.  相似文献   
6.
In this study, a systematic survey of cultural airborne fungi was carried out in the occurrence environments of wall paintings that are preserved in the Tiantishan Grottoes and the Western Xia Museum, China. A bio-aerosol sampler was used for sampling in four seasons in 2016. Culture-dependent and -independent methods were taken to acquire airborne fungal concentration and purified strains; by the extraction of genomic DNA, amplification of fungal ITS rRNA gene region, sequencing, and phylogenetic analysis, thereafter the fungal community composition and distribution characteristics of different study sites were clarified. We disclosure the main environmental factors which may be responsible for dynamic changes of airborne fungi at the sampling sites. The concentration of cultural airborne fungi was in a range from 13 to 1,576 CFU/m3, no significant difference between the two sites at the Tiantishan Grottoes, with obvious characteristics of seasonal variation, in winter and spring were higher than in summer and autumn. Also, there was a significant difference in fungal concentration between the inside and outside of the Western Xia Museum, the outside of the museum was far more than the inside of the museum in the four seasons, particularly in the winter. Eight fungal genera were detected, including Cladosporium, Penicillium, Alternaria, and Filobasidium as the dominant groups. The airborne fungal community structures of the Tiantishan Grottoes show a distinct characteristic of seasonal variation and spatial distribution. Relative humidity, temperature and seasonal rainfall influence airborne fungal distribution. Some of the isolated strains have the potential to cause biodeterioration of ancient wall paintings. This study provides supporting information for the pre-warning conservation of cultural relics that are preserved at local sites and inside museums.  相似文献   
7.
以漳江口国家级红树林自然保护区、九龙江口省级红树林自然保护区和泉州湾省级湿地保护区3个红树林分布区作为研究对象,研究底栖软体动物在中等尺度下的共存格局及机制。利用非度量多维标度分析、基于蒙特卡洛的零模型和网络分析探讨底栖软体动物的共存格局,利用raup-crik零模型识别决定性过程和随机过程测定底栖软体动物的相对重要性。结果显示,漳江口、九龙江口和泉州湾湿地共发现37种底栖软体动物,底栖软体动物的平均密度、物种数及群落物种组成差异显著。软体动物的共存格局呈现非随机分散和模块化,这些格局同时受到决定性过程和随机过程的调控作用,其中,物种竞争起主导作用。  相似文献   
8.
Foreign aid has arguably played a significant role in the economic development of the Pacific Island countries. Tonga is deemed to have succeeded in implementing the principles of the Paris Declaration of 2005, especially Ownership, Alignment and Harmonisation, by aid funding channelled through state institutions. Nevertheless, it is still unknown if the aid effect has reached civil society, and how community members perceive aid. This article examines the perceptions of community members towards foreign aid, using the case study of the archipelago of Vava'u, Tonga.  相似文献   
9.
以广州市20个典型社区为例,构建多元线性回归模型,探讨了绿色空间对居民休闲性体力活动的影响,并剖析绿色空间对不同类型社区居民休闲性体力活动影响的作用机制。结果发现:1)不同类型社区居民的休闲性体力活动水平存在明显差异,低档社区的为782.99 MET-min/w,中档社区的为871.70 MET-min/w,高档社区的为1 227.91 MET-min/w;2)影响不同类型社区居民休闲性体力活动水平的绿色空间因素有所不同,低档社区、中档社区和高档社区的显著因子分别为“到最近公园广场距离”“邻里绿化覆盖率”和“健身活动设施数量”;3)绿色空间对休闲性体力活动水平影响的作用机制存在社区差异,低档社区居民休闲性体力活动主要受到成本作用的约束,中档社区居民主要受绿化环境作用影响,而高档社区居民主要受机会作用影响。  相似文献   
10.
陈宇 《世界地理研究》2021,30(5):893-902
领土疆域是现代国家存在和发展的客观地理空间实体,对国家的长治久安和繁荣稳定具有根本现实意义。在人类社会发展的演进历程中,人类社会共同体在有着固定地理边界的领土上建立了现代国家,然而随着人类探索世界的范围因认知能力、科学技术及经济全球化等变化而扩展,国家地理疆域的形态也由此发生了深刻变化。现代国家的地理疆域已经从领土、近海和近空传统地理空间,向深海、远洋、太空、极地和信息等现代地理空间延展。总的来看,现代国家疆域的形态特征的变化主要表现为,由二维空间向多维空间的变化,从确定性空间向不确定空间延展,从地理空间迈向向利益空间发展,从主权空间向战略空间扩展。在现代国家的地理疆域已经发生变化的前提下,中国在构建人类命运共同体中提出了以和平、主权、普惠、共治为治理原则来治理国际关系新疆域,这不仅为治理国际关系新疆域贡献了中国智慧,也有效推动了中国疆域治理迈向更高台阶。  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