收费全文 | 30340篇 |
免费 | 345篇 |
国内免费 | 457篇 |
安全科学 | 951篇 |
废物处理 | 1439篇 |
环保管理 | 3675篇 |
综合类 | 5066篇 |
基础理论 | 7952篇 |
环境理论 | 18篇 |
污染及防治 | 8020篇 |
评价与监测 | 2070篇 |
社会与环境 | 1773篇 |
灾害及防治 | 178篇 |
2023年 | 134篇 |
2022年 | 320篇 |
2021年 | 316篇 |
2020年 | 240篇 |
2019年 | 284篇 |
2018年 | 478篇 |
2017年 | 477篇 |
2016年 | 742篇 |
2015年 | 568篇 |
2014年 | 885篇 |
2013年 | 2416篇 |
2012年 | 1060篇 |
2011年 | 1434篇 |
2010年 | 1168篇 |
2009年 | 1196篇 |
2008年 | 1437篇 |
2007年 | 1485篇 |
2006年 | 1263篇 |
2005年 | 1093篇 |
2004年 | 982篇 |
2003年 | 1075篇 |
2002年 | 949篇 |
2001年 | 1238篇 |
2000年 | 863篇 |
1999年 | 517篇 |
1998年 | 347篇 |
1997年 | 358篇 |
1996年 | 356篇 |
1995年 | 423篇 |
1994年 | 446篇 |
1993年 | 352篇 |
1992年 | 374篇 |
1991年 | 352篇 |
1990年 | 386篇 |
1989年 | 341篇 |
1988年 | 296篇 |
1987年 | 278篇 |
1986年 | 223篇 |
1985年 | 248篇 |
1984年 | 265篇 |
1983年 | 256篇 |
1982年 | 241篇 |
1981年 | 222篇 |
1980年 | 175篇 |
1979年 | 194篇 |
1978年 | 176篇 |
1975年 | 140篇 |
1974年 | 117篇 |
1972年 | 130篇 |
1971年 | 131篇 |
Polychlorinated biphenyls (PCBs) contaminate 19% of US Superfund sites and represent a serious risk to human and environmental health. One promising strategy to remediate PCB-contaminated sediments utilizes organohalide-respiring bacteria (OHRB) that dechlorinate PCBs.
However, functional genes that act as biomarkers for PCB dechlorination processes (i.e., reductive dehalogenase genes) are poorly understood. Here, we developed anaerobic sediment microcosms that harbor an OHRB community dominated by the genus Dehalococcoides. During the 430-day microcosm incubation, Dehalococcoides 16S rRNA sequences increased two orders of magnitude to 107 copies/g of sediment, and at the same time, PCB118 decreased by as much as 70%. In addition, the OHRB community dechlorinated a range of penta- and tetra-chlorinated PCB congeners including PCBs 66, 70?+?74?+?76, 95, 90?+?101, and PCB110 without exogenous electron donor. We quantified candidate reductive dehalogenase (RDase) genes over a 430-day incubation period and found rd14, a reductive dehalogenase that belongs to Dehalococcoides mccartyi strain CG5, was enriched to 107 copies/g of sediment. At the same time, pcbA5 was enriched to only 105 copies/g of sediment. A survey for additional RDase genes revealed sequences similar to strain CG5’s rd4 and rd8. In addition to demonstrating the PCB dechlorination potential of native microbial communities in contaminated freshwater sediments, our results suggest candidate functional genes with previously unexplored potential could serve as biomarkers of PCB dechlorination processes.
相似文献The sigma (SIG) coordinate system in ocean circulation simulation models results inevitably in horizontal pressure gradient error. This problem also emerges in models of deep lakes or reservoirs with the same characteristics of underwater terrain mutation. SIG coordinates reflect vertical relative stratification but cannot be used to calculate horizontal pressure gradient force in places with drastic topographic changes; this results in vertical water temperature and circulation errors. In deep lakes or reservoirs, differences in water density caused by the temperature difference between upper and lower water bodies is the primary cause of thermal stratification phenomena. Lake Mead was used as a case study on steep topography based on Environmental Fluid Dynamics Code (EFDC) model in this study. SIG coordinates result in close agreement between the calibrated temperature time series at the top and middle water layers, but disparity in the bottom water layer. The error emerges in the horizontal pressure gradient error due to the SIG coordinate transformation. Neither increasing the vertical resolution nor adjusting the horizontal viscosity coefficient resolve this error. We test the sigma-zed (SGZ) coordinate which combines Z coordinate and SIG coordinate as a replacement for the SIG coordinate to find that they effectively reduce the model’s runtime and simulation efficiency. The vertical temperature distribution in SGZ coordinate mode is more accurate than the distribution in SIG coordinate mode. The Navier-Stokes horizontal gradient and advection diffusion equation results under SIG coordinates are very sensitive to the pressure gradient. The replacement also enhances resolution near the thermocline, facilitates reclosing of the water bottom and the equal sigma surface, lends significant advantages in terms of vertical temperature in the simulation for local deep water with steep terrain, and shortens runtime for 0.14 h. SGZ mixed coordinates are recommended in the simulation of deep lakes or reservoirs wherein the underwater topography is large (with abundant continuous deep trenches or reefs).
相似文献