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   20871篇
  免费   236篇
  国内免费   128篇
安全科学   531篇
废物处理   903篇
环保管理   2824篇
综合类   3411篇
基础理论   5892篇
环境理论   10篇
污染及防治   5369篇
评价与监测   1290篇
社会与环境   872篇
灾害及防治   133篇
  2021年   143篇
  2019年   132篇
  2018年   237篇
  2017年   262篇
  2016年   409篇
  2015年   315篇
  2014年   471篇
  2013年   1628篇
  2012年   586篇
  2011年   840篇
  2010年   676篇
  2009年   668篇
  2008年   848篇
  2007年   876篇
  2006年   781篇
  2005年   677篇
  2004年   656篇
  2003年   636篇
  2002年   627篇
  2001年   775篇
  2000年   576篇
  1999年   330篇
  1998年   264篇
  1997年   284篇
  1996年   280篇
  1995年   336篇
  1994年   302篇
  1993年   287篇
  1992年   279篇
  1991年   280篇
  1990年   296篇
  1989年   278篇
  1988年   249篇
  1987年   242篇
  1986年   224篇
  1985年   200篇
  1984年   255篇
  1983年   213篇
  1982年   271篇
  1981年   221篇
  1980年   184篇
  1979年   201篇
  1978年   184篇
  1977年   153篇
  1976年   139篇
  1975年   143篇
  1974年   159篇
  1973年   164篇
  1972年   148篇
  1971年   146篇
排序方式: 共有10000条查询结果,搜索用时 10 毫秒
121.
Major and trace elements of selected pedons in the USA   总被引:6,自引:0,他引:6  
Few studies of soil geochemistry over large geographic areas exist, especially studies encompassing data from major pedogenic horizons that evaluate both native concentrations of elements and anthropogenically contaminated soils. In this study, pedons (n = 486) were analyzed for trace (Cd, Co, Cr, Cu, Hg, Mn, Ni, Pb, Zn) and major (Al, Ca, Fe, K, Mg, Na, P, Si, Ti, Zr) elements, as well as other soil properties. The objectives were to (i) determine the concentration range of selected elements in a variety of U.S. soils with and without known anthropogenic additions, (ii) illustrate the association of elemental source and content by assessing trace elemental content for several selected pedons, and (iii) evaluate relationships among and between elements and other soil properties. Trace element concentrations in the non-anthropogenic dataset (NAD) were in the order Mn > (Zn, Cr, Ni, Cu) > (Pb, Co) > (Cd, Hg), with greatest mean total concentrations for the Andisol order. Geometric means by horizon indicate that trace elements are concentrated in surface and/or B horizons over C horizons. Median values for trace elements are significantly higher in surface horizons of the anthropogenic dataset (AD) over the NAD. Total Al, Fe, cation exchange capacity (CEC), organic C, pH, and clay exhibit significant correlations (0.56, 0.74, 0.50, 0.31, 0.16, and 0.30, respectively) with total trace element concentrations of all horizons of the NAD. Manganese shows the best inter-element correlation (0.33) with these associated total concentrations. Total Fe has one of the strongest relationships, explaining 55 and 30% of the variation in total trace element concentrations for all horizons in the NAD and AD, respectively.  相似文献   
122.
Indicators of ecosystem recovery   总被引:6,自引:0,他引:6  
  相似文献   
123.
Excessive nitrogen (N) loading to N-sensitive waters such as the Neuse River estuary (North Carolina) has been shown to promote changes in microbial and algal community composition and function (harmful algal blooms), hypoxia and anoxia, and fish kills. Previous studies have estimated that wet atmospheric deposition of nitrogen (WAD-N), as deposition of dissolved inorganic nitrogen (DIN: NO3-, NH3/NH4+) and dissolved organic nitrogen, may contribute at least 15% of the total externally supplied or "new" N flux to the coastal waters of North Carolina. In a 3-yr study from June 1996 to June 1999, we calculated the weekly wet deposition of inorganic and organic N at eleven sites on a northwest-southeast transect in the watershed. The annual mean total (wet DIN + wet organics) WAD-N flux for the Neuse River watershed was calculated to be 956 mg N/m2/yr (15026 Mg N/yr). Seasonally, the spring (March-May) and summer (June-August) months contain the highest total weekly N deposition; this pattern appears to be driven by N concentration in precipitation. There is also spatial variability in WAD-N deposition; in general, the upper portion of the watershed receives the lowest annual deposition and the middle portion of the watershed receives the highest deposition. Based on a range of watershed N retention and in-stream riverine processing values, we estimate that this flux contributes approximately 24% of the total "new" N flux to the estuary.  相似文献   
124.
Der Beitrag ist aus einem Vortrag hervorgegangen, den der Verfasser am 23. 9. 2009 bei der Jahrestagung der Gesellschaft Deutscher Chemiker, Fachgruppe Umweltchemie und ?kotoxikologie, in Trier gehalten hat. Wenngleich gewichtige Sachgründe für eine fachübergreifende rechts- und naturwissenschaftliche Kooperation sprechen, so stellt sie doch in der gegenw?rtigen universit?ren Forschungslandschaft eher eine Rarit?t dar. Zu den Ursachen hierfür geh?rt, dass eine derartige Kooperation mit erheblichen Schwierigkeiten verbunden ist, die aus der zunehmenden Spezialisierung der Wissenschaft und der damit einhergehenden Herausbildung divergierender F?cherkulturen resultieren. Hinzu kommt, dass die aktuelle Drittmittelf?rderung, die nicht zuletzt auf die Initiierung und Intensivierung von Interdisziplinarit?t zielt, auf Rechtswissenschaftler nur eine vergleichsweise geringe Anreizwirkung ausübt. Doch mehren sich die Anzeichen dafür, dass sich hier alsbald ein Wandel vollzieht. Das Chemikalienrecht bietet jedenfalls (ebenso wie beispielsweise das Naturschutzrecht) etliche Ansatzpunkte für sachlich ertragreiches kooperatives Zusammenwirken von Rechts- und Naturwissenschaftlern.  相似文献   
125.
The development of process-based models to estimate ammonia emissions from animal feeding operations (AFOSs) is sought to replace costly and time-consuming direct measurements. Critical to process-based model development is conducting sensitivity analysis to determine the input parameters and their interactions that contribute most to the variance of the model output. Global and relative sensitivity analyses were applied to a process-based model for predicting ammonia emissions from the surface of anaerobic lagoons for treating and storing manure. The objectives were to compare global sensitivity analysis (GSA) to relative (local) sensitivity analysis (RSA) on a process-based model for ammonia emissions. Based on the first-order coefficient, both GSA and RSA showed the model input parameters in order of importance in process model for ammonia emissions from lagoon surfaces were: (i) pH, (ii) lagoon liquid temperature, (iii) wind speed above the lagoon surface, and (iv) the concentration of ammoniacal nitrogen in the lagoon. The GSA revealed that interactions between model parameters accounted for over two-thirds of the model variance, a result that cannot be achieved using traditional RSA. Also, the GSA showed that parameter interactions involving liquid pH had more impact on the model output variance than the single parameters: (i) temperature, (ii) wind speed, or (iii) total ammoniacal nitrogen. This study demonstrates that GSA provides a more complete analysis of model input parameters and their interactions on the model output compared to RSA. A comprehensive tutorial regarding the application of GSA to a process model is presented.  相似文献   
126.
Regional Environmental Change - Louisiana faces extensive coastal land loss which threatens the livelihoods of marginalized populations. These groups have endured extreme disruptive events in the...  相似文献   
127.
Species turnover patterns can be inconsistent due to differences in the dispersal ability of different growth forms. Here, species of trees, shrubs, herbs, and bryophytes in the Xiaoqinling National Nature Reserve in China were analyzed to determine patterns of species turnover along an elevation and spatial gradient. Variance partitioning was used to assess the relative contribution of topographic heterogeneity and dispersal limitation to species turnover. Our results suggest that the effect of dispersal limitation is more important than topographic heterogeneity on species turnover in temperate mountane ecosystems in the study area. Dispersal limitation has a greater effect on trees species turnover than on shrubs, herbs or bryophytes species turnover.  相似文献   
128.
Diet, food preferences and main ecophysiological characteristics such like energy requirements and thermoregulation characteristics of Mus spicilegus spicilegus (Petenyi, 1882) were studied. The most preferred foods for the mice were seeds of weed species that composed more than 85% of their diet. The results of food selection experiments shows that the total daily consumption by mice is 2.77 ± 0.76 g/animal/day or 5.5 kJ/g/day or 84.9 kJ/animal/day. From the total daily energy consumption 16.4% go back to nature in the form of feces and urine and the rest 83.6% animals utilized for assimilation. The results of the temperature preferences for Mus s. spicilegus shows preferred temperature zone from around 26 to 36°C where mice spent about 72% of the experimental time. The lowest value of oxygen consumption for resting metabolism rate (RMR) was registered at 30°C–3.20 ± 0.71 cm3 O2/g/h. It is possible to consider that the thermoneutral zone is around these temperature values. The obtained results give reason to conclude that from an ecophysiologycal point of view the climate in the south boundary of distribution provides optimal conditions for species development. The main cause for population decreasing probably is the loss of open habitats including natural steppe grasslands.  相似文献   
129.
Quantitative parameters of cenopopulations of Lobaria pulmonaria, an endangered cyanolichen, have been studied in spruce phytocenoses of Karelia recovering after disturbance through the successional stage of aspen stands for a period ranging from 80 to 450 years. The results show that, as the time since disturbance increases, the total number of L. pulmonaria thalli and the number of colonized substrate units increase as well, with no stabilization of these parameters being observed in the series of communities studied. The total area of thalli in the phytocenoses is restored within approximately 200 years after the last disturbance. In old-growth forests (>400 years), L. pulmonaria thalli colonize a broad spectrum of tree species in different life states, including lower branches of young spruce trees. Even if they serve as temporary substrates, this can markedly strengthen the potential of the species for further expansion within the community in case of shortage in the main phorophyte (aspen). Thus, the proportion of regressive populations decreases, while that of colonizing populations increases, which is indicative of successful reproduction of the species.  相似文献   
130.
Sustainable groundwater quality is a key global concern and has become a major issue of disquiets in most parts of the world including Bangladesh. Hence, the assessment of groundwater quality is an important study to ensure its sustainability for various uses. In this study, a combination of multivariate statistics, geographical information system (GIS) and geochemical approaches was employed to evaluate the groundwater quality and its sustainability in Joypurhat district of Bangladesh. The results showed that the groundwater samples are mainly Ca–Mg–HCO3 type. Principal component analysis (PCA) results revealed that geogenic sources (rock weathering and cation exchange) followed by anthropogenic activities (domestic sewage and agro-chemicals) were the major factors governing the groundwater quality of the study area. Furthermore, the results of PCA are validated using the cluster analysis and correlation matrix analysis. Based on the groundwater quality index (GWQI), it is found that all the groundwater samples belong to excellent to good water quality domains for human consumption, although iron, fluoride and iodide contaminated to the groundwater, which do not pose any significant health hazard according to World Health Organization’s and Bangladesh’s guideline values. The results of irrigation water quality index including sodium adsorption ratio (SAR), permeability index and sodium percentage (Na %) suggested that most of the groundwater samples are good quality water for agricultural uses. The spatial distribution of the measured values of GWQI, SAR, Fe (iron), EC (electrical conductivity) and TH (total hardness) were spatially mapped using the GIS tool in the study area.  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