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   553篇
  免费   135篇
  国内免费   207篇
测绘学   3篇
大气科学   14篇
地球物理   61篇
地质学   639篇
海洋学   107篇
综合类   36篇
自然地理   35篇
  2024年   5篇
  2023年   33篇
  2022年   34篇
  2021年   51篇
  2020年   32篇
  2019年   24篇
  2018年   24篇
  2017年   21篇
  2016年   24篇
  2015年   28篇
  2014年   46篇
  2013年   52篇
  2012年   31篇
  2011年   44篇
  2010年   38篇
  2009年   36篇
  2008年   29篇
  2007年   31篇
  2006年   39篇
  2005年   35篇
  2004年   37篇
  2003年   20篇
  2002年   20篇
  2001年   30篇
  2000年   16篇
  1999年   15篇
  1998年   13篇
  1997年   15篇
  1996年   16篇
  1995年   7篇
  1994年   12篇
  1993年   11篇
  1992年   4篇
  1991年   8篇
  1990年   2篇
  1989年   2篇
  1988年   6篇
  1987年   1篇
  1985年   1篇
  1983年   1篇
  1979年   1篇
排序方式: 共有895条查询结果,搜索用时 15 毫秒
1.
The aim of this study is to evaluate the feasibility of sludge digester effluent as feeding solution to enrich anaerobic ammonium oxidation (anammox) bacteria. The performance of the two parallel pilot scale‐upflow packed bed anammox reactors (UPBAn1 and UPBAn2) are examined in terms of the enrichment of anammox bacteria. The control experiment is set up conducting synthetic wastewater as feeding solution in the UPBAn1 reactor whereas, the sludge digester effluent is fed to the nitritation reactor and then the partially nitrated digester effluent to the UPBAn2 reactor. Anammox activities are evaluated by mass balances based on ammonium (NH4+), nitrite (NO2?), and nitrate (NO3?) analysis and NRR. Microbial community of anammox bacteria is analyzed using real‐time polymerase chain reaction (PCR). The results demonstrate that UPBAn 1 and UPBAn2 reactors are successfully enriched on days 64 and 40 with NRRs of 19.54 and 19.43 g N m?3 per day, respectively. This study reveals that both synthetic wastewater and digester effluent are suitable for the enrichment of anammox bacteria; however, digester effluent as feeding solution for enrichment of anammox bacteria based on the ease of process control and process stability is more advisable.  相似文献   
2.
马莲河流域地处鄂尔多斯盆地南部黄土高原区,按储水介质和埋藏条件可将地下水分为黄土潜水、基岩潜水和基岩承压水三类,利用2015—2018年流域的295组地下水水化学数据,分析了地下水中锶(Sr2+)的质量浓度水平、垂向分布及沿流向分布的特征,并采用因子相关性研究地下水中Sr2+的来源和富集成因。结果表明:马莲河流域地下水富锶现象显著,黄土潜水、基岩潜水和基岩承压水中锶的平均质量浓度分别为1.14 mg/l、1.39 mg/l和5.24 mg/l;地下水中锶富集成因主要是含锶矿物天然状态下的水-岩作用,黄土潜水、基岩潜水和基岩承压水三类地下水中Sr2+的主要来源分别为碳酸盐岩与铝硅酸盐岩、碳酸盐岩与硫酸盐和碳酸盐矿物的溶滤作用;地下水中Sr2+的质量浓度沿流向呈逐渐增大的趋势,在不同区域中随深度变化的趋势不一致。  相似文献   
3.
The Beetaloo Sub-basin, northern Australia, is considered the main depocentre of the 1,000-km scale Mesoproterozoic Wilton package of the greater McArthur Basin – the host to one of the oldest hydrocarbon global resources. The ca. 1.40–1.31 Ga upper Roper Group and the latest Mesoproterozoic to early Neoproterozoic unnamed group of the Beetaloo Sub-basin, together, record ca. 500 million years of depositional history within the North Australia Craton. Whole-rock shale Sm–Nd and Pb isotope data from these sediments reveal sedimentary provenance and their evolution from ca. 1.35 to 0.85 Ga. Furthermore, these data, together with shale major/trace elements data from this study and pyrolysis data from previous publications, are used to develop a dynamic tectonic geography model that links the organic carbon production and burial to an enhanced weathering of nutrients from a large igneous province. The ca. 1.35–1.31 Ga Kyalla Formation of the upper Roper Group is composed of isotopically evolved sedimentary detritus that passes up, into more isotopically juvenile Pb values towards the top of the formation. The increase in juvenile compositions coincides with elevated total organic carbon (TOC) contents of these sediments. The coherently enriched juvenile compositions and TOC the upper portions of the Kyalla Formation are interpreted to reflect an increase in nutrient supply associated with the weathering of basaltic sources (e.g. phosphorous). Possible, relatively juvenile, basaltic sources include the Wankanki Supersuite in the western Musgraves and the Derim Derim–Galiwinku large igneous province (LIP). The transition into juvenile, basaltic sources directly before a supersequence-bounding unconformity is here interpreted to reflect uplift and erosion of the Derim Derim–Galiwinku LIP, rather than an influx of southern Musgrave sources. A new baddeleyite crystallisation age of 1,312.9 ± 0.7 Ma provides both a tight constraint on the age of this LIP, along with its associated magmatic uplift, as well as providing a minimum age constraint for Roper Group deposition. The unconformably overlying lower and upper Jamison sandstones are at least 300 million years younger than the Kyalla Formation and were sourced from the Musgrave Province. An up-section increase in isotopically juvenile compositions seen in these rocks is interpreted to document the progressive exhumation of the western Musgrave Province. The overlying Hayfield mudstone received detritus from both the Musgrave and Arunta regions, and its isotopic geochemistry reveals affinities with other early Neoproterozoic basins (e.g. Amadeus, Victoria and Officer basins), indicating the potential for inter-basin correlations.  相似文献   
4.
为探究重金属在红树林沉积物及红树植物中的分布累积及迁移规律,选取了徐闻南山镇红树林为研究对象,通过测定红树林沉积物及红树植物不同部位(根、茎、叶)的重金属质量分数,运用富集因子、生物富集系数、转移系数及相关性分析等方法进行分析。结果表明:1)红树林沉积物重金属质量分数表现为铬(Cr)>锌(Zn)>镍(Ni)>铜(Cu)>铅(Pb)>砷(As)>汞(Hg)>镉(Cd),为中等变异程度;除了镍(Ni)元素外,其余7种重金属未超过国家一级标准,除了铅(Pb)元素外,其余7种重金属均超过广东省土壤环境背景值,说明研究区沉积物中重金属具有一定的积累效应。2)沉积物中砷(As)、铜(Cu)、锌(Zn)、汞(Hg)、镍(Ni)、铬(Cr)富集因子值均>1.5,说明受到轻微人为活动影响;各站位镍(Ni)富集因子值均>5,结合研究区背景,反映了镍(Ni)受到自然和人为输入的共同影响。3)白骨壤体内重金属主要集中在根部,而红海榄体内重金属在根茎叶中分布相对均匀。白骨壤根茎叶部位的大多数重金属质量分数远高于红海榄,说明白骨壤对重金属的吸附能力比红海榄强。汞(Hg)集中分布在植物的叶片部位,且与其他重金属之间相关性不明显;推测汞(Hg)主要通过叶片吸收进入植物体内,与交通运输污染有关。4)不同红树植物对不同重金属富集能力各异,白骨壤对重金属的富集能力表现为:镉(Cd)>砷(As)>铜(Cu)>锌(Zn)>汞(Hg)>铅(Pb)>镍(Ni)>铬(Cr),红海榄表现为:镉(Cd)>铜(Cu)>汞(Hg)>锌(Zn)>铅(Pb)>砷(As)>镍(Ni)>铬(Cr)。白骨壤和红海榄对汞(Hg)的运移能力都较强;红海榄对镉(Cd)的富集能力和转运能力都较强,而白骨壤对镉(Cd)富集能力较强,转运能力却较弱,这说明红树植物对重金属元素的富集能力与转运能力不存在正比关系。  相似文献   
5.
文章选择鹿回头近岸海域常见的板叶角蜂巢珊瑚(Favites complanata)和十字牡丹珊瑚(Pavona decussata)为研究对象, 采用室内连续培养的方法, 探究两种不同造礁石珊瑚对酸化和溶解有机碳(DOC)加富的响应。结果表明: 酸化(pH 7.6)并不会影响两种珊瑚的钙化速率和生长速率; 但DOC加富(524.03±78.42μmol•L-1)使两种珊瑚的钙化速率分别降低67%和47%、生长速率降低59%和40%。当二者共同作用时, 两种珊瑚的钙化速率降低30%和11%、生长速率降低46%和59%, 大多没有DOC单独作用时强烈, 表现出一定的拮抗作用。两种珊瑚共生虫黄藻叶绿素荧光指数(Fv/Fm)均升高后降低, 板叶角蜂巢珊瑚Fv/Fm最先降低。实验表明, 这两种珊瑚虽然对海洋酸化的敏感度不高, 但是对有机物加富有不同的响应, 板叶角蜂巢珊瑚更为敏感, 可能导致这两种珊瑚在未来环境变化中有不同命运。  相似文献   
6.
Fluorine is in the dynamic balance of two geochemical processes, enrichment and leaching, reflecting the adsorption and desorption of fluoride by clay minerals, respectively. The two geochemical processes of fluorine in soil are influenced by factors including geochemical characteristics of soil and clay minerals, pH and sanity of soil solutions, climate, grazing and agriculture activities. Main factors controlling enrichment and leaching process of fluorine in soil can be found with interaction consideration.  相似文献   
7.
The aim of this study was to investigate the effects of light intensity and enhanced nitrogen supply on the growth and photosynthesis of the green-tide macroalga, Ulva prolifera. Thalli of U. prolifera were grown in natural or NH 4 +-enriched seawater under two different light intensities for 7 days, and then the growth rate, pigmentation, and photosynthetic performance of the thalli were evaluated. The results show that the relative growth rate(RGR) was markedly higher under the high light level than under the low light level. Enrichment with NH 4 + enhanced the RGR under high light intensity, but did not affect RGR under low light intensity. In low light conditions, NH 4 +-enrichment resulted in a marked decrease in the maximal photosynthetic rate( P m) and the maximum carbon fixation rate( V max), but it did not affect the half saturation constant for carbon( K 0.5) or the ratio of V max to K 0.5, which reflects the carbon acquisition efficiency. In high light conditions, P m, K 0.5, and the dark respiration rate( R d) increased under NH 4 + enrichment, but V max and the V max / K 0.5 ratio decreased. Regardless of the light intensity, NH 4 +-enrichment did not affect the apparent photosynthetic efficiency( α), which refl ects the ability of the alga to use light energy at low light levels. Under both low and high light intensities, the chlorophyll a(Chl a), chlorophyll b(Chl b), and carotenoids(Car) contents in thalli were higher in NH 4 +-enriched than in natural seawater, except that there was a decrease in the Chl b content of thalli in NH 4 +-enriched seawater under low light intensity. Therefore, NH 4 + enrichment improved the growth and photosynthetic performance of U. prolifera under high light intensity, but not under low light intensity. We discuss the possible mechanisms underlying these physiological responses.  相似文献   
8.
Based on introducing geological structure of Tanggu area, the authors analyzed the sedimentary environment of Paleogene Dongying Formation, and further analyzed its geothermal occurrence. The studied area spans two grade IV tectonic units which are Beitang depression and Banqiao depression. The studied area is mainly located in the Tanggu nose-like structure which is the secondary structure of Beitang depression. Its existence affects the sedimentary distribution of the part strata of Paleogene. The depositions of Dongying Ⅲ ( SQEd3 ) and Dongying Ⅱ ( SQEd2 ) are mainly delta front facies and lake sand bars which are propitious to the geothermal fluid enrichment. The favorable enrichment region of geothermal fluid is located in south Hetou- Tanggu. Most of Dongying Ⅰ ( SQEd1 ) changes into swamp plain deposition, only the northeast part of this area is the distributary riverway facies with developed sand layers. There is the favorable enrichment region of geothermal fluid. The hydraulic connection is weak among the geothermal reservoirs of Dongying Formation and its overlying Guantao and Minghuazhen formations. The underlying Shahejie Formation geothermal reservoir pro- vides a steady stream of fluid supply and ground pressure protection for Dongying Formation geothermal reservoir.  相似文献   
9.
以代表性暗棕壤垂直剖面土壤地球化学数据为依据,研究了长白山区特定的森林景观条件下成土过程中元素的分异与富集特征。研究结果表明,在岩石风化过程中主要以CaO、 Na2 O、 Sr淋溶为主,有机酸淋溶过程中SiO2、 Al2 O3、 K2 O部分被淋溶,部分游离铁下渗到土体中下部氧化成铁锰胶膜;微量元素在风化过程中被次生黏土矿物吸附,不同程度地富集,在腐殖质积累过程中Cd、 Zn、Pb、 Mo、 Hg、 Co、 Mn、 Cr等亲生物元素进一步富集于表层土壤, Ce、 Ni、 Sb、 Ti、 V、 Y、 Zr在表生环境中地球化学性质稳定,残留富集于土壤表层。母质层继承和保留了基岩元素地球化学分布的基本特征。  相似文献   
10.
金黄色葡萄球菌(Staphylococcus aureus)是近岸海域海水中的主要病原菌,严重威胁接触者的安全。抗生素处理是治疗金黄色葡萄球菌感染的重要手段,其耐药性的发生受到了高度重视。采用全基因组重测序与KEGG富集分析结合的方法,对红霉素(erythromycin)、氯霉素(chloramphenicol)和万古霉素(vancomycin)处理后的耐盐金黄色葡萄球菌(S.aureus ZS01)和不耐盐金黄色葡萄球菌(S.aureus 502A)进行耐药机制研究。结果表明,S.aureus 502A经抗生素处理后发生突变的程度大于S.aureus ZS01,二者在经过氯霉素处理发生了更大程度的突变。红霉素、氯霉素和万古霉素处理主要影响了金黄色葡萄球菌的致病能力;红霉素和氯霉素可能通过影响金黄色葡萄球菌脂类的代谢引起其耐药性的变化。除此之外,三种抗生素处理均出现了较多TIGR01741家族蛋白和假设蛋白基因的突变,推测与菌株的耐药性和致病性相关。耐盐金黄色葡萄球菌可通过外排系统作用产生红霉素耐药性,不耐盐菌株因细胞壁成分相关基因的突变提高了对万古霉素的耐受性。研究结果可为耐盐和不耐盐金黄色葡萄球菌的耐药机制及抗生素对金黄色葡萄球菌致病性影响的研究提供基础数据。  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