全文获取类型
收费全文 | 3735篇 |
免费 | 213篇 |
国内免费 | 119篇 |
专业分类
林业 | 364篇 |
农学 | 190篇 |
基础科学 | 258篇 |
413篇 | |
综合类 | 2348篇 |
农作物 | 83篇 |
水产渔业 | 81篇 |
畜牧兽医 | 172篇 |
园艺 | 104篇 |
植物保护 | 54篇 |
出版年
2024年 | 26篇 |
2023年 | 103篇 |
2022年 | 146篇 |
2021年 | 125篇 |
2020年 | 125篇 |
2019年 | 141篇 |
2018年 | 82篇 |
2017年 | 105篇 |
2016年 | 162篇 |
2015年 | 111篇 |
2014年 | 246篇 |
2013年 | 275篇 |
2012年 | 385篇 |
2011年 | 347篇 |
2010年 | 296篇 |
2009年 | 257篇 |
2008年 | 226篇 |
2007年 | 232篇 |
2006年 | 196篇 |
2005年 | 154篇 |
2004年 | 85篇 |
2003年 | 79篇 |
2002年 | 51篇 |
2001年 | 46篇 |
2000年 | 27篇 |
1999年 | 17篇 |
1998年 | 11篇 |
1997年 | 4篇 |
1996年 | 2篇 |
1994年 | 1篇 |
1993年 | 1篇 |
1992年 | 1篇 |
1991年 | 1篇 |
1987年 | 1篇 |
排序方式: 共有4067条查询结果,搜索用时 31 毫秒
91.
黄土高原地区的生态建设与政策创新 总被引:14,自引:6,他引:8
生态环境建设是西部大开发的根本切入点。黄土高原地区是我国生态环境最为严峻的地区 ,目前仍有 2 9× 10 4 km2 的土地有待治理 ,任务十分艰巨。本区生态环境建设的主要阻力是生态系统脆弱、经济系统贫困、科技文化水平落后、生态建设效益滞后、投资巨大等。提出的对策是稳定落实“退田还林”补贴政策 ,完善多元化的生态建设投资体系 ,建立公平合理的资源开发政策体系 ,加强生态环境建设的科技支撑作用 ,严格控制人口的快速增长等。强调排除阻力、促进生态环境建设的关键是调整思路 ,在各项政策和措施上都有所创新 ,有所突破 相似文献
92.
Human-made contour banks are a central component of theShikim water harvesting system in Israels Negev Desert.Efficient water capture depends on the presence of a stable microphytic crustwhich directs surplus surface runoff into the banks where it is stored. We usedsimulated rainfall to examine the impact of soil surface disturbance on runoffand sediment transport, and the effect of this on the efficiency of resourcecapture within the Shikim system. Two disturbance regimes:1) removal of the microphytic crust only, and 2) removal of the crust and shrubpatches by cultivation, were compared with an undisturbed control. In theundisturbed state, 32% of rainfall was redistributed as runoff. This runoffpenetrated approximately 27% deeper under the shrub patches compared with themicrophytic crust. When the microphytic crust was destroyed by simulatedtrampling, the runoff coefficient declined to 13%, and there was no significantdifference in water penetration between shrub and crust patches. Completedestruction of the shrub hummocks and crust by cultivation resulted in adeclinein the runoff coefficient to 6%. The result of sustained disturbance in thesepatchy Negev shrublands is a breakdown in spatial heterogeneity, a loss ofecosystem function, a reduction in ecosystem goods and services such as plantdiversity and production, and ultimately a reduction in pastoral productivity.These results reinforce the view that microphytic crusts are critical for theefficient operation of the Shikim water harvesting system.Given that practices such as cultivation and trampling which disturbmicrophyticcrusts result in enhanced infiltration, crusts should be left intact tomaximisethe water harvesting efficiency in these desert landscapes.This revised version was published online in May 2005 with corrections to the Cover Date. 相似文献
93.
首先搭建了一个基于循环经济的企业科技创新动力模式的分析框架,通过分析循环经济的利益相关主体之间的关系和利益形成过程,进一步深入分析在发展循环经济中企业科技创新乏力的原因并提出动力机制建立的思路和建议。 相似文献
94.
以提高自主创新能力为主题,分析了我国兽药研究开发现状,提出了今后我国兽药行业及生产企业进行新兽药研究开发的战略目标设想、内容和建议。 相似文献
95.
陕北黄土高原生态资产与区域生态安全程度研究 总被引:1,自引:1,他引:1
在GIS支持下,通过测定陕北黄土高原植被生产和生态调节的物质量及生态资产价值.结果表明:①通过计算,陕北黄土高原每年的生态资产总价值为7.00×1010元,其中释放氧气的价值最高,占总价值的29.51%;土壤保持价值最小,占总价值的1.5%.而就植被类型而言,果树的生态价值最高,占总价值的43.89%;最小是沼泽,占总价值的0.4%.②通过地理信息系统与生态学的结合,对陕北黄土高原的生态资产进行定量测量,并利用GIS软件的空间分析功能,绘制了陕北黄土高原生态资产价值空间分布图.③陕北黄土高原生态安全程度为0.45,处于生态不安全状态,负荷超载,要狠抓生态建设.④针对陕北黄土高原环境现状,提出相应的治理对策. 相似文献
96.
Quantifying the nitrogen retention capacity of natural wetlands in the large-scale drainage basin of the Baltic Sea 总被引:3,自引:0,他引:3
We estimate the nitrogen retention capacity of natural wetlands in the 1.7 million km2 Baltic Sea drainage basin, using a wetland GIS data base. There are approximately 138,000 km2 of wetlands (bogs and fens) in the Baltic Sea drainage basin, corresponding to 8% of the area. The input of nitrogen to natural wetlands from atmospheric deposition was estimated to 55,000–161,000 ton y1. A map of the deposition of both wet and dry nitrogen is presented. The input from the human population was estimated to 255,000 ton y1 in terms of excretory release in processed sewage water. There may also be leakage from forests and agricultural land into the wetlands. Due to lack of data on hydrology and topography, such potential nitrogen sources are not accounted for here. The capacity of the wetlands to retain the atmospheric deposition of nitrogen was estimated to 34,000–99,000 ton y1. The potential retention by wetlands was estimated to 57,000–145,000 ton y1 when the nitrogen input from the human population was added. If drained wetlands were to be restored and their area added to the present wetland area, the nitrogen retention capacity was estimated to increase to 196,000–261,000 ton y1. Our results indicate that existing natural wetlands in the Baltic Sea drainage basin annually can retain an amount of nitrogen which corresponds to about 5–13% of annual total (natural and anthropogenic) nitrogen emissions entering the Baltic Sea. The ecosystem retention service performed by wetlands accounts for a substantial nitrogen removal, thereby reducing the eutrophication of the Baltic Sea. 相似文献
97.
Urban forests (UF) provide a range of important ecosystem services (ES) for human well-being. Relevant ES delivered by UF include urban temperature regulation, runoff mitigation, noise reduction, recreation, and air purification. In this study the potential of air pollution removal by UF in the city of Florence (Italy) was investigated. Two main air pollutants were considered – particulate matter (PM10) and tropospheric ozone (O3) – with the aim of providing a methodological framework for mapping air pollutant removal by UF and assessing the percent removal of air pollutant.The distribution of UF was mapped by high spatial resolution remote sensing data and classified into seven forest categories. The Leaf Area Index (LAI) was estimated spatially using a regression model between in-field LAI survey and Airborne Laser Scanning data and it was found to be in good linear agreement with estimates from ground-based measurements (R2 = 0.88 and RMSE% = 11%). We applied pollution deposition equations by using pollution concentrations measured at urban monitoring stations and then estimated the pollutant removal potential of the UF: annual O3 and PM10 removal accounted for 77.9 t and 171.3 t, respectively. O3 and PM10 removal rates by evergreen broadleaves (16.1 and 27.3 g/m2), conifers (10.9 and 28.5 g/m2), and mixed evergreen species (15.8 and 31.7 g/m2) were higher than by deciduous broadleaf stands (4.1 and 10 g/m2). However, deciduous forests exhibited the largest total removal due to the high percentage of tree cover within the city. The present study confirms that UF play an important role in air purification in Mediterranean cities as they can remove monthly up to 5% of O3 and 13% of PM10. 相似文献
98.
方嘉珍 《农业图书情报学刊》2010,22(10):344-346
在分析弱势群体构成和文化需求的基础上,以徐州地区为例论述了基层图书馆为农村弱势群体服务的若干举措和实践活动,探讨为农村弱势群体服务的方式方法,并对今后工作提出具体建议。 相似文献
99.
在分析我国农机制造业发展现状和存在的问题的基础上,指出我国农机制造业的可持续发展最终要靠技术创新来实现,并提出了促进我国农机制造业技术创新的对策. 相似文献
100.
从构建和谐社会探索县级图书馆服务创新 总被引:2,自引:0,他引:2
刘学香 《农业图书情报学刊》2008,20(7)
县级图书馆发挥自身优势,在构建和谐社会中,大胆创新,拓宽服务领域,提高服务质量,是提升县级图书馆事业的需要,也是建设中国特色社会主义社会的需要.新时期县级图书馆服务创新,是发展图书馆事业服务于和谐社会的必由之路. 相似文献