全文获取类型
收费全文 | 6197篇 |
免费 | 414篇 |
国内免费 | 451篇 |
专业分类
林业 | 924篇 |
农学 | 484篇 |
基础科学 | 421篇 |
864篇 | |
综合类 | 2770篇 |
农作物 | 542篇 |
水产渔业 | 87篇 |
畜牧兽医 | 405篇 |
园艺 | 245篇 |
植物保护 | 320篇 |
出版年
2024年 | 25篇 |
2023年 | 148篇 |
2022年 | 256篇 |
2021年 | 236篇 |
2020年 | 244篇 |
2019年 | 287篇 |
2018年 | 139篇 |
2017年 | 292篇 |
2016年 | 360篇 |
2015年 | 283篇 |
2014年 | 457篇 |
2013年 | 369篇 |
2012年 | 542篇 |
2011年 | 501篇 |
2010年 | 350篇 |
2009年 | 350篇 |
2008年 | 267篇 |
2007年 | 293篇 |
2006年 | 291篇 |
2005年 | 215篇 |
2004年 | 178篇 |
2003年 | 141篇 |
2002年 | 109篇 |
2001年 | 132篇 |
2000年 | 99篇 |
1999年 | 88篇 |
1998年 | 75篇 |
1997年 | 55篇 |
1996年 | 47篇 |
1995年 | 46篇 |
1994年 | 31篇 |
1993年 | 32篇 |
1992年 | 28篇 |
1991年 | 25篇 |
1990年 | 20篇 |
1989年 | 14篇 |
1988年 | 13篇 |
1987年 | 7篇 |
1986年 | 6篇 |
1985年 | 2篇 |
1984年 | 1篇 |
1982年 | 2篇 |
1981年 | 2篇 |
1980年 | 1篇 |
1977年 | 1篇 |
1962年 | 1篇 |
1956年 | 1篇 |
排序方式: 共有7062条查询结果,搜索用时 180 毫秒
81.
82.
83.
Agroforestry research and development in southern Africa during the 1990s: Review and challenges ahead 总被引:4,自引:7,他引:4
F. Kwesiga F.K. Akinnifesi P.L. Mafongoya M.H. McDermott A. Agumya 《Agroforestry Systems》2003,59(3):173-186
In light of the large number of empirical studies of agroforestry adoption published during the last decade, we believe it is time to take stock and identify general determinants of agroforestry adoption. In reviewing 120 articles on adoption of agricultural and forestry technology by small holders, we find five categories of factors that explain technology adoption within an economic framework: preferences, resource endowments, market incentives, biophysical factors, and risk and uncertainty. By selecting only empirical analyses that focus on agroforestry and related investments, we narrow our list down to 32 studies primarily from tropical areas. We apply vote-counting based meta-analysis to these studies and evaluate the inclusion and significance of the five adoption factors. Our analysis shows that preferences and resource endowments are the factors most often included in studies. However, adoption behavior is most likely to be significantly influenced by risk, biophysical, and resource factors. In our conclusion, we discuss specific recommendations for the next generation of adoption studies and meta-analyses that include considering a fuller menu of variables, reporting key statistics and marginal probabilities, and conducting weighted meta-regressions.This revised version was published online in November 2005 with corrections to the Cover Date. 相似文献
84.
Stable isotopes of nitrogen are potentially a valuable tool for regional assessments of nitrogen saturation because they provide an integrated measure of the past nitrogen cycling history of a site. We measured δ15N of soil and litter, as well as net nitrification potential, at three sites across a nitrate-loss gradient in the White Mountains, New Hampshire to test the hypotheses: (1) that δ15N in soil and litter increase across a spatial gradient of nitrate loss; and (2) that δ15N in soil and litter is elevated when nitrification is elevated. δ15N was found not to vary significantly among the three sites. Patterns of leaf litter and forest floor δ15N, however, were strongly influenced by species composition in individual plots. Beech litter had significantly higher δ15N than yellow birch, sugar maple, and red maple. The conifer-dominated plots had significantly lower δ15N in both the organic soil horizons and in litter than did the hardwood-dominated plots. When we adjusted for spatial heterogeneity in mineral soil δ15N values by using an enrichment factor, δ15Nfoliar − δ15NBs, in place of absolute soil δ15N values, a positive relationship was found with net nitrification for hardwoods. δ15N may also be a useful tool for evaluating species differences in nitrogen cycling and nitrogen uptake. The distinct pattern we observed of decreasing δ15N across the continuum from hardwood-dominated to conifer-dominated sites may suggest that local drivers (for example, nitrification rate) regulate the absolute value of foliar δ15N, while species-driven factors (e.g., timing and type of uptake) control the foliar δ15N value of one species relative to another in the same plot. 相似文献
85.
江门市东湖公园北园区绿化景观设计 总被引:1,自引:0,他引:1
江门市东湖公园北园区是东湖公园的新扩区,在绿化景观设计上,突出自身特点,营造自然、鲜明的绿化景观,并与其它园区相协调和互相衔接,构建了优美的城市景观。 相似文献
86.
湖北乔灌复层防火林带模式研究 总被引:2,自引:0,他引:2
通过对湖北乔灌复层防火林带的研究表明:1、乔灌复层防火林带中乔、灌不同树种对土壤、气候等环境条件的要求相同或近似,灌木树种要有一定的耐阴能力,防火树种以乡土树种为主。2、防火林带内树种的基本配置方式有凸形配置和凹形配置2种,二者各有优势,在实际应用中,应根据具体情况综合运用,以提高林带防火能力。3、林带走向应与防火季节主风向垂直,在湖北绝大多数林带的走向以东西向为宜,林中应设计与坡向垂直的林带,林带造林密度大于常规造林密度。 相似文献
87.
为逐步改善省会的空气质量,研究了火炬树在石家庄市沙地立地条件下的生长适应性,调查了在沙地条件下耐寒、耐旱、耐热、耐阴、抗病虫及极限条件下的生长情况。提出了火炬树与其它防风固沙植物的配置形式,从而形成乔灌草多层次组合的防风固沙生态体系。 相似文献
88.
89.