全文获取类型
收费全文 | 1905篇 |
免费 | 118篇 |
国内免费 | 70篇 |
专业分类
林业 | 490篇 |
农学 | 72篇 |
基础科学 | 81篇 |
393篇 | |
综合类 | 781篇 |
农作物 | 36篇 |
水产渔业 | 14篇 |
畜牧兽医 | 114篇 |
园艺 | 65篇 |
植物保护 | 47篇 |
出版年
2023年 | 29篇 |
2022年 | 31篇 |
2021年 | 55篇 |
2020年 | 58篇 |
2019年 | 74篇 |
2018年 | 29篇 |
2017年 | 61篇 |
2016年 | 71篇 |
2015年 | 72篇 |
2014年 | 102篇 |
2013年 | 110篇 |
2012年 | 129篇 |
2011年 | 125篇 |
2010年 | 112篇 |
2009年 | 146篇 |
2008年 | 129篇 |
2007年 | 138篇 |
2006年 | 129篇 |
2005年 | 75篇 |
2004年 | 65篇 |
2003年 | 35篇 |
2002年 | 40篇 |
2001年 | 40篇 |
2000年 | 29篇 |
1999年 | 38篇 |
1998年 | 24篇 |
1997年 | 26篇 |
1996年 | 26篇 |
1995年 | 11篇 |
1994年 | 13篇 |
1993年 | 12篇 |
1992年 | 14篇 |
1991年 | 15篇 |
1990年 | 12篇 |
1989年 | 9篇 |
1988年 | 6篇 |
1987年 | 3篇 |
排序方式: 共有2093条查询结果,搜索用时 21 毫秒
1.
Spatiotemporal patterns of mountain whitefish (Prosopium williamsoni) in response to a restoration of longitudinal connectivity 下载免费PDF全文
Peter M. Kiffney Ben Cram Paul L. Faulds Karl Burton Michele Koehler Thomas P. Quinn 《Ecology of Freshwater Fish》2018,27(4):1037-1053
To examine the role of longitudinal connectivity on the spatial and temporal dynamics of mountain whitefish (Prosopium williamsoni), we quantified movement and population dynamics following installation of the Landsburg Dam fishway, Cedar River, WA, USA. Mountain whitefish is widely distributed, poorly studied and not the focus of restoration. Before the fishway, mountain whitefish were not observed above the dam. Here, we focus on snorkel counts collected at reach and mesohabitat (e.g. pools) scales over 11 summers on the 20‐km above‐dam segment following restoration. A camera within the ladder provided number, size and movement timing, thereby informing on behaviour and recolonisation. Segment‐scale abundance increased following fish passage reaching an asymptote in 7 years, and mountain whitefish were detected throughout the main stem in 10 years. Annual movement through the ladder increased over time and was positively correlated with instream abundance and discharge, but negatively correlated with water temperature. About 60% of fish movements occurred in spring and early summer, potentially for foraging opportunities. Reach‐scale abundance peaked between 7 and 10 km from the dam; deep, cool (~10.6 to 11.6°C) conditions characterised these reaches. At the mesohabitat scale, mountain whitefish detection increased with depth and velocity after accounting for distance from the dam. Our results show how restoring longitudinal connectivity allowed this nontarget species to colonise newly available habitat. Their response supports the critical roles of longitudinal connectivity and environmental conditions, that manifest at different spatial scales, in dictating how freshwater fish respond to habitat disturbance. 相似文献
2.
3.
为解决祁连山区高寒草地牧草返青期季节性超载过牧问题,自2015年5月至7月起连续两年在青海省祁连县默勒镇瓦日尕村的冬春放牧地上进行了为期60d的返青期休牧试验,休牧期间放牧家畜全部采用舍饲圈养,并于2016年8月中旬对休牧草地进行植被调查。结果表明,返青期休牧可使草地群落的总盖度、总植物量显著增加(P0.05),使禾本科牧草株高、分盖度和植物量显著增加(P0.05),对莎草科植物影响不显著(P0.05);阔叶型毒杂草的株高、盖度、生物量均有下降的趋势。休牧两年与休牧一年相比,群落中禾本科植物的株高、盖度、植物量均在继续增加,阔叶型毒杂草的各项指标继续下降。结果证明,返青期休牧可有效促进祁连山区草原化草甸退化草地的植被恢复,并大幅度提高草地生产力。 相似文献
4.
5.
经过多年多点示范,总结出高山秋种马铃薯高产栽培技术。即秋种马铃薯以选择排灌水方便的沙壤土或黄壤土为宜,忌与茄科作物连作;8月上中旬,采用单垄双行双三角形播种,密度5.25万~6.00万株/hm2;合理施肥,注意防治病虫害。 相似文献
6.
华北刺槐林与自然恢复植被土壤微生物量碳、氮含量四季动态 总被引:8,自引:0,他引:8
【目的】土壤微生物量碳、氮是植被所需碳、氮的重要“源”或“库”,是公认的综合评价土壤质量或肥力的重要指标,也是土壤生态系统变化的预警及敏感指标,研究其动态变化,可为退耕还林及后期管理决策提供科学依据,并为深入研究林地碳氮循环及温室气体排放提供参考。【方法】以农田( FL)为对照,研究华北土石山区10年生刺槐林、43年生刺槐林、自然恢复植被( NRV)土壤微生物量碳、氮的四季动态变化,并对各样地微生物量碳、氮对土壤营养库的贡献率进行对比研究。【结果】各样地微生物量碳、氮随土层加深而逐渐下降,其季节动态变化差异显著;农田、自然恢复植被、10和43年生刺槐林地0~20 cm 土层微生物量碳、氮含量四季均值分别为251.94,290.68,150.66,197.34 mg·kg -1和30.95,46.46,36.55,45.27 mg·kg -1。其中:自然恢复植被的微生物量碳、氮含量四季均值最高,其微生物量碳含量分别是农田、10和43年生刺槐林的1.15,1.93和1.47倍,微生物量氮含量分别是它们的1.50,1.27和1.03倍;土壤微生物量碳、氮含量随刺槐树龄增大而升高,43年生刺槐林0~20 cm 土层的微生物量碳、氮含量是10年生刺槐林的1.31和1.24倍。各植被样地不同层次土壤微生物量碳氮比季节差异明显,农田、自然恢复植被、10年和43年生刺槐林 0~20 cm 土层碳氮比四季均值分别为8.64,6.26,4.12 和4.36;10,43年生刺槐林碳氮比分别是农田的0.48和0.50倍,是自然恢复植被的0.66和0.70倍。在 0~20 cm 土层中,农田、自然恢复植被、10和43年生刺槐林地微生物量碳对土壤有机碳平均贡献率分别为1.88%,2.00%,1.54%和1.24%,土壤微生物量氮对土壤全氮的平均贡献率分别为1.21%,5.44%,3.55%和2.26%。【结论】各样地土壤微生物量碳、氮之间显著相关,它们与土壤全氮、有机质和速效钾含量均显著相关;除此之外,土壤微生物量碳还与土壤硝态氮含量显著相关。随着树龄的增加刺槐林土壤微生物量尤其是微生物量氮含量显著提高,因而土壤的生物肥力也显著提高;由土壤微生物量碳、氮含量及其对土壤营养库的贡献率可知,自然恢复植被更利于土壤微生物结构、功能的恢复和生物活性的改善。 相似文献
7.
8.
9.
- As a result of ancient introductions, some alien species are erroneously considered native. Salvelinus alpinus (Linnaeus, 1758) is hypothesized to have survived as a postglacial relic in the northern European Alps, although documented ancient introductions contradict its native status in a few mountain lakes from the southern Alps (Trentino Alto Adige, Italy). Regardless of its uncertain origin, its recent expansion across the southern Alps (introduced into >170, mainly originally fishless, high‐altitude lakes) was the result of management and alleged conservation actions.
- The present study is a review of the origin of S. alpinus in the southern Alps, and of the appropriateness of management and conservation actions.
- The autochthony of S. alpinus in this region is rejected by multiple lines of evidence and its presence is most likely the result of historical introductions. Only the origin of two populations occurring at lower altitude is uncertain, as they may be native.
- Residual uncertainty makes it difficult to evaluate the conservation status and value of the Italian populations of S. alpinus, but it does not prevent the highlighting of several errors in the past and the present management of this species.
- Past and present introductions have altered the original genetics of the ancient populations and have contributed to the invasion of montane aquatic habitats by modern domestic fish. Although it is now timely to consider the adverse ecological consequences of fish stocking, the many introductions of S. alpinus are associated with a misleading conservation rhetoric, providing ambiguous educational messages to people and relevant stakeholders, and probably diverting resources intended for biodiversity conservation.
- A non‐intervention approach (i.e. not to stock or fish S. alpinus) would have been the wiser strategy to preserve ancient populations, their supposed conservation value, and their actual cultural value. Even now, this strategy is probably the best option, while planning definitive research.
10.
邢涛 《国际沙棘研究与开发》2016,(4):75-77
山洪灾害防治非工程措施项目是保障人民群众生命财产安全的重点项目,也是中央财政部门大力支持的项目,但是山洪灾害防治非工程措施项目能否可持续地、正常地发挥其最大效益,与后期的运行管理有直接相关性.文章主要结合山东省枣庄市山亭区的实际情况,分析非工程措施在山洪灾害防治体系建设中的应用. 相似文献