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   16435篇
  免费   5644篇
  国内免费   67篇
林业   1905篇
农学   1004篇
基础科学   988篇
  7049篇
综合类   7954篇
农作物   350篇
水产渔业   454篇
畜牧兽医   1020篇
园艺   235篇
植物保护   1187篇
  2025年   229篇
  2024年   687篇
  2023年   641篇
  2022年   679篇
  2021年   636篇
  2020年   727篇
  2019年   846篇
  2018年   595篇
  2017年   969篇
  2016年   1106篇
  2015年   858篇
  2014年   1079篇
  2013年   1182篇
  2012年   1570篇
  2011年   1423篇
  2010年   1117篇
  2009年   1122篇
  2008年   949篇
  2007年   1088篇
  2006年   865篇
  2005年   714篇
  2004年   523篇
  2003年   415篇
  2002年   302篇
  2001年   301篇
  2000年   228篇
  1999年   195篇
  1998年   184篇
  1997年   138篇
  1996年   115篇
  1995年   127篇
  1994年   125篇
  1993年   97篇
  1992年   79篇
  1991年   79篇
  1990年   59篇
  1989年   44篇
  1988年   23篇
  1987年   18篇
  1986年   3篇
  1985年   3篇
  1984年   1篇
  1982年   1篇
  1975年   1篇
  1962年   2篇
  1956年   1篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
1.
ABSTRACT:     The biomass of bighand thornyhead Sebastolobus macrochir was increased by the high recruitment success of the 1999–2002 year classes off the Pacific coast of northern Honshu, Japan. In this study, the growth of bighand thornyhead was examined over a 9-year period from 1996 to 2004 in this area. The growth of the 1999 year class and the 2000–2002 year classes was reduced at 3 and 2 years old, respectively, while the 1999–2002 year classes were smaller than the 1993–1998 year classes. In 2-, 3- and 4-year-old fish, the relationship between abundance and mean standard length was expressed by negative linear regressions, while fish became smaller when abundance of the year class was larger. Mean bottom temperatures were stable at depths of 350–900 m; variations in water temperature were small in the main distribution area of bighand thornyhead. We discuss the factors affecting the growth of bighand thornyhead via changes in the demersal fish community and feeding habits.  相似文献   
2.
采用大型通用有限元分析程序ABAQUS进行了方钢管混凝土轴压短柱力学性能模拟,分析了各个阶段构件的承载能力。结果表明:有限元计算的极限荷载和荷载-平均纵向应变曲线与试验结果符合较好,说明网格划分、边界条件、材料本构关系的选取基本正确,适用于钢管混凝土短柱的非线性分析。  相似文献   
3.
农用地分等与定级估价研究--以呼和浩特市清水河县为例   总被引:2,自引:2,他引:2  
在认识清水河县自然、社会经济条件的基础上,对清水河县农用地质量特别是耕地质量进行评定,探讨农用地分等与定级估价计算模型及方法,为加强农用土地资源管理与土地资产管理打下良好基础。  相似文献   
4.
农用地估价的常用方法有:收益还原法、收益倍数法、成本法、假设开发法、市场比较法、影子价格法、享乐定价法、土壤潜力估价法、标准田法等。这些估价方法对我国农用地的可操作性、适用范围.学术界有不同的看法,文章对这些观点进行了综述与分析。  相似文献   
5.
Objective: To describe diagnostics, therapy, and sequelae of acute carbon monoxide (CO) toxicity because of a motor vehicle generator in 4 dogs and 2 cats. Series summary: Four dogs and 2 cats presented for recumbency, disorientation, dyspnea, and stiffness after an estimated 6–8 hour exposure to exhaust from a generator. Diagnostics included a serum carboxyhemoglobin levels evaluation, arterial blood gas analysis, pulse oximetry readings, and blood pressure measurements. Initial therapy included oxygen (O2) administration, intravenous bronchodilators, fluids, and a hemoglobin‐based O2 carrying (HBOC) molecule. Following administration of the HBOC, 4 of the 6 animals showed dramatic clinical improvement. Two weeks after hospital discharge, the owner reported potential hearing deficits in all animals. Brain auditory evoked response (BAER) tests were conducted in all surviving animals and some degree of hearing impairment was documented in all cases, with complete clinical resolution noted 6 weeks later. Unique information provided: This report describes the therapeutic use of an HBOC in acute isolated CO toxicity (i.e. without the complications of smoke inhalation). In addition, delayed nervous system dysfunction was documented in all surviving animals.  相似文献   
6.
The dusuns of Central Maluku which improve fallows by planting spice trees such as clove (Syzygium aromaticum syn. Eugenia aromatica), nutmeg (Myristica fragrans), or coconut (Cocos nucifera) and tending spontaneous regeneration, are an example of a species-rich forest garden system in the eastern hemisphere of the Indonesian archipelago. In this paper we report the influence of site conditions, as slope inclination, soil type, and soil depth, on species composition, tree species diversity, and stand structure in old dusuns in four villages on Saparua, a small island south of Seram. In addition, we compared the floristic composition and stand structure of old dusuns with those of primary forest and advanced fallow vegetation (∼15-yr old). Soil conditions, particularly the depth of the mineral soil layer, and the relief have a major influence on species selection in dusuns. The most common species in old dusuns were absent both in primary forest and forest fallow. Floristic similarity between old dusuns and the unmanaged stands was low. However, in two study villages species richness of old dusuns was similar to that of the primary forest. Old dusuns showed an open structure with an average reduction of basal area by about half compared to the primary forest. Less than half of all individuals found above 10 cm DBH in old dusuns were planted. Only very few species were exclusively planted. Most species were both planted and emerged from spontaneous regeneration. The results indicate the important role of spontaneous regeneration in this forest garden system. This revised version was published online in June 2006 with corrections to the Cover Date.  相似文献   
7.
土地闲置原因分析及对策研究   总被引:1,自引:0,他引:1  
刘志华  孙鹏举 《山西农业科学》2011,39(4):345-347,369
2010年3月8日《国土资源部关于加强房地产用地供应和监管有关问题的通知》中提出,2010年3—7月在全国范围内开展房地产用地专项整治工作,8月19日国土资源部通报了房地产用地专项整治情况,整治过程中,发现违法用地面积近1.26万hm2,其中,闲置土地占90%以上,已成当前最大问题。以城市建设用地为主要研究对象,分析了当前房地产用地中土地闲置产生的原因及其所带来的危害,并对如何预防土地闲置、盘活已闲置土地进行了探讨。  相似文献   
8.
    
The paper provides an overview of the—evolving methodology for the land degradation neutrality (LDN) assessment based on studies at national, regional, and local levels in Russia. A review of more than one hundred publications in Russian language over the past 6–7 years allowed for analysis of the following areas: LDN terminology, LDN assessment at different levels, adapting a transition matrix; using the LDN concept for economic valuation of land, estimating LDN baseline, and using LDN as an integral indicator for sustainable land management. With the LDN concept, a global approach to monitoring land degradation has become applicable beyond the limited geographic scope of the drylands. The paper observes how the LDN concept has been broadened with the introduction of the LDN Index proposed to evaluate the rate of LDN achievement; a proposal on reconstructing transition matrices and adding land cover sub-categories; approach of integrating traditional national sectoral systems for assessing land quality with an LDN add-on. The broader relevance of the paper includes the justification that it provides for using the LDN concept by policy-makers at national and subnational levels, in particular in Russian-speaking countries. It includes the application of additional indicators to capture soil erosion, salinity, soil depletion, aridity, etc., and using different site-specific LDN baselines, not only those time-based but also factoring natural background trends like climate change, natural succession cycles linked with geological and geomorphological processes. Approaches for LDN-based economic valuation of lands and typology of sustainable land management practices and models were also fruitful.  相似文献   
9.
The Aeta Magbukún of Mariveles are one of the least known and researched Indigenous peoples remaining on the fringe of the bay, and within the remaining forests in Bataan province on Luzon Island in the Philippines. This work describes the unique cultural systems and language of the Aeta Magbukún tribe in Biaan, Mariveles, and both the traditional forest resource use and the evolving new subsistence practices developed to adapt to the encroachment of non-Indigenous peoples onto ancestral lands. The Aeta's forest resource use practices are discussed from a sustainable Indigenous development context within unique socioeconomic, cultural, and environmental circumstances in Bataan.  相似文献   
10.
Dry forests represent the most endangered ecosystem in tropical regions and continue to be one of the most sought after environments for human colonization, development and production. In spite of this, dry forests are one of the least well studied tropical habitats. This special issue is dedicated to reviewing much of the information that exists about tropical dry forest in the Americas. The introduction summarizes the articles presented herein, highlighting management strategies that are suggested throughout the special issue that may be useful for conserving tropical dry forest environments.  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