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   890篇
  免费   96篇
  国内免费   139篇
林业   79篇
农学   44篇
基础科学   47篇
  320篇
综合类   387篇
农作物   8篇
水产渔业   190篇
畜牧兽医   35篇
园艺   4篇
植物保护   11篇
  2024年   8篇
  2023年   31篇
  2022年   26篇
  2021年   30篇
  2020年   21篇
  2019年   18篇
  2018年   32篇
  2017年   56篇
  2016年   69篇
  2015年   52篇
  2014年   59篇
  2013年   72篇
  2012年   80篇
  2011年   101篇
  2010年   73篇
  2009年   85篇
  2008年   59篇
  2007年   50篇
  2006年   55篇
  2005年   43篇
  2004年   28篇
  2003年   17篇
  2002年   9篇
  2001年   11篇
  2000年   9篇
  1999年   6篇
  1998年   3篇
  1997年   3篇
  1996年   9篇
  1995年   1篇
  1994年   2篇
  1993年   2篇
  1992年   2篇
  1990年   2篇
  1989年   1篇
排序方式: 共有1125条查询结果,搜索用时 15 毫秒
1.
The newly formed extensive flooded land associated with the Three Gorges Reservoir hold great potentials to provide ecosystem services (ESs) for surrounding cities and communities. Conflicting values and conventional management strategies may complicate and inhibit broad-scale decision-making for this novel ecosystem. Public attitudes are a crucial influence in the way this process plays out. This study was conducted to assess how the reservoir bank is perceived by the general public of Chongqing Municipality in terms of importance of riparian ESs, and preference for corresponding landscape design styles through photo-questionnaire interviewing. Although Chongqing citizens know little of the “ecosystem services” concept, they do recognise riparian environmental and ecological benefits especially the bank protection and pollution interception functions, while benefits of habitats for wildlife and flood attenuation were of less concern. Recreational values were least appreciated, which is inconsistent with other studies. This may be attributed to a wider, intensive environmental education in various social media in today’s China. But publicity has been dominated by toxic air, water and soil pollution control rather than the benefits of nature conservation and personal contact with nature. In line with recognition of ESs, there is quite strong preference for the forested design scenario and particularly vegetation components across the socio-demographic spectrum. Overall, socio-demographic variables have very low explanatory power for the perceptual responses. Our study also revealed misunderstandings around riparian functions and a gap between public expectations and special abiotic and biotic prerequisites in restoring novel ecosystems. Finally, we discuss the relevance of this research to urban novel ecosystems in general, and implications for raising public awareness, engaging appropriate public values in sustainable design and management of such ecosystems.  相似文献   
2.
2018年4月10日、6月11日、8月14日在桓仁水库的上游(江南九队)、中游(砬砬岗子)和下游(泗河大地)3个站位用500 mL具塞磨口瓶盛装采水器采集的水库水样,进行限制性营养盐原位试验,同时测定水中溶解氧和初级生产力,并检测分析水体理化指标,以查明辽宁桓仁水库氮磷分布、营养盐限制和初级生产力状况,保护桓仁水库生态系统的结构和功能完整性。试验结果显示,营养盐加富试验中,0.5 mg/mL氮+0.3 mg/mL磷的添加组可显著提高水体中的溶解氧水平(P<0.05);上游和中游氮磷比为2∶1、下游氮磷比为4∶1可显著提高水体中的溶解氧水平(P<0.05);初级生产力随季节变化特征明显,原始P/R系数均小于试验组。根据试验结果分析得知,桓仁水库营养盐限制因素受季节变化的影响显著,具有氮、磷双限制等特征;根据初级生产力判断桓仁水库为自养代谢型水体。  相似文献   
3.
三峡库区沉积物中重金属化学形态分布特征与相关性分析   总被引:1,自引:1,他引:0  
为全面了解三峡库区干支流沉积物中重金属的赋存形态,更好地为评价及保护三峡库区水生态环境提供科学依据,于2015年6月在三峡库区采集沉积物样品67个,通过ICP-MS等仪器测定了沉积物中各重金属元素的不同赋存形态量,运用SPSS、Matlab统计分析软件对重金属赋存形态进行对比分析。结果表明,库区表层沉积物中Cr、Ni元素主要以残渣态为主(占总量的75%以上);Cu、Zn、As元素主要以残渣态和可还原态为主(合计占总量80%~95%);Cd元素主要以酸可提取态和可还原态为主(分别占总量的55%和23%);Pb元素主要以可还原态和残渣态为主(各占总量的71%和18%)。Cd和Pb元素生物可利用态量较高,均超过该元素的长江沉积物背景值;该种形态下的重金属元素容易进入水体,并且在地球化学条件变化时转化成能被生物吸收和利用的形态,对生物造成较大危害。干流上游段重庆主城区及下游段,渠系河、大宁河、磨刀溪、沿渡河应当是重金属生态危害研究与防治的重点区域。  相似文献   
4.
Abstract Diel changes in littoral fish assemblage were studied in four reservoirs in the Czech Republic (Central Europe). The sampling was performed by beach seining in an unstructured littoral zone. Perch, Perca fluviatilis L., roach, Rutilus rutilus (L.), bream, Abramis brama (L.), carp, Cyprinus carpio L., ruffe, Gymnocephalus cernua (L.), pikeperch, Sander lucioperca (L.), eel, Anguilla anguilla (L.) and A. brama × R. rutilus hybrids exhibited higher densities at night. Only bleak, Alburnus alburnus (L.) exhibited higher densities during the day. The number of species was higher in night hauls, and mean body size of roach and bleak was also higher at night. Diel changes in fish densities resulted in the change in species composition between day and night. The study has implication for the sampling design to assess littoral fish assemblages using beach seine netting and recommends night sampling for a representative assessment or sampling during both diel periods for a robust assessment.  相似文献   
5.
某水库建设项目地质灾害危险性评估   总被引:2,自引:0,他引:2  
通过野外调查、室内分析研究了评估区的地质环境条件,主要包括环境地质、水文地质和工程地质;结合建设项目可行性研究报告、岩土工程勘察报告,对某水库建设项目地质灾害危险性进行了现状评估、预测评估以及综合性分区评估;对拟建场地进行了适宜性评价,并提出了地质灾害防治措施。  相似文献   
6.
为了解三峡库区花岗岩母质不同经济林模式的土壤养分状况,研究了各模式下土壤pH、有机质、全量养分和速效养分的特征,并利用主成分分析法对不同模式的土壤质量进行了综合评价。结果表明,除土壤pH外,经济林土壤有机质、全氮、全磷、全钾、碱解氮、有效磷、速效钾等含量的变异较宽,为中等变异。随土壤层次的增加,各模式下土壤有机质、全氮、速效氮、速效磷和速效钾含量呈递减趋势,而全磷和全钾则存在不规律变化。土壤有机质、全氮和速效钾对经济林土壤质量的高低产生着主要作用,土壤养分综合排名表现为柑橘(精细管理)>柑橘(常规管理)>板栗林>茶园。因此,利用生物措施和精细化管理措施,可有效提高该区域经济林的土壤质量。  相似文献   
7.
结合水库移民工作实践经验,构建了后扶项目多项目管理组织机构并初步分析了多项目管理在后扶项目管理中开展方法.  相似文献   
8.
本文在对白石水库水源富营养化进行评价的基础上,对水库供水可靠性进行了分析.从现状水质评价结果看,白石水库的水质满足《地表水环境质量标准》(GB 3838-2002)中的Ⅲ类用水水质要求,可以作为集中式生活饮用水水源地和工业用水水源地.经过水量调节计算,可知白石水库供水水质是可靠的.  相似文献   
9.
Modern cities use straightened and concrete watercourses with simple greening for riparian zones, which has been criticised for insensitivity to natural system integrity and cultural identity. This increases the challenge to urban ecologists, landscape designers and managers to conceive innovative and effective design solutions that do not jeopardise hydraulic efficiency yet are culturally acceptable to local communities. This paper introduces the field of local ecological knowledge (LEK) as rich sources of inspiration and design solutions to meet this challenge. LEK refers to knowledge and practices of various local cultures about the relationship of living beings (including humans) with their environments. Wepropose a pragmatic framework that integrates LEK with modern landscape design. It includes steps: (1) investigate existing practices, skills andmeasures relating to LEK; (2) explore how and whereLEKcan inspire or integrate withdesign. Here we propose three aspects for integration in accordance with the nature of landscaping process: overall structure, component and maintenance.(3)evaluate and test the integration in terms of its acceptability by local communities. Taking the riparian revegetation project of Hanfeng Lake Urban Wetland Park (Chongqing Municipality) as an example, we illustrate the utility of this framework. 11 local common practices relating to LEK were identified. According to thethree aspects for integration, alternative design solutions were offered. The survey showed that LEK based designwas more highly valued than other conventional ways in terms of useracceptance. We conclude that LEK provides actionable ecologically sound and culturally desirablesolutions for urbanlandscape. We also recognise that LEK evolveswith changing environments and new harmonious and dynamic human-nature relationships arecontinually re-forming. The process of acquisition and application of LEK could encourage multidisciplinary and synthetic approaches to urban ecosystems. Such adaptability and interdisciplinary engagementare crucial totoday’schanging and complexurban environments.  相似文献   
10.
为阐明漳泽水库水环境要素的时空变化规律,依据历史资料和现场监测进行研究。结果显示,漳泽水库水环境要素年际分布的差异性较大,外源污染物的排放量是影响漳泽水库水质的首要原因。近10年来,漳泽水库水环境要素年际间均呈不显著的上升趋势,但月度变化差异较大;水质指标(BOD5、CODMn和DO)含量月度最高值出现在枯水期的2、3月份;7月份,DO和NH3-N处于最低值。TP在枯水期含量最低,平水期最高,丰水期则居中。温度、降雨量、蓄水量等因子的季节变化对水环境要素年内变化的影响比较显著。水温分层特性增强了水环境要素在垂向的分异性。  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