全文获取类型
收费全文 | 11358篇 |
免费 | 401篇 |
国内免费 | 423篇 |
专业分类
林业 | 4580篇 |
农学 | 312篇 |
基础科学 | 229篇 |
1212篇 | |
综合类 | 4335篇 |
农作物 | 219篇 |
水产渔业 | 304篇 |
畜牧兽医 | 435篇 |
园艺 | 295篇 |
植物保护 | 261篇 |
出版年
2024年 | 32篇 |
2023年 | 121篇 |
2022年 | 185篇 |
2021年 | 191篇 |
2020年 | 201篇 |
2019年 | 288篇 |
2018年 | 95篇 |
2017年 | 252篇 |
2016年 | 320篇 |
2015年 | 311篇 |
2014年 | 522篇 |
2013年 | 465篇 |
2012年 | 816篇 |
2011年 | 933篇 |
2010年 | 789篇 |
2009年 | 874篇 |
2008年 | 852篇 |
2007年 | 923篇 |
2006年 | 692篇 |
2005年 | 605篇 |
2004年 | 458篇 |
2003年 | 389篇 |
2002年 | 319篇 |
2001年 | 252篇 |
2000年 | 202篇 |
1999年 | 200篇 |
1998年 | 115篇 |
1997年 | 118篇 |
1996年 | 101篇 |
1995年 | 101篇 |
1994年 | 94篇 |
1993年 | 82篇 |
1992年 | 79篇 |
1991年 | 55篇 |
1990年 | 55篇 |
1989年 | 38篇 |
1988年 | 33篇 |
1987年 | 18篇 |
1986年 | 3篇 |
1985年 | 1篇 |
1981年 | 1篇 |
1962年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
41.
Maurice Keith R. Welch Joan M. Brown Christopher P. Latham Roger Earl 《Landscape Ecology》2004,19(6):603-620
The Pocono mesic till barrens (PMTB) are a unique assemblage of fire-maintained shrub communities that support numerous rare species. Historically these barrens covered a large area in the vicinity of Long Pond, Pennsylvania, USA. However, due largely to regional fire suppression instituted in the early 1960s, over 70% of the area covered by barrens succeeded to fire-intolerant forest that does not support the rare species. We investigated the influence of forest proximity on barrens succession across three geomorphic types during periods of high fire frequency and fire suppression, testing the hypothesis that forest processes such as seed rain, shading, and detrital enrichment of soils enhances barrens succession through a contagion effect. Evidence of a forest contagion effect should be shown by increased rates of barrens succession with increasing proximity to the nearest forest edge. In order to detect a forest contagion effect, barrens persistence and barrens succession were modeled in proximity zones of 0-50 m, 50-100 m, 100-200 m, and greater than 200 m from the nearest forest edge. We used existing GIS data layers for fire, geomorphology, and vegetation distribution in 1938, 1963, and 1992. The layers were modified and overlain using ArcView software to determine persistence and succession rates for each unique combination of layers in each proximity zone from 1938 to 1963 (pre-fire suppression) and 1963 to 1992 (post-fire suppression). ANCOVA results indicate that proximity to the nearest forest edge significantly affected barrens persistence rates in both time periods, but succession rates were significantly affected in 1938 to 1963 only. Twenty-eight percent of the 1938 barrens succeeded to forest by 1963; 56% of the 1963 barrens became forest by 1992. Results support previous findings that barrens persistence is enhanced by increased fire frequency, and that barrens persist longer where they overlie flat glacial till than on other geomorphology types. 相似文献
42.
43.
44.
通过野外调查和查阅文献资料,得出石门国家森林公园共有野生观赏植物122科267属377种,其中蕨类植物19科23属28种,裸子植物4科5属7种,被子植物99科239属342种。分析了石门国家森林公园野生观赏植物资源的基本特征和用途,并探讨其保护和开发利用。 相似文献
45.
首先,提出了大连国家生态工业示范园净水厂建设的必要性;然后,分析项目使用林地对当地森林资源、生物多样性、生态效能、景观风貌、环境质量及林业发展的影响;最后,结合园区供水安全现状、相关法规及其自然地理情况,综合分析该项目使用林地的可行性,为林业管理者科学管理提供参考依据。分析表明,为了保障大连国家生态工业示范园区供水安全,建设大连国家生态工业示范园净水厂使用林地是可行的。 相似文献
46.
47.
Han Shu-Xian 《山东饲料》2015,(6)
The lead time compression is the core of supply chain management with time competition and the powerful source of competitive advantage of supply chain.As an emerging technology,The Internet of things ... 相似文献
48.
通过分析森林生物多样性与森林病虫害的现状及其对森林可持续发展的意义,提出了提高意识、加强管理,依靠科技手段保护生物多样性和防治病虫害等措施建议,以期可持续发展森林保护与利用。 相似文献
49.
Ammonia oxidation, the first step of nitrification, is mediated by both ammonia-oxidizing archaea (AOA) and bacteria (AOB); however, the relative contributions of AOA and AOB to soil nitrification are not well understood. In this study we used 1-octyne to discriminate between AOA- and AOB-supported nitrification determined both in soil-water slurries and in unsaturated whole soil at field moisture. Soils were collected from stands of red alder (Alnus rubra Bong.) and Douglas-fir (Pseudotsuga menziesii Mirb. Franco) at three sites (Cascade Head, the H.J. Andrews, and McDonald Forest) on acidic soils (pH 3.9–5.7) in Oregon, USA. The abundances of AOA and AOB were measured using quantitative PCR by targeting the amoA gene, which encodes subunit A of ammonia monooxygenase. Total and AOA-specific (octyne-resistant) nitrification activities in soil slurries were significantly higher at Cascade Head (the most acidic soils, pH < 5) than at either the H.J. Andrews or McDonald Forest, and greater in red alder compared with Douglas-fir soils. The fraction of octyne-resistant nitrification varied among sites (21–74%) and was highest at Cascade Head than at the other two locations. Net nitrification rates of whole soil without NH4+ amendment ranged from 0.4 to 3.3 mg N kg−1 soil d−1. Overall, net nitrification rates of whole soil were stimulated 2- to 8-fold by addition of 140 mg NH4+-N kg−1 soil; this was significant for red alder at Cascade Head and the H.J. Andrews. Red alder at Cascade Head was unique in that the majority of NH4+-stimulated nitrifying activity was octyne-resistant (73%). At all other sites, NH4+-stimulated nitrification was octyne-sensitive (68–90%). The octyne-sensitive activity—presumably AOB—was affected more by soil pH whereas the octyne-resistant (AOA) activity was more strongly related to N availability. 相似文献
50.
结合生态农村园发展的要求及对同安造水村的综合考察,从多角度来分析市场形势和造水村的人文、地理等各条件的综合状况,为造水村观光农业旅游资源的合理开发提供较为科学的理论依据,同时,为同安区汀溪镇发展成为厦门市的后花园提供借鉴。 相似文献