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   29902篇
  免费   340篇
  国内免费   248篇
安全科学   1015篇
废物处理   1458篇
环保管理   3982篇
综合类   4485篇
基础理论   7805篇
环境理论   10篇
污染及防治   7367篇
评价与监测   2136篇
社会与环境   2049篇
灾害及防治   183篇
  2023年   135篇
  2022年   262篇
  2021年   324篇
  2020年   195篇
  2019年   254篇
  2018年   443篇
  2017年   466篇
  2016年   726篇
  2015年   535篇
  2014年   861篇
  2013年   2475篇
  2012年   1020篇
  2011年   1360篇
  2010年   1114篇
  2009年   1158篇
  2008年   1419篇
  2007年   1340篇
  2006年   1214篇
  2005年   1083篇
  2004年   1040篇
  2003年   993篇
  2002年   940篇
  2001年   1092篇
  2000年   773篇
  1999年   484篇
  1998年   366篇
  1997年   383篇
  1996年   400篇
  1995年   466篇
  1994年   398篇
  1993年   344篇
  1992年   377篇
  1991年   356篇
  1990年   328篇
  1989年   322篇
  1988年   297篇
  1987年   241篇
  1986年   247篇
  1985年   248篇
  1984年   267篇
  1983年   255篇
  1982年   269篇
  1981年   221篇
  1980年   165篇
  1979年   181篇
  1978年   163篇
  1977年   133篇
  1975年   137篇
  1973年   167篇
  1972年   142篇
排序方式: 共有10000条查询结果,搜索用时 171 毫秒
901.
902.
In this study, habitat surveys were undertaken on 50 grass-based farms in SE Ireland and data digitised onto aerial photography. Additional data i.e. stocking rates, and participation (or otherwise) in the Irish Rural Environment Protection Scheme (REPS) were collected and analysed as possible explanatory variables for farm habitat composition.Results indicated that approximately 14.3% of the land area of sampled farms comprised of semi-natural habitat types, a proportion substantially greater than has been reported for many other European countries. The most frequently recorded semi-natural habitats included, field boundaries, scrub, and deciduous and riparian woodlands.Multivariate analysis of farm habitat configuration showed a strong dichotomy between dairy and non-dairy farming systems. Habitats such as intensively managed grassland and built ground were closely associated with dairy-based enterprises. In contrast, the incidence of other habitat types was associated with non-dairy and/or REPS participating enterprises. The ecological quality of field boundaries as assessed by the Field Boundary Evaluation and Grading System (FBEGS) Index was significantly greater on dairy, compared with dry-stock farms.This dichotomy in farm habitat composition is not reflected within current Agri-Environment (AE) policy. Integration of locally important drivers of habitat diversity into the design and implementation of AE policy, is integral to the successful performance of AE schemes.  相似文献   
903.
The effects on different physico-chemical, chemical and biological soil properties of the incorporation of different organic materials into a calcareous vineyard soil were studied in an experiment during three growing seasons. The organic materials used were sheep manure and four composts derived from the treatment of winery and distillery wastes. A multivariate technique, factor analysis (FA), was also used for interpreting the data obtained from the samplings carried out throughout the three vine growing seasons. The application of the organic materials induced an increase in the activity of the soil microorganisms and in the soil macro and micronutrient contents, as well as a slow release of inorganic N. Through FA of the soil properties studied, three factors that explained 74.7% of the variability were established and the effects of the treatments after the whole experimental period were assessed.  相似文献   
904.
In the extremely arid (∼150 mm yr−1) eastern Canary Islands of Lanzarote, Fuerteventura and La Graciosa, agriculture has been sustained for decades by a traditional runoff-capture (RC) farming system known as “gavias”. Although the main goal of these systems is to increase water supply for crops, making unnecessary conventional irrigation, a secondary and equally important factor is that this system allows for sustainable agricultural production without addition of chemical or organic fertilizers. A field study was conducted to assess the impact of long-term agriculture (>50 yr) on soil fertility and to evaluate key factors affecting the nutrient sustainability of RC agricultural production. Soil fertility and nutrient dynamics were studied through chemical characterization of the arable layer (0-25 cm) of RC agricultural plots, adjacent natural soils (control) not affected by runoff and cultivation, and sediments contributed by a series of RC events. Results showed that RC soils have enhanced fertility status, particularly because they are less affected by salinity and sodicity (mean electrical conductivity = 1.8 dS m−1 vs. 51.0 dS m−1 in control soils; mean exchangeable sodium percentage = 11.1% vs. 30.6% in control soils), and have higher water and nutrient holding capacities (mean clay plus silt contents ≈87% vs. 69% in control soils). In general, sediments transported with the runoff and deposited in RC plots (average sediment yield ≈ 46 ton ha−1 yr−1), contain sufficient nutrients to prevent a progressive reduction of essential plant nutrients below natural levels in spite of nutrient uptake and removal by the harvested crop. Average additions of nitrogen, phosphorus and potassium with the runoff sediments were 33.6, 35.3 and 48.8 kg ha−1 yr−1, respectively. Results of this study show how a crop production system can be sustained in the long term by natural hydrological and biogeochemical catchment processes. This system maintains a nutrient balance that is not based on energy-intensive inputs of fertilizers, but is integrated in natural nutrient cycling processes, unlike other tropical farming agroecosystems.  相似文献   
905.
Achievement of at least “good ecological status” in all waterbodies under the EU Water Framework Directive by 2015 will in some cases be a challenge. The twin challenge is to manage expectations of policy makers for such waterbodies as to a realistic length of time required for improvement in water quality. Hence, understanding the source, transformation processes and residence time of nitrate in a hydrological system is an essential part of meeting such challenges. On a dairy farm with 24 shallow groundwater wells, the dual isotopic composition of nitrate (δ15N and δ18O) was used to clarify nitrate sources, to assess spatial and temporal variability in nitrate concentrations and to determine if and where denitrification was occurring. Vertical travel time was estimated to correlate nitrate concentrations with management practices. Organically derived nitrogen was the predominant source contributing to groundwater nitrate concentrations. Denitrification was identified as prevalent within specific regions of the study site. The distinct low temporal variability in the isotopic data suggests constancy among nitrate sources and processes over time across the study site. Vertical travel times of up to 3 years were estimated on site indicating the influence of recent management practices on nitrate concentrations. Very slow horizontal migration of groundwater (decades) indicates a legacy of older management practices. Stable isotope techniques, together with an understanding of time lag, provide an extra mechanism to test the efficacy of monitoring and mitigation programmes.  相似文献   
906.
Plant biomass is known to increase in response to elevated atmospheric CO2 concentration (pCO2); however, no experiments have quantified the trajectory of crop fertilization across the full range of pCO2 levels estimated for the next 300 years. Here we quantify the above- and below-ground biomass response of Raphanus sativus (common radish) across eight pCO2 levels ranging from 348 to 1791 ppmv. We observed a large net biomass increase of 58% above ground and 279% below ground. A large part of the net increase (38% of the above-ground and 53% of the below-ground) represented biomass fertilization at the high levels of pCO2 (700–1791 ppmv) predicted if fossil fuel emissions continue unabated. The trajectory of below-ground fertilization in R. sativus greatly exceeded a trajectory based on extrapolation of previous experiments for plants grown at pCO2 < 800 ppmv. Based on the experimental parameters used to grow these plants, we hypothesize that these experiments represent the maximum CO2 fertilization that can be achieved for this plant growing under low light levels. If the below-ground biomass enhancement that we have quantified for R. sativus represents a generalized root-crop response that can be extrapolated to agricultural systems, below-ground fertilization under very high pCO2 levels could dramatically augment crop production in some of the poorest nations of the world, provided that water resources are sufficient and sustainable.  相似文献   
907.
Alternatives assessment is becoming increasingly popular to evaluate the potential environmental and human health hazards of materials. A three step process was used to identify and evaluate alternative products for a children's furniture manufacturer. An alternatives assessment framework was developed to analyze alternative mattresses. The framework specifically addresses those environmental and safety attributes applicable to the product and the product components in accordance with the product’s intended use. The result of the assessment allowed the manufacturer to select the most environmentally friendly alternative and eliminate polyvinyl chloride from their product.  相似文献   
908.
The off-gas handling system is a vital component of a smelter and it is essential that it operates in accordance with environmental regulations. A key feature of the off-gas management system is maintaining a slightly negative system pressure to prevent gas leakage, such as SO2, out into the work and external environments. To enhance sustainable development, there is also growing interest in capturing and reusing the significant amount of heat and CO2 contained within off-gas. The potential incorporation of heat and CO2 recovery systems places importance on enhancing control to minimize the risk of both gas leakage and disturbance of the smelting process. With these requirements in mind, an easily implementable feedforward control enhancement to existing control schemes is proposed to help provide improved performance.  相似文献   
909.
Hazardous chemicals are pervasive in household disinfectant products. Many ingredients have established associations with acute and chronic human health conditions as well as with environmental damage. Although these associations are suggested but not proven, they are of great concern. This article describes the application of the precautionary principle to the selection of an anti-microbial active ingredient for a botanical disinfectant when significant uncertainty exists around the hazard and risk of traditional disinfectant active ingredients. We show that application of the precautionary principle does not stifle innovation and facilitates a responsible approach to product development.  相似文献   
910.
Environmental radon exposure of residents of domestic premises in the United Kingdom (UK) and elsewhere in Europe is estimated on the basis of the measured radon concentrations in, and the relative occupancies of, the principal living room and bedroom. While studies on radon concentration variability in the individual units in apartment blocks in various countries have been described, little data has been reported on variability in two-storey single-family dwellings, and the majority of extant studies consolidate living room and bedroom data early in the analysis. To investigate this further, detailed analysis was made of radon concentration data from a set of thirty-four homes situated in areas of Northamptonshire known to exhibit high radon levels. All homes were of typical UK construction of brick/block/stone walls under a pitched tile/slate roof. Approximately 50% of the sample were detached houses, the remainder being semi-detached (duplex) or terraced (row-house). Around 25% of the sample possessed cellars, while 12% were single-storey dwellings (bungalows), reflecting the typical incidence of this type of dwelling in England. In the two-storey homes, all monitored bedrooms were on the upper floor. Distribution of the ratios of bedroom/living room radon concentrations (BR/LR ratio) in individual properties was left-skewed (mean 0.67, median 0.73, range 0.05-1.05) with a tail extending to just above 1.0. The mean is consistent with the outcome of earlier extensive studies in England, while the variability depends principally on the characteristics of the property, and not on seasonal factors. In a small set of homes, the BR/LR ratio was anomalously low, (mean 0.3). BR/LR ratios in single-storey homes clustered around a value of 1.0, indicating that house design, rather than lifestyle, is the dominant factor in determining bedroom radon concentrations. Homes with higher mean annual radon concentrations showed lower BR/LR ratios, supporting our proposal that, in some homes, radon emanation from building materials may comprise a significant component of the overall radon level.  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