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   226篇
  免费   27篇
  国内免费   52篇
  2024年   2篇
  2023年   7篇
  2022年   8篇
  2021年   6篇
  2020年   7篇
  2019年   9篇
  2018年   9篇
  2017年   8篇
  2016年   7篇
  2015年   7篇
  2014年   22篇
  2013年   20篇
  2012年   25篇
  2011年   21篇
  2010年   10篇
  2009年   11篇
  2008年   16篇
  2007年   13篇
  2006年   18篇
  2005年   12篇
  2004年   15篇
  2003年   10篇
  2002年   5篇
  2001年   6篇
  2000年   1篇
  1999年   6篇
  1998年   4篇
  1997年   3篇
  1996年   1篇
  1995年   3篇
  1994年   2篇
  1993年   1篇
  1992年   1篇
  1991年   2篇
  1990年   1篇
  1989年   1篇
  1988年   1篇
  1985年   1篇
  1984年   1篇
  1981年   2篇
排序方式: 共有305条查询结果,搜索用时 15 毫秒
231.
土壤质量及其评价   总被引:104,自引:10,他引:94  
保持和提高土壤质量是实现农业可持续发展的基础,为此,必须清楚土壤质量的概念以及土壤质量的评价方法,本文综述了土壤质量的概念及其研究进展,土壤质量评价要以土壤的功能为基础,不同土壤的评价应采用不同的标准,土壤质量的评价是相对的而不是绝对的,介绍了国际上几种常用的土壤质量评价的方法,并对耕地土壤质量评价的指标选择进行了探讨。  相似文献   
232.
中山市小榄镇城区绿地植物调查与评价   总被引:3,自引:0,他引:3  
本文对小榄镇城区绿地植物种类进行了调查与研究.调查发现,小榄镇植物种类丰富,有105科433种.其中,公园绿地和苗圃区植物种类最多,街头与街道附属绿地植物种类较单调:乔木与灌木应用较多,藤本植物应用较少:乔木花卉于公园中应用较多;人工造林的林相、林型单一.旧城区绿地严重缺乏;部分工厂、学校对绿化不重视.研究提出,合理选择绿化树种,将木兰科植物作为乡土类的重点,紫葳科、木棉科和豆科作为外来树种引进的重点;对贺榄山现存桉树、相思疏残林进行林相改造,选择樟科、壳斗科、山茶科等作为骨干树种分期、分批进行混植.最后,筛选出适合各种类型绿地应用的绿化树种.  相似文献   
233.
以分析区域农业资源利用的持续与高效的关系为基础,建立红黄壤地区农业资源持续高效利用指标评价体系,应用指标体系对红黄壤地区中低产地区的22个典型县进行评价研究。  相似文献   
234.
浮游甲壳类定量采样计数方法评价   总被引:1,自引:0,他引:1  
1引言浮游生物是养殖鱼类特别是滤食性鱼类的主要饵料,同时又是评价水生态系统质量优劣的重要指标.在大水面增养殖研究中,常常通过水体中浮游生物的采样分析,计算水体生物生产力.而在池塘生态系统中则又常常借助于浮游生物的采样分析来判定养鱼水质,决定增施饵肥料...  相似文献   
235.
摘要 目的:探讨贵州省1例家族性高胆固醇血症(FH)家系基因突变筛查与评估结果。方法:参照荷兰脂质诊断网络(DLCN)指南诊断标准于2021年11月-2022年11月贵阳市某三甲医院选取1例临床诊断FH的患者进行研究,分析其家系系谱,采用C8000型全自动生化分析仪检测家系成员总胆固醇(TC)、甘油三酯(TG)、LDL-C、高密度脂蛋白胆固醇(HDL-C)水平;并采集家系临床相关数据,收集血清样本提取白细胞DNA进行全基因组外显子测序致病基因,筛选出4个FH相关基因[低密度脂蛋白受体(LDLR)、载脂蛋白B(ApoB)、西布曲明9a型(PCSK9)、LDLR衔接蛋白1(LDLRAP1)]的单核苷酸多态性(SNP)位点情况,采用Polyphen-2和SIFT软件对SNP位点进行致病性分析;同时回顾性分析双重滤过血浆置换(DFPP)吸附治疗期间的心血管疾病发生情况及治疗前后的载脂蛋白A(ApoA)、载脂蛋白E(ApoE)、载脂蛋白B(ApoB)、脂蛋白a(LPa)、TC、TG、HDL-C、LDL-C、游离脂肪酸(FFA)水平变化情况。结果:该家系Ⅰ-1成员血清LDL-C达8.16 mmol/L,Ⅱ-1成员血清LDL-C达7.0 mmol/L,Ⅱ-2成员血清LDL-C达3.45 mmol/L,按照DLCN标准,Ⅰ-1评7分,Ⅱ-1评6分,均提示患FH可能性大,达到FH临床诊断标准。利用Polyphen-2预测,该家系ApoB基因中rs676210、rs679899分别得分0.999、0.998,1个新位点c.10094A>T得分为0.829,均提示可能致病,SIFT软件预测以上三个位点均有害。DFPP治疗后患者病情稳定,至今无心绞痛严重心血管事件发生。与吸附治疗前比较,吸附治疗后ApoA、ApoE、ApoB、LPa、TG、HDL-C、LDL-C、TC水平较低FFA水平较高,差异有统计学意义(P<0.05)。结论:ApoB基因SNP位点突变很可能是该家系引起FH的主要原因,而DFPP治疗有利于调节FH患者血脂水平,进而降低心血管疾病的发生风险。  相似文献   
236.
The purpose of this study is to employ a nonlinear dynamic evaluation method to assess the tourism sustainability of Tibet Autonomous Region (TAR), China, a new emerging tourism destination. The methodology draws on system dynamics and Back Propagation (BP) neural network. According to 7 setting principles, this study identifies 13 tourism sustainability indicators including conventional tourism income, tourism resources stock, pollution stock, etc., as well as specific residents’ tourism cognition, seasonal difference, accessibility, etc. Then a system dynamics model including the 13 indicators (variables) and other relevant auxiliary variables is established. Based on the numerical simulation, using a three layers BP neural network optimized by genetic algorithm and particle swarm algorithm, this study evaluates the future sustainability dynamically and compares the sustainability evolution from 2014 to 2050 under different development strategies. The research results not only provide information useful for the dynamic control and scientific management of the future sustainable tourism development, but also provide a systems approach to evaluate regional tourism sustainability.  相似文献   
237.
This study evaluated the extent to which natural protected areas (NPAs) in Mexico have been effective for preventing land use/land cover change, considered as a major cause of other degradation processes. We developed an effectiveness index including NPA percentage of transformed areas (agriculture, induced vegetation, forestry plantations, and human settlements) in 2002, the rate and absolute extent of change in these areas (1993–2002), the comparison between rates of change observed inside the NPA and in an equivalent surrounding area, and between the NPA and the state(s) in which it is located. We chose 69 terrestrial federal NPAs, decreed before 1997, that were larger than 1,000 ha, not urban/reforested with non-native vegetation, not islands and not coastal strips, and estimated the extent of transformed areas using 1993 and 2002 land use/land cover maps. Over 54% of NPAs were effective, and were heterogeneously distributed by management categories: 65% of Biosphere Reserves, 53% of Flora and Fauna Protection Areas, and 45% of National Parks. 23% of NPAs were regarded as weakly effective, and the remaining 23% as non-effective. We recognize the importance of NPAs as a relevant conservation instrument, as half of NPAs analyzed (particularly biosphere reserves) prevented natural vegetation loss compared with their geographic context. Our results suggest that conservation based on NPAs in Mexico still faces significant challenges. Our approach can be expanded for evaluating the effectiveness of NPA in other regions, as land use/land cover maps are now available almost worldwide.  相似文献   
238.
Improving crop water productivity is necessary for ensuring food and ecological security. To quantify the water utilization in grain production from multiple perspectives, gross inflow water productivity (WPg), generalized water productivity (WPu), evapotranspiration water productivity (WPET) and irrigation water productivity (WPI) were screened in this study. Then, we calculated and analyzed the temporal and spatial variation of the water productivity indices (WPs) in the irrigated land of 31 provinces of China based on the data collected from 459 irrigation districts. The results show that the national values of the four indices were 0.694, 0.860, 1.314, and 1.361 kg/m3, and almost all of the provincial WPs increased from 1998 to 2010. The Moran's I analysis shows all of the WP indices were significantly clustered, both globally and locally, in the study years. WPs of the provinces in and around Huang-Huai-Hai Plain are high, while provinces in the northeast, south of the Yangtze River and the northwest of China showed lower water use efficiency. There is a large difference among the four indices in the scientific connotation and numerical performance, and the degree of the spatial difference was WPI > WPET > WPu > WPg. It is necessary to promote WP in the irrigated land of China, and the provinces in the northeast should be chosen as the key regions to develop water-saving agriculture because of the high production of grain and the low value of WP. Policy decision-making based on the spatial pattern of WP and their relationships among indices for specified regions are also discussed in this paper.  相似文献   
239.
With the increase in industrial development and improper land uses, Taiwan has faced the extinction crisis and is seeing the signs of nature, revenge in the forms of frequent landslides, floods, and debris flows in recent years. The government has strongly promoted the concept of ecotechnology in the hope of maintaining the ecosystem and its integrity. However, the application of ecotechnology is still in its infancy, so there is currently no objective evaluation standard to assess its effectiveness. Deficiencies in the scientific approach to quantify the results are due to current disputes and bottlenecks in its implementation. In order to seek reasonable and feasible methods to overcome the deficiencies in the implementation of the scientific approach, this study collected domestic and international ecological evaluation models to develop innovative processes that utilize ecological, safety, and cost indices. First, the commonly used ecotechnology methods for river embankment were acquired from the governmental departments. Second, the ecological, safety and cost indices for the acquired ecotechnology methods and construction sites were analyzed and quantified using fuzzy theory. Third, indices of ecotechnology methods were linked with those of construction sites for the PROMETHEE technique evaluation for suitable ecotechnology method selection. A practical construction case located in Shihmen reservoir watershed was chosen for evaluation and verification. The results show that the developed model can provide useful information for decision making while performing ecotechnology engineering.  相似文献   
240.
Red fluorescent proteins (RFPs) are useful tools for live cell and multi-color imaging in biological studies. However, when labeling proteins in secretory pathway, many RFPs are prone to form artificial puncta, which may severely impede their further uses. Here we report a fast and easy method to evaluate RFPs fusion properties by attaching RFPs to an environment sensitive membrane protein Orai1. In addition, we revealed that intracellular artificial puncta are actually colocalized with lysosome, thus besides monomeric properties, pKa value of RFPs is also a key factor for forming intracellular artificial puncta. In summary, our current study provides a useful guide for choosing appropriate RFP for labeling secretory membrane proteins. Among RFPs tested, mOrange2 is highly recommended based on excellent monomeric property, appropriate pKa and high brightness.  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