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   137篇
  免费   2篇
  国内免费   1篇
安全科学   4篇
废物处理   5篇
环保管理   17篇
综合类   14篇
基础理论   26篇
污染及防治   34篇
评价与监测   25篇
社会与环境   11篇
灾害及防治   4篇
  2023年   1篇
  2022年   1篇
  2021年   1篇
  2020年   1篇
  2019年   2篇
  2018年   4篇
  2017年   5篇
  2016年   4篇
  2015年   5篇
  2014年   3篇
  2013年   5篇
  2012年   5篇
  2011年   9篇
  2010年   9篇
  2009年   3篇
  2008年   15篇
  2007年   9篇
  2006年   9篇
  2005年   7篇
  2004年   3篇
  2003年   4篇
  2002年   6篇
  2001年   1篇
  2000年   2篇
  1999年   8篇
  1997年   1篇
  1996年   1篇
  1995年   2篇
  1994年   4篇
  1993年   1篇
  1991年   1篇
  1990年   1篇
  1989年   1篇
  1987年   1篇
  1979年   1篇
  1976年   2篇
  1973年   1篇
  1964年   1篇
排序方式: 共有140条查询结果,搜索用时 140 毫秒
31.
To determine if using a checklist of specific ultrasound image criteria to screen the fetal heart improves the cardiac exam completion rate, defined as the ability to classify the heart as normal or abnormal. This is a retrospective cohort study of patients with singleton pregnancies who underwent a fetal anatomy survey between 18 and 28 weeks' gestation. A checklist was used from 1 September 2015 to 31 March 2016 to categorize exams as complete-normal, complete-abnormal, or incomplete. Performance was compared with a 7-month period prior to checklist introduction (1 December 2014 to 30 June 2015). Checklist utilization improved the cardiac exam completion rate by 8.9%. With the checklist, 1083 of 1202 exams (90.1%) were completed compared to 987 of 1193 (82.7%) pre-checklist, P < .001. We did not detect a change in cases classified as abnormal and referred for echocardiography: 25 (2.1%) with the checklist and 16 (1.3%) pre-checklist, P = .16. We did not detect more congenital heart disease (CHD), 12 (1.0%) with checklist screening, 5 (0.4%) pre-checklist, P = .14. Critical CHD was not missed in either group. Using the checklist improved the cardiac exam completion rate. There was no change in congenital heart disease detection.  相似文献   
32.
Human activities threaten the biodiversity of aquatic mammals across the globe. Conservation of these species hinges on the ability to delineate movements and foraging behaviors of animals, but gaining such insights is hampered by difficulties in tracing individuals over their lives. We determined isotope ratios in teeth (87Sr/86Sr, 13C/12C, and 18O/16O) to examine lifelong movement and resource-use patterns of a unique freshwater population of a wide-ranging pinniped species (harbor seal [Phoca vitulina]) that resides in Iliamna Lake, Alaska (U.S.A.). This population's potentially unique migratory behavior and use of different trophic resources are unknown. The isotope ratios we measured in teeth showed that seals were born in the lake, remained lifelong residents, and relied principally on resources produced from in the lake, even when seasonally abundant and nutrient-dense spawning anadromous fish (i.e., sockeye salmon [Oncorhynchus nerka]) were available in the lake. Our results illustrate how serial isotope records in teeth, particularly 87Sr/86Sr ratios, can be used to quantify how coastal mammal populations exploit both freshwater and marine ecosystems. Understanding lifelong patterns of habitat and resource use is essential information when designing effective conservation plans for threatened coastal mammals. We present the Iliamna Lake harbor seals as a unique case study into how isotope records within teeth can help reveal the cryptic ecology of such a population residing in an intact ecosystem. The results also provide critical baseline information for the Kvichak River system, which is facing an uncertain future due to proposed large-scale industrial development and a rapidly changing climate.  相似文献   
33.
Thompson JN  Fernandez CC 《Ecology》2006,87(1):103-112
Variation among sites and years in the local ecological outcome of interspecific interactions can generate a geographic mosaic of coevolution, as indicated by recent mathematical models. We evaluated whether local temporal dynamics of ecological outcome in the interaction between the moth Greya politella (Prodoxidae) and its host plant Lithophragma parviflorum (Saxifragaceae) are likely to mitigate or magnify geographic differences in ecological outcome found in earlier studies. The moths are highly host-specific pollinating floral parasites, and the mutualism can be swamped in some populations by the presence of effective co-pollinators. Hence, differing community contexts can shift the outcome of the interaction from mutualism to commensalism or antagonism. During each of four years, we evaluated the effect of Greya oviposition on seed development through a paired design that controlled for plant genotype and microenvironment. At Turnbull National Wildlife Refuge in Washington State, the interaction was significantly mutualistic in all four years. Mutualism in this population was indicated by a higher probability of development of capsules visited by ovipositing Greya than capsules not visited by Greya on the same plant. At Rapid River, Idaho, the interaction was commensalistic in three years and antagonistic in one year. Antagonism in this population was indicated by selective withering of capsules containing Greya eggs. Overall, the results suggest stable geographic differences in the range of ecological outcomes in this plant-insect interaction under different community contexts.  相似文献   
34.
35.
Chronic elevated nitrogen (N) deposition has altered the N status of temperate forests, with significant implications for ecosystem function. The Bear Brook Watershed in Maine (BBWM) is a whole paired watershed manipulation experiment established to study the effects of N and sulfur (S) deposition on ecosystem function. N was added bimonthly as (NH4)2SO4 to one watershed from 1989 to 2016, and research at the site has studied the evolution of ecosystem response to the treatment through time. Here, we synthesize results from 27 years of research at the site and describe the temporal trend of N availability and N mineralization at BBWM in response to chronic N deposition. Our findings suggest that there was a delayed response in soil N dynamics, since labile soil N concentrations did not show increases in the treated watershed (West Bear, WB) compared to the reference watershed (East Bear, EB) until after the first 4 years of treatment. Labile N became increasingly available in WB through time, and after 25 years of manipulations, treated soils had 10× more extractable ammonium than EB soils. The WB soils had 200× more extractable nitrate than EB soils, driven by both, high nitrate concentrations in WB and low nitrate concentrations in EB. Nitrification rates increased in WB soils and accounted for ~?50% of net N mineralization, compared to ~?5% in EB soils. The study provides evidence of the decadal evolution in soil function at BBWM and illustrates the importance of long-term data to capture ecosystem response to chronic disturbance.  相似文献   
36.
Regional Environmental Change - Mediterranean forests are found in the Mediterranean basin, California, the South African Cape Province, South and southwestern Australia and parts of Central Chile....  相似文献   
37.
This paper analyses the relationship between nitrous oxide (N2O) emissions, economic growth, agricultural land used and exports in Germany. We use time‐series data between 1970 and 2012 and the Autoregressive Distributed Lag (ARDL) methodology to test for cointegration in the long run. Results show that there is a quadratic long run relationship between N2O emissions and economic growth, confirming the existence of an Environmental Kuznets Curve (EKC) for Germany. Agricultural land area affects N2O emissions positively, whereas exports affect emissions negatively. The turning point is $27,880, which is within the sample and implies that Germany is in the decreasing part of the curve of environmental degradation. The paper shows that, contrary to testing the EKC in less developed countries, mitigation of N2O emissions does not negatively affect growth in Germany. As such, it is feasible to undertake any conservative policy in order to reduce emissions without major consequences on economic sectors.  相似文献   
38.
Hexachlorobenzene (HCB) was measured in adipose tissue intraoperatively collected from 387 subjects over 16 years old undergoing surgery in two hospitals in Granada (Southern Spain). HCB was quantified in 90.7% of subjects. The concentrations and frequencies of HCB were similar to those reported in adipose tissue samples in other recent European studies. Exposure patterns differed between females and males: higher HCB concentrations were found in females than in males (geometric mean 18.3 vs. 6.8 ng/g, p<0.001). The relationship between HCB concentrations and exposure risk factors was assessed by multivariate analysis stratifying by gender. In men, HCB concentrations were predicted (r(2)=0.45) by age, body mass index (BMI), place of residence, smoking, consumption of fish, chicken and cheese, occupation related to agriculture, and family involvement in construction activities. In women, HCB concentrations were predicted (r(2)=0.50) by age, BMI, consumption of milk and cheese, and occupation related to industry. The finding that women had three-fold higher levels of HCB than the men deserves further investigation.  相似文献   
39.
The objective of this work was to study the use of new biodegradable films in agriculture under open field conditions. Three biodegradable mulch films made from modified biodegradable polyester of different thicknesses and colors (black and white) and a conventional low density polyethylene (LDPE) mulch film were used to cover the beds of tomato plants. Changes in physical appearance of the films were recorded as well as changes in their mechanical, optical, and physical properties. Once tomato harvest was completed, the conventional LDPE mulch film was removed and all the tomato plants were cut using a mower. The biodegradable mulch films were plowed into the soil. The change in the appearance of the film was recorded and samples of each film after plowing were characterized according to the properties mentioned above. After the biodegradable films photodegraded, cross-link formation occurred within the films which promoted brittleness. Titanium dioxide, an additive used to produce white color in the films, catalyzed the photodegradation, while carbon black used for black color stabilized the photodegradation. The white films started to degrade after two weeks while it took about eight weeks for the black films to significantly degrade. The black biodegradable film seems to be a more promising alternative as a mulch film because of the comparable yields and weed suppression ability to conventional mulch film.  相似文献   
40.
In a previous paper, we demonstrated that the main mechanism of degradation of poly(butylene adipate-co-terephthalate) (PBAT) biodegradable mulch films when exposed to field conditions was crosslinking due to the photodegradation from solar radiation. The aim of this work was to determine the effect of crosslinking on the biodegradability of PBAT samples. PBAT films were subjected to UV photodegradation in laboratory simulated conditions to investigate the effects of crosslinking and other major changes in the structure and mechanical properties of the films. Crosslinking caused the films to become more brittle and produced a reduction of the tensile strength and percent elongation. Besides the crosslinking degradation mechanism, chain scission also occurred in the samples. After 45d of biodegradation test, the non-crosslinked PBAT sample reached 60% of mineralization. However, the percent mineralization was reduced when samples were crosslinked. The percent mineralization of samples with 10%, 30%, 50%, and 70% gel content was 36%, 43%, 21%, and 24%, respectively. Our results indicate that crosslinking is a key process underlying the degradation of the PBAT film and did affect the biodegradability of the films, since the samples with greater amount of gel content generally showed less percent mineralization in the biodegradation tests.  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