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   19903篇
  免费   144篇
  国内免费   183篇
安全科学   474篇
废物处理   833篇
环保管理   2030篇
综合类   4067篇
基础理论   4509篇
环境理论   9篇
污染及防治   4927篇
评价与监测   1495篇
社会与环境   1789篇
灾害及防治   97篇
  2022年   189篇
  2021年   182篇
  2020年   112篇
  2019年   150篇
  2018年   298篇
  2017年   326篇
  2016年   444篇
  2015年   318篇
  2014年   577篇
  2013年   1485篇
  2012年   646篇
  2011年   832篇
  2010年   660篇
  2009年   744篇
  2008年   825篇
  2007年   871篇
  2006年   775篇
  2005年   707篇
  2004年   656篇
  2003年   649篇
  2002年   609篇
  2001年   807篇
  2000年   537篇
  1999年   331篇
  1998年   206篇
  1997年   227篇
  1996年   221篇
  1995年   252篇
  1994年   234篇
  1993年   177篇
  1992年   210篇
  1991年   227篇
  1990年   232篇
  1989年   190篇
  1988年   189篇
  1987年   128篇
  1986年   169篇
  1985年   169篇
  1984年   162篇
  1983年   150篇
  1982年   156篇
  1981年   153篇
  1980年   128篇
  1979年   140篇
  1978年   125篇
  1976年   112篇
  1974年   126篇
  1972年   122篇
  1971年   111篇
  1964年   112篇
排序方式: 共有10000条查询结果,搜索用时 400 毫秒
991.
Using the long-tailed tit (Aegithalos c. caudatus) as an example of the so-called irruptive species, the ecophysiological characteristics (body mass, fat reserves) and migration speed during autumn period were analyzed. The mean body mass and the median fat score did not depend on the number of migrating individuals. There was no evidence of the so-called stress in birds, participating in migration: migrants had even higher fat reserves than residents, while no significant difference in the body mass was recorded. The increase in fat reserves in the long-tailed tit during migration period was similar to the seasonal changes in energy reserves known for typical migrants. This makes it possible for late-migrating individuals to maintain higher total speed of migration compared to early-migrating ones. Basing on the obtained data, we conclude that the ecophysiological characteristics of the long-tailed tits migrating in autumn are similar to those of regular migrants.  相似文献   
992.
On the basis of long-term studies on the ecology of pied flycatcher in Karelia, it has been shown that male nonparticipation in nestling feeding is due to the course of postnuptial molt rather than to the mode of mating relationships, as it was considered previously. This is confirmed by the results of selective capture of birds that have stopped to feed their brood. It is suggested that reduction of parental care is associated with changes in the hormonal balance of molting birds.  相似文献   
993.
994.
The abundance of terrestrial arthropods in the Orenburgskii Nature Reserve has decreased since its establishment in 1989. At present, this parameter fluctuates considerably, and the species composition of arthropods is changing in favor of xerobiontic and eurybiontic species. These changes result from the absence of grazing load as well as from periodic fires. Therefore, it is necessary to organize controlled livestock grazing and reliable fire protection in reserved areas.  相似文献   
995.
Biodiversity conservation has undergone a profound change in philosophy, policies and management approaches over the last forty years. The traditional top–down approach to nature protection has been widely criticized for failing to include critical social elements in management practices, and is being gradually replaced by a slew of participatory strategies under the rubric of bottom-up conservation. The new approach recognizes local communities as key partners in wildlife management and seeks their participation in social development and biodiversity conservation. However, every social context is different in its structure and functions, and in the way social groups respond to calls for participation. In order to gain a better understanding of the approach and the barriers encountered in its implementation, a questionnaire survey of 188 households was employed in the communities of the Upper Mustang extension of Annapurna Conservation Area (ACA) in Nepal. The study provides a comparative analysis of community participation and its barriers between Non-Tourist (NT) and Tourist (TV) villages. The results revealed important differences between the two groups in terms of their participation in community programs, barriers to participation, and perception of benefits from participation. Owing to their distinct spatial, demographic and attitudinal differences, the two village groups have their own sets of needs, values and motivation factors which cannot be generalized and treated as such. The research clearly identifies the need for the conservation agency to be creative in devising strategies and initiatives appropriate to specific social groups so as to optimize their input in participatory conservation.  相似文献   
996.
Shaded perennial agroforestry systems contain relatively high quantities of soil carbon (C) resulting from continuous deposition of plant residues; however, the extent to which the C is sequestered in soil will depend on the extent of physical protection of soil organic C (SOC). The main objective of this study was to characterize SOC storage in relation to soil fraction-size classes in cacao (Theobroma cacao L.) agroforestry systems (AFSs). Two shaded cacao systems and an adjacent natural forest in reddish-yellow Oxisols in Bahia, Brazil were selected. Soil samples were collected from four depth classes to 1 m depth and separated by wet-sieving into three fraction-size classes (>250 μm, 250–53 μm, and <53 μm)—corresponding to macroaggregate, microaggregate, and silt-and-clay size fractions—and analyzed for C content. The total SOC stock did not vary among systems (mean: 302 Mg/ha). On average, 72% of SOC was in macroaggregate-size, 20% in microaggregate-size, and 8% in silt-and-clay size fractions in soil. Sonication of aggregates showed that occlusion of C in soil aggregates could be a major mechanism of C protection in these soils. Considering the low level of soil disturbances in cacao AFSs, the C contained in the macroaggregate fraction might become stabilized in the soil. The study shows the role of cacao AFSs in mitigating greenhouse gas (GHG) emission through accumulation and retention of high amounts of organic C in the soils and suggests the potential benefit of this environmental service to the nearly 6 million cacao farmers worldwide.  相似文献   
997.
Bio-economic farm models are tools to evaluate ex-post or to assess ex-ante the impact of policy and technology change on agriculture, economics and environment. Recently, various BEFMs have been developed, often for one purpose or location, but hardly any of these models are re-used later for other purposes or locations. The Farm System Simulator (FSSIM) provides a generic framework enabling the application of BEFMs under various situations and for different purposes (generating supply response functions and detailed regional or farm type assessments). FSSIM is set up as a component-based framework with components representing farmer objectives, risk, calibration, policies, current activities, alternative activities and different types of activities (e.g., annual and perennial cropping and livestock). The generic nature of FSSIM is evaluated using five criteria by examining its applications. FSSIM has been applied for different climate zones and soil types (criterion 1) and to a range of different farm types (criterion 2) with different specializations, intensities and sizes. In most applications FSSIM has been used to assess the effects of policy changes and in two applications to assess the impact of technological innovations (criterion 3). In the various applications, different data sources, level of detail (e.g., criterion 4) and model configurations have been used. FSSIM has been linked to an economic and several biophysical models (criterion 5). The model is available for applications to other conditions and research issues, and it is open to be further tested and to be extended with new components, indicators or linkages to other models.  相似文献   
998.
Principles of good practice for collaborative resource management were derived from the literature and their use studied in a range of integrated catchment management processes. Desk-based reviews and interviews with participants allowed the principles to be refined and described within a framework that illustrates the interrelationships between core principles, enabling principles, precursors to a project and the influence of external factors on such collaborative processes. The findings illustrate the importance of these relationships in understanding how success is defined and under what conditions successful outcomes can be achieved. Understanding how these procedural aspects influence outcomes contributes to the wider literature on collaborative resource management that often treats processes separately from their context.  相似文献   
999.
Anaerobic lagoons are commonly used for the treatment of swine wastewater. Although these lagoons were once thought to be relatively simple, their physical, chemical, and biological processes are very complex. This study of anaerobic lagoons had two objectives: (i) to quantify denitrification enzyme activity (DEA) and (ii) to evaluate the influence of lagoon characteristics on the DEA. The DEA was measured by the acetylene inhibition method. Wastewater samples and physical and chemical measurements were taken from the wastewater column of nine anaerobic swine lagoons from May 2006 to May 2009. These lagoons were typical for anaerobic swine lagoons in the Carolinas relative to their size, operation, and chemical and physical characteristics. Their mean value for DEA was 87 mg N2O-N m(-3) d(-1). In a lagoon with 2-m depth, this rate of DEA would be compatible with 1.74 kg N ha(-1) d(-1) When nonlimiting nitrate was added, the highest DEA was compatible with 4.38 kg N ha(-1) d(-1) loss. Using stepwise regression for this treatment, the lagoon characteristics (i.e., soluble organic carbon, total nitrogen, temperature, and NO3-N) provided a final step model R2 of 0.69. Nitrous oxide from incomplete denitrification was not a significant part of the system nitrogen balance. Although alternate pathways of denitrification may exist within or beneath the wastewater column, this paper documents the lack of sufficient denitrification enzyme activity within the wastewater column of these anaerobic lagoons to support large N2 gas losses via classical nitrification and denitrification.  相似文献   
1000.
To investigate the role of particle size on the oxidation, bioavailability, and adverse effects of manufactured Cu nanoparticles (NPs) in soils, we exposed the earthworm Eisenia ferida to a series of concentrations of commercially produced NPs labeled as 20- to 40-nm or < 100-nm Cu in artificial soil media. Effects on growth, mortality, reproduction, and expression of a variety of genes associated with metal homeostasis, general stress, and oxidative stress were measured. We also used X-ray absorption spectroscopy and scanning X-ray fluorescence microscopy to characterize changes in chemical speciation and spatial distribution of the NPs in soil media and earthworm tissues. Exposure concentrations of Cu NPs up to 65 mg kg(-1) caused no adverse effects on ecologically relevant endpoints. Increases in metallothionein expression occurred at concentrations exceeding 20 mg kg(-1) of Cu NPs and concentrations exceeding 10 mg kg(-1) of CuSO4. Based on the relationship of Cu tissue concentration to metallothionein expression level and the spatial distribution and chemical speciation of Cu in the tissues, we conclude that Cu ions and oxidized Cu NPs were taken up by the earthworms. This study suggests that oxidized Cu NPs may enter food chains from soil but that adverse effects in earthworms are likely to occur only at relatively high concentrations (> 65 mg Cu kg(-1) soil).  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