全文获取类型
收费全文 | 1256篇 |
免费 | 85篇 |
国内免费 | 156篇 |
专业分类
林业 | 126篇 |
农学 | 72篇 |
基础科学 | 43篇 |
371篇 | |
综合类 | 590篇 |
农作物 | 22篇 |
水产渔业 | 15篇 |
畜牧兽医 | 106篇 |
园艺 | 13篇 |
植物保护 | 139篇 |
出版年
2024年 | 8篇 |
2023年 | 34篇 |
2022年 | 53篇 |
2021年 | 44篇 |
2020年 | 28篇 |
2019年 | 40篇 |
2018年 | 39篇 |
2017年 | 49篇 |
2016年 | 69篇 |
2015年 | 58篇 |
2014年 | 99篇 |
2013年 | 99篇 |
2012年 | 100篇 |
2011年 | 99篇 |
2010年 | 116篇 |
2009年 | 88篇 |
2008年 | 91篇 |
2007年 | 77篇 |
2006年 | 68篇 |
2005年 | 50篇 |
2004年 | 38篇 |
2003年 | 33篇 |
2002年 | 16篇 |
2001年 | 29篇 |
2000年 | 9篇 |
1999年 | 8篇 |
1998年 | 4篇 |
1997年 | 8篇 |
1996年 | 3篇 |
1995年 | 4篇 |
1994年 | 4篇 |
1993年 | 6篇 |
1992年 | 10篇 |
1991年 | 6篇 |
1990年 | 3篇 |
1988年 | 3篇 |
1987年 | 2篇 |
1986年 | 1篇 |
1985年 | 1篇 |
排序方式: 共有1497条查询结果,搜索用时 31 毫秒
141.
142.
143.
新疆南疆属于极端干旱生态脆弱带,土壤风蚀已严重制约着南疆绿洲农业的可持续发展。在分析了造成南疆绿洲区土壤风蚀主要因素的基础上,探讨了风蚀造成的危害,提出了保护、恢复与重建绿洲外缘的自然植被、加强绿洲内部防护林体系的建设、调整种植结构和进行农作物留茬覆盖等防治土壤风蚀的综合治理措施。 相似文献
144.
145.
王胜利 《干旱区资源与环境》2006,20(6):13-16
文章从现代化视角出发,从经济结构和社会结构两个领域,对1949年—2002年关中地区现代化构成要素的七项指标,进行数量统计和分析。经济结构中的人均国民生产总值、农业总产值占国民总产值比例、服务业产值占国民生产总值比例、农业劳动力比重等四项现代化要素增长。社会结构中的服务业总产值占国民生产总值比例、每千人口中的医生人数、每百万人口中的学生人数、城市化率四项现代化指标提高。这一研究对“一线两带”建设具有极强的现实意义。 相似文献
146.
利用陕南白山羊与关中奶山羊、布尔山羊三品种杂交试验,分别测定初生重、3月龄、6月龄、12月龄四个阶段的胸围、体长、体高和体重,将测定的各项指标分别与布陕杂一代、关陕杂一代和陕南白山羊的同阶段指标进行对比分析,结果表明,布关陕杂种羊在不同生长阶段的体重优于布陕杂一代、关陕杂一代和陕南白山羊。布关陕杂种羊的初生重比布陕杂一代、关陕杂一代、陕南白山羊分别提高18.8%、48.2%、66.2%;3月龄体重分别提高17.2%、28.1%、35.1%;6月龄体重分别提高17.7%、51.3%、65.6%;12月龄体重分别提高15.2%、52.3%、58.0%。 相似文献
147.
Goal, Scope and Background
In the nineteen nineties most European countries issued legislation on soil protection, including soil contamination. In the case of a presumed contamination, soil investigation mostly follows a stepwise approach starting with a preliminary investigation, then an in-depth investigation and, finally, remediation. Soil clean-up standards are often foreseen as trigger criteria to determine the need for an in-depth soil investigation or for remediation. There are however large differences in soil clean-up standards. This is partly because of the different roles of soil clean-up standards in each legislative framework and partly because of differences in the soil clean-up standard (SCS) derivation procedures. Despites these differences there are large similarities in the concepts of the derivation procedures for clean-up standards. A better understanding of the differences in clean-up standards is needed. In order to clarify the variation, the background of the clean-up standards for a selected number of countries was investigated. The objective of this paper is to investigate the underlying reason why country-wide generic soil and groundwater clean-up standards of eight trace elements (cadmium, chromium (III), copper, mercury, lead, nickel, zinc and arsenic) differ between the selected countries.Main Features
To avoid misinterpretation of the differences in application of the clean-up standards, a short overview of the legislative role of clean-up standards is given first. Differences in model concepts and parameter values are discussed, followed by a comparison of the generic soil and groundwater clean-up standards for trace elements and a discussion on the sources of variation. The influence of the use of ecotoxicological criteria and data for the derivation of soil clean-up standards will be discussed in more detail. Selected countries were Canada, the Flemish Region (Belgium), France, Germany, Great-Britain, the Netherlands, Norway, Sweden, Switzerland and United States of America (USA).Results and Discussion
When soil clean-up standards for eight trace elements (cadmium, chromium (III), copper, mercury, lead, nickel, zinc and arsenic) were compared between the selected countries differences of more than a factor 1000 arose. Notwithstanding the use of similar derivation procedures, differences were caused by the use of different software models with their specific input data, boundary conditions and applied protection criteria for humans and the ecology. Ecotoxicological criteria tend to lower the soil clean-up standard for the selected trace elements.Conclusions
In the countries that are included in this study, clean-up standards are used is different ways, this is for the determination of the necessity for remediation or for the need for further soil investigation. This paper shows a wide variation in the clean-up standards, which has further implication on the decision for remediation or further investigation and, hence, the financial costs of soil management. All the clean-up standards have as primary goal the protection of human health. A number of countries also include the protection of the ecological function of the soil. Differences in selected software model, (standard) parameters values, selected human toxicological and ecotoxicological criteria, are reason for a substantial variation in the clean-up standards for trace elements.Recommendations and Perspective
. Is this variation justified? The derivation of soil clean-up standards involves - besides scientific elements - political elements, like differentiation in landuse types (agricultural, residential, recreational, industrial), receptor at risk or protection level. It is obvious that harmonization of these elements will be complicated. However, a European action programme, like the thematic strategy for soil protection, could initiate this process of harmonization. Nevertheless, soil-clean-up standards could never be uniform over the whole of Europe because they include country specific elements (geographical, ethnological) and political decisions. 相似文献148.
利用宁夏自治区第三次至第七次森林资源普查数据及林业生态建设工程数据,采用材积源生物量法分析研究了宁夏森林及林业生态建设工程的固碳能力和潜力。结果表明:"十五"和"十一五"林业生态建设与保护工程实施期间,宁夏自治区森林主要面积、蓄积量、森林总生物量和碳贮量等均呈增加趋势。林业生态建设与保护工程增加森林面积、天然林林分面积和人工林林分面积分别为18.30万hm2、0.88万hm2、5.76万hm2;增加活立木蓄积量、林分蓄积量分别为300.29万m3、267.48万m3,其中人工林林分蓄积量增加为207.24万m3。"十五"和"十一五"期间,宁夏林业生态建设与保护工程共增加森林碳贮量124.99万t,其中三北防护林工程、天然林保护工程和经济林工程合计增加森林碳贮量91.45万t。预计到2020年,宁夏森林总碳贮量将达到430.52万t。 相似文献
149.
基于对内蒙古锡林郭勒盟镶黄旗牧民家庭的入户调查数据,分析了2008、2009年西部地区牧民家庭的收入与消费结构。结果表明:牧民收入来源单一,以家畜出售为主,抗风险能力差;消费支出中饲草料购置比重最大,且增长迅速,其次是食品、教育、医疗等支出,住房方面政策补贴较多。牧民的收入与消费特点是由自然因素、市场因素、政策因素和自身因素共同决定的,基于此,提出了牧民提高收入、改善消费结构的对策建议:牧民要提高养殖技术,科学养殖,增加收入,且要转变消费观念,适度储蓄,增强抗风险能力;政府要加大对牧区的财政补贴力度,加强对牧民的教育、培训和医疗保健投入,提高牧民素质。 相似文献
150.
通过对关中地区经济发展与渭河流域(关中段)水环境质量状况的分析,模拟了关中经济发展和渭河水环境质量演变关系,研究了演变关系的趋势,并提出相应的建议,以期为两者之间协调发展提供科学决策的依据。结果表明,关中地区人均GDP与高锰酸钾指数曲线近似"倒U型"曲线的右半段,走势良好。而人均GDP与氨氮指标的曲线与传统的EKC曲线不相吻合,类似"倒U+U"形。说明关中地区在渭河的治理过程中投入较大,环保措施得当,但近两年在氨氮控制上还需加大力度。 相似文献