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   9313篇
  免费   98篇
  国内免费   245篇
安全科学   197篇
废物处理   401篇
环保管理   1053篇
综合类   2164篇
基础理论   2065篇
污染及防治   2127篇
评价与监测   629篇
社会与环境   960篇
灾害及防治   60篇
  2023年   61篇
  2022年   140篇
  2021年   106篇
  2020年   70篇
  2019年   101篇
  2018年   149篇
  2017年   168篇
  2016年   223篇
  2015年   166篇
  2014年   269篇
  2013年   769篇
  2012年   292篇
  2011年   393篇
  2010年   325篇
  2009年   351篇
  2008年   417篇
  2007年   422篇
  2006年   325篇
  2005年   315篇
  2004年   305篇
  2003年   304篇
  2002年   289篇
  2001年   345篇
  2000年   216篇
  1999年   117篇
  1998年   99篇
  1997年   95篇
  1996年   109篇
  1995年   177篇
  1994年   148篇
  1993年   123篇
  1992年   121篇
  1991年   122篇
  1990年   110篇
  1989年   95篇
  1988年   104篇
  1987年   100篇
  1986年   76篇
  1985年   79篇
  1984年   78篇
  1983年   72篇
  1982年   69篇
  1981年   68篇
  1980年   48篇
  1979年   52篇
  1973年   53篇
  1972年   53篇
  1916年   53篇
  1914年   71篇
  1913年   107篇
排序方式: 共有9656条查询结果,搜索用时 234 毫秒
781.
A major bottleneck for data-based policy making is that data sources are collected, managed, and distributed by different institutions, residing in different locations, resulting in conceptual and practical problems. The use of dispersed data for agricultural systems research requires the integration of data sources, which means to ensure consistency in data interpretations, units, spatial and temporal scales, to respect legal regulations of privacy, ownership and copyright, and to enable easy dissemination of data. This paper describes the SEAMLESS integrated database on European agricultural systems. It contains data on cropping patterns, production, farm structural data, soil and climate conditions, current agricultural management and policy information. To arrive at one integrated database, a shared ontology was developed according to a collaborative process, which facilitates interdisciplinary research. The paper details this process, which can be re-used in other research projects for integrating data sources.  相似文献   
782.
Long term trend analysis of bulk precipitation, throughfall and soil solution elemental fluxes from 12 years monitoring at 10 ICP Level II forest sites in the UK reveal coherent national chemical trends indicating recovery from sulphur deposition and acidification. Soil solution pH increased and sulphate and aluminium decreased at most sites. Trends in nitrogen were variable and dependant on its form. Dissolved organic nitrogen increased in bulk precipitation, throughfall and soil solution at most sites. Nitrate in soil solution declined at sites receiving high nitrogen deposition. Increase in soil dissolved organic carbon was detected - a response to pollution recovery, changes in soil temperature and/or increased microbial activity. An increase of sodium and chloride was evident - a possible result of more frequent storm events at exposed sites. The intensive and integrated nature of monitoring enables the relationships between climate/pollutant exposure and chemical/biological response in forestry to be explored.  相似文献   
783.
Abstract: Finding an adequate measure of hunting sustainability for tropical forests has proved difficult. Many researchers have used urban bushmeat market surveys as indicators of hunting volumes and composition, but no analysis has been done of the reliability of market data in reflecting village offtake. We used data from urban markets and the villages that supply these markets to examine changes in the volume and composition of traded bushmeat between the village and the market (trade filters) in Equatorial Guinea. We collected data with market surveys and hunter offtake diaries. The trade filters varied depending on village remoteness and the monopoly power of traders. In a village with limited market access, species that maximized trader profits were most likely to be traded. In a village with greater market access, species for which hunters gained the greatest income per carcass were more likely to be traded. The probability of particular species being sold to market also depended on the capture method and season. Larger, more vulnerable species were more likely to be supplied from less‐accessible catchments, whereas there was no effect of forest cover or human population density on probability of being sold. This suggests that the composition of bushmeat offtake in an area may be driven more by urban demand than the geographic characteristics of that area. In one market, traders may have reached the limit of their geographical exploitation range, and hunting pressure within that range may be increasing. Our results demonstrate that it is possible to model the trade filters that bias market data, which opens the way to developing more robust market‐based sustainability indices for the bushmeat trade.  相似文献   
784.
Abstract: Reintroduction of captive‐reared animals has become increasingly popular in recent decades as a conservation technique, but little is known of how demographic factors affect the success of reintroductions. We believe whether the increase in population persistence associated with reintroduction is sufficient to warrant the cost of rearing and relocating individuals should be considered as well. We examined the trade‐off between population persistence and financial cost of a reintroduction program for Crested Coots (Fulica cristata). This species was nearly extirpated from southern Europe due to unsustainable levels of hunting and reduction in amount and quality of habitat. We used a stochastic, stage‐based, single‐sex, metapopulation model with site‐specific parameters to examine the demographic effects of releasing juveniles or adults in each population for a range of durations. We parameterized the model with data from an unsuccessful reintroduction program in which juvenile captive‐bred Crested Coots were released between 2000 and 2009. Using economic data from the captive‐breeding program, we also determined whether the strategy that maximized abundance coincided with the least expensive strategy. Releasing adults resulted in slightly larger final abundance than the release of nonreproductive juveniles. Both strategies were equally poor in achieving a viable metapopulation, but releasing adults was 2–4 times more expensive than releasing juveniles. To obtain a metapopulation that would be viable for 30 years, fecundity in the wild would need to increase to the values observed in captivity and juvenile survival would need to increase to almost unity. We suggest that the most likely way to increase these vital rates is by increasing habitat quality at release sites.  相似文献   
785.
Carbofuran (2,3-dihydro-2,2-dimethylbenzofuran-7-yl methylcarbamate) has been used within the Nzoia River Basin (NRB), especially in Bunyala Rice Irrigation Schemes, in Kenya for the control of pests. In this study, the capacity of native bacteria to degrade carbofuran in soils from NRB was investigated. A gram positive, rod-shaped bacteria capable of degrading carbofuran was isolated through liquid cultures with carbofuran as the only carbon and nitrogen source. The isolate degraded 98% of 100-μg mL?1 carbofuran within 10 days with the formation of carbofuran phenol as the only detectable metabolite. The degradation of carbofuran was followed by measuring its residues in liquid cultures using high performance liquid chromatography (HPLC). Physical and morphological characteristics as well as molecular characterization confirmed the bacterial isolate to be a member of Bacillus species. The results indicate that this strain of Bacillus sp. could be considered as Bacillus cereus or Bacillus thuringiensis with a bootstrap value of 100% similar to the 16S rRNA gene sequences. The biodegradation capability of the native strains in this study indicates that they have great potential for application in bioremediation of carbofuran-contaminated soil sites.  相似文献   
786.
One-year quantitative chemical data set consisting of water-soluble constituents (NH4+, Na+, K+, Mg2+, Ca2+, Cl?, NO3?, SO42? and HCO3?), crustal and trace elements (Al, Fe, Ca, Mg, K, Mn, Zn, Pb) and carbonaceous species (OC, EC) in ambient aerosols, collected over an urban site located in a high-dust semi-arid region of western India, reveals excellent linear relationship (r2 = 0.92; slope = 0.96 ± 0.05) between gravimetrically assessed TSP (total suspended particulates) and chemically analyzed aerosol mass. The TSP abundance ranging from 60 to 250 μg m?3, over a period of 12 months (January–December), is dominated by mineral dust (~70%); whereas contribution from sea-salts, anthropogenic and carbonaceous species exhibits significant temporal variability depending upon the wind regimes. The mineral dust is enriched in Ca, Mg and Fe with respect to upper continental crust (UCC); whereas Zn and Pb exhibit a characteristic anthropogenic source and high enrichment factors. The carbonaceous species show significant seasonality; with dominance of OC (range: 4.6–28 μg m?3; average: 12.8 μg m?3; SD: 6.8) and minor contribution from EC (range: 0.3–4.4 μg m?3; average: 2.4 μg m?3; SD: 1.4). The observed concentrations are significantly lower than those reported for the metro cities in South Asia but the OC/EC ratios (range: 4.3–35; average: 8.3; SD: 5.7) are significantly higher than the characteristic ratio (~2–4) reported for the urban atmosphere. Such quantitative chemical characterization of aerosols is essential in assessing their role in atmospheric chemistry and climate change. This study could also be useful in understanding the physical and optical aerosol properties documented from the same site and thus, in validating regional climate models.  相似文献   
787.
The use of futures in copper marketing became common during the 1990s, but little attention has been put on the reasons for it and its relationship with other elements of marketing such as forward sales and premiums. This study focuses on these issues and the benefit of futures relative to annual forward contracts. It argues futures complement annual forward contracts to secure the average market price rather than to maximize revenue. The paper analyzes empirical evidence for the six largest Chilean copper producers. This sample reflects well the entire Chilean industry during the 1990s. The analysis shows that forward sales became more volatile and futures seem to be the instrument to avoid income fluctuation at the end of the marketing process. However, the benefits of futures compared to forward sales are very modest. In the last section we suggest a better marketing instrument to maximize revenue may be premiums.  相似文献   
788.
ABSTRACT

To test the possible use of composted food waste and wastewater sludge as biofilters to treat gas-phase volatile organic compounds (VOCs), batch experiments were conducted with an isolated strain that could degrade aromatic compounds under aerobic conditions. A benzene and trichloroethylene (TCE) mixture was used as the gas-phase pollutant in experiments with composted food waste, sludge, and soil. Under aerobic conditions, benzene was degraded as a primary substrate and TCE was degraded cometabolically, with water contents varying from 6 to 60% (volume of water added/volume of solid). Optimal water content for VOC removal was 12% for the soil, 36% for the composted food waste, and 48% for the sludge.

The extent of VOC sorption and biodegradation at the optimal water content was different for each material. With the same initial VOC concentration, more VOCs were removed by sorption onto the composted food waste and the sludge, while less VOCs were biodegraded in comparison with the results using soil. The reason the biodegradation in the soil was greater may be partly attributed to the fact that, due to less sorption, the aqueous-phase concentration of VOCs, which microorganisms could utilize as a carbon source or cometabolize, was higher. We also speculate that the distribution of microorganisms in each medium affects the rate of biodegradation. A large number of microorganisms were attached to the composted food waste and sludge. Mass transfer of VOCs and oxygen to these microorganisms, which appear to have been heterogeneously distributed in clusters, may have been limited, resulting in hindered biodegradation.  相似文献   
789.
South Africa faces several development challenges including those linked to climate change. Energy usage in South Africa, for example, is already constrained because of a range of development challenges (the dependence on cheap coal as a heating source; energy availability; access; affordability of alternative energy sources; and a range of health impacts, including air pollution). Notwithstanding calls for a transition to a low carbon economy, there have been few, recent assessments in South Africa of the costs associated with such a transition, particularly the social and economic costs for the poor who use energy in a variety of ways. In this paper we focus on trying to unravel some of the complex energy-use behaviour including understanding what drives consumers in resource-poor areas to choose and use persistently risky energy options. Analysis of qualitative data in two township areas, Doornkop and Kwaguqa, shows that township households, whether electrified or not, continue to burn coal. In both study areas, an estimated 80% of electrified households burn coal for space heating and cooking and also make use of multiple fuel sources for a range of activities. Although the major obstacles preventing people from discontinuing domestic coal combustion are poverty, the ready availability and social acceptability of coal together with other social customs cannot be underrated. This paper therefore highlights some of the persistent challenges associated with sustainable energy transitions in South Africa including implications for improved mitigation and adaptation for the energy sector in wider climate change efforts.  相似文献   
790.
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