全文获取类型
收费全文 | 12392篇 |
免费 | 849篇 |
国内免费 | 1502篇 |
专业分类
林业 | 1278篇 |
农学 | 647篇 |
基础科学 | 542篇 |
5104篇 | |
综合类 | 5641篇 |
农作物 | 133篇 |
水产渔业 | 88篇 |
畜牧兽医 | 341篇 |
园艺 | 135篇 |
植物保护 | 834篇 |
出版年
2024年 | 60篇 |
2023年 | 311篇 |
2022年 | 458篇 |
2021年 | 387篇 |
2020年 | 408篇 |
2019年 | 470篇 |
2018年 | 365篇 |
2017年 | 688篇 |
2016年 | 780篇 |
2015年 | 620篇 |
2014年 | 805篇 |
2013年 | 875篇 |
2012年 | 1131篇 |
2011年 | 1037篇 |
2010年 | 821篇 |
2009年 | 805篇 |
2008年 | 696篇 |
2007年 | 782篇 |
2006年 | 659篇 |
2005年 | 515篇 |
2004年 | 398篇 |
2003年 | 313篇 |
2002年 | 211篇 |
2001年 | 219篇 |
2000年 | 144篇 |
1999年 | 129篇 |
1998年 | 121篇 |
1997年 | 80篇 |
1996年 | 66篇 |
1995年 | 68篇 |
1994年 | 81篇 |
1993年 | 60篇 |
1992年 | 45篇 |
1991年 | 49篇 |
1990年 | 40篇 |
1989年 | 17篇 |
1988年 | 17篇 |
1987年 | 6篇 |
1986年 | 1篇 |
1985年 | 3篇 |
1984年 | 1篇 |
1962年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 109 毫秒
51.
我国特大城市建成区土地集约利用研究 总被引:2,自引:0,他引:2
运用因子分析法对我国16个特大城市建成区的土地集约利用水平进行综合评价和排序.结果表明:我国特大城市建成区土地集约利用效率呈现总体水平偏低、地区差异明显及各大城市影响因素侧重点不同等3大特征.因此,需合理调整各大城市产业结构,以促进城市经济可持续发展;积极开展城市土地整理,以提高土地利用效率;加大城市生态环境保护力度,以优化土地利用生态效益. 相似文献
52.
贫困地区基于农民专业合作社的土地流转模式及选择——以贵州、四川、甘肃三省为例 总被引:1,自引:1,他引:0
基于贵州、四川、甘肃三省贫困连片区的调研,采用问卷调查、关键人访谈和统计分析相结合的方法,总结出农户与农民专业合作社间土地流转模式主要有转包、转让、出租、入股、出租-入股5种,同时分析了每种模式的运作机制和利弊,并在此基础上为不同类型农户的土地流转模式选择提出建议。 相似文献
53.
54.
55.
公园绿地作为"唯一有生命力"的城市基础设施,一直以来其建设和投资标准不像道路、管线等基础睡设施那般明确,而随着近年来城市发展对生态环境的日益重视,各类公园绿地项目逐渐增多。为了加强公园绿地项目在前期设计和审批阶段的科学性、合理性,文章结合北京市当前园林绿化的实际情况,按照"一亩地公园"模型的思路,根据不同的投资工程,建立了投资标准测算模型,测算了投资指标,尝试为公园绿地的投资标准确定提供一种新的思路。 相似文献
56.
57.
Land surface temperature retrieval for arid regions based on Landsat-8 TIRS data:a case study in Shihezi,Northwest China 总被引:1,自引:0,他引:1
Lei YANG ;YunGang CAO ;XiaoHua ZHU ;ShengHe ZENG ;GuoJiang YANG ;JiangYong HE ;XiuChun YANG 《干旱区科学》2014,(6):704-716
Scientific interest in geophysical information about land surface temperature (LST) is ever increasing, as such information provides a base for a large number of applications, including environmental and agricultural monitoring. Therefore, the research of LST retrieval has become a hot topic. Recent availability of Landsat-8 satel- lite imagery provides a new data source for LST retrieval. Hence, exploring an adaptive method with reliable ac- curacy seems to be essential. In this study, basing on features of Landsat-8 TIRS thermal infrared channels, we re-calculated parameters in the atmospheric transmittance empirical models of the existing split-window algorithm, and estimated the ground emissivity with the help of the land cover classification map of the study area. Further- more, a split-window algorithm was rebuilt by virtual of the estimation model of the updated atmospheric transmit- tance and the ground emissivity, and then a remote sensing retrieval for the LST of Shihezi city in Xinjiang Uygur autonomous region of Northwest China was conducted on the basis of this modified algorithm. Finally, precision validation of the new model was implemented by using the MODIS LST products. The results showed that the LST retrieval from Landsat-8 TIRS data based on our algorithm has a higher credibility, and the retrieved LST is more consistent with the MODIS LST products. This indicated that the modified algorithm is suitable for retrieving LST with competitive accuracy. With higher resolutions, Landsat-8 TIRS data may provide more accurate observation for LST retrieval. 相似文献
58.
59.
A. S. Gregory G. J. D. Kirk C. A. Keay B. G. Rawlins P. Wallace A. P. Whitmore 《Soil Use and Management》2014,30(1):10-22
It is estimated that half the soil carbon globally is in the subsoil, but data are scarce. We updated estimates of subsoil organic carbon (OC) in England and Wales made by Bradley et al. (2005) using soil and land‐use databases and compared the results with other published data. We estimated that the soils of England and Wales contained 1633, 1143 and 506 Tg of OC at 0–30, 30–100 and 100–150 cm depths, respectively. Thus, half of the soil OC was found below 30 cm depth. Peat soils accounted for the largest proportion, containing 44% of all the OC below 30 cm despite their small areal extent, followed by brown soils, surface‐water gley soils, ground‐water gley soils and podzolic soils. Peat soils had more than 25% of their profile OC per unit area in the 100–150 cm depth, whereas most other soils had <8% at this depth. The differences between soil types were consistent with differences in soil formation processes. Differences in depth distributions between land uses were small, but subsoil OC stocks in cultivated soils were generally smaller than in soils under grassland or other land uses. Data on subsoil OC stocks in the literature were scarce, but what there was broadly agreed with the findings of the above database exercise. There was little evidence by which to assess how subsoil OC stocks were changing over time. 相似文献
60.
Soil protection for a sustainable future: options for a soil monitoring network for Ireland 下载免费PDF全文
The increased recognition of the importance of soil is reflected in the UN Post‐2015 Development Agenda with sustainable development goals that directly and indirectly relate to soil quality and protection. Despite a lack of legally binding legislation for soil protection, the European Commission remains committed to the objective of soil protection. However, the achievement of a legally binding framework for soil protection relies on the implementation of a soil monitoring network (SMN) that can detect changes to soil quality over time. As beneficiaries do not pay for the provision of soil information, the options for soil monitoring are limited. The use of existing data sets should be considered first. Using Ireland as an example, this research explored the opportunities for a SMN for Ireland considering three existing national data sets. The options for a SMN are considered in terms of their spatial and stratified distribution, the parameters to be measured and an economic analysis of the options proposed. This research finds that for Ireland, either a 10 or a 16 km2 grid interval stratified by land use and drainage class offers the best potential in relation to the spatial distribution of existing data sets to reflect local data at a national level. With existing data, the stratified SIS data using the 16 km2 grid offers the best value for money, with baseline costs for analysis, excluding field costs, of between €706 481 and €2.8 million. Acknowledging the impossibility of measuring all parameters with ideal frequency, this study proposes a two‐tier system for optimized monitoring frequency. Parameters must anticipate future policy requirements. Finally, the implementation of a SMN must be accompanied by standardized methods, defined thresholds and action mandates to maintain soil quality within allowable limits. 相似文献