全文获取类型
收费全文 | 1048篇 |
免费 | 35篇 |
国内免费 | 64篇 |
专业分类
林业 | 322篇 |
农学 | 42篇 |
基础科学 | 98篇 |
98篇 | |
综合类 | 405篇 |
农作物 | 27篇 |
水产渔业 | 11篇 |
畜牧兽医 | 94篇 |
园艺 | 17篇 |
植物保护 | 33篇 |
出版年
2024年 | 1篇 |
2023年 | 12篇 |
2022年 | 26篇 |
2021年 | 30篇 |
2020年 | 20篇 |
2019年 | 27篇 |
2018年 | 13篇 |
2017年 | 23篇 |
2016年 | 28篇 |
2015年 | 37篇 |
2014年 | 45篇 |
2013年 | 45篇 |
2012年 | 79篇 |
2011年 | 84篇 |
2010年 | 74篇 |
2009年 | 70篇 |
2008年 | 70篇 |
2007年 | 54篇 |
2006年 | 42篇 |
2005年 | 48篇 |
2004年 | 44篇 |
2003年 | 35篇 |
2002年 | 32篇 |
2001年 | 25篇 |
2000年 | 15篇 |
1999年 | 27篇 |
1998年 | 18篇 |
1997年 | 10篇 |
1996年 | 14篇 |
1995年 | 13篇 |
1994年 | 19篇 |
1993年 | 17篇 |
1992年 | 15篇 |
1991年 | 13篇 |
1990年 | 8篇 |
1989年 | 5篇 |
1988年 | 5篇 |
1987年 | 2篇 |
1985年 | 1篇 |
1984年 | 1篇 |
排序方式: 共有1147条查询结果,搜索用时 15 毫秒
101.
The relative importance of litter quality and site heterogeneity on population dynamics of decomposer food webs was investigated in a semi-natural mixed deciduous forest in Denmark. Litterbags containing beech or ash leaves were placed in four plots. Plots were located within gaps and under closed canopies at two topographically different sites, above and below a slope, respectively, to cover variable environmental conditions. Litter was collected after 2, 4 and 9 months of decomposition. Extensive decay prevented analysis of ash after 9 months. Density of bacteria (CFU), active fungal mycelium (FDA), protozoa (MPN) and nematodes were 4-15-fold higher in ash leaves than in beech leaves in accordance with the higher resource quality of ash. Similar effects of site on density of decomposers were evident in both litter types: with some exceptions, decomposers were higher at the low site and stimulated in gaps. Taxonomic diversity of nematodes increased during decomposition and functional diversity of nematodes followed a pattern often encountered, i.e. opportunistic bacterial-feeders were gradually replaced by fungal-feeders and slower growing bacterial-feeders while predators and omnivors peaked at the end of the study period. At the first sampling, where bacterial activity prevailed, the relative abundance of the two dominant bacterial-feeders, Rhabditidae (fast growing) and Plectus spp. (slower growing), depended more on site than litter type. At the second sampling where fungal activity became more important, the proportions of bacterial and fungal-feeding nematodes also depended more on site than on litter type. At the third sampling individual nematode taxa responded differently to site. In summary, we conclude that although litter quality had a major influence on the density of organisms in the decomposer food web, site effects were also detected and nematode functional groups responded more to site than to litter quality early on in the decomposition process. 相似文献
102.
Hong Li Robert J. Lascano Jill Booker L. Ted Wilson Kevin F. Bronson 《Soil & Tillage Research》2001,58(3-4):245-258
Landscape variability associated with topographic features affects the spatial pattern of soil water and N redistribution, and thus N uptake and crop yield. A landscape-scale study was conducted in a center pivot irrigated field on the southern High Plains of Texas in 1999 to assess soil water, soil NO3-N, cotton (Gossypium hirsutum L.) lint yield, and N uptake variability in the landscape, and to determine the spatial correlation between these landscape variables using a state-space approach. The treatments were irrigation at 50 and 75% cotton potential evapotranspiration (ET). Neutron access tubes were placed at a 15-m interval along a 710 m (50% ET) and 820 m (75% ET) transect across the field. Soil NO3-N in early spring was autocorrelated at a distance varying between 60 and 80 m. Measured soil volumetric water content (WC), total N uptake, and lint yield were generally higher on lower landscape positions. Cotton lint yield was significantly correlated to soil WC (r=0.76), soil NO3-N (r=0.35), and site elevation (r=−0.54). Differences of site elevation between local neighboring points explained the soil water, NO3-N and lint yield variability at the micro-scale level in the landscape. Soil WC, cotton lint yield, N uptake, and clay content were crosscorrelated with site elevation across a lag distance of ±30–40 m. The state-space analysis showed that cotton lint yield was positively weighted on soil WC availability and negatively weighted on site elevation. Cotton lint yield state-space models give insights on the association of soil physical and chemical properties, lint yield, and landscape processes, and have the potential to improve water and N management at the landscape-scale. 相似文献
103.
104.
渝东南地区独特的地貌为寺观园林选址创造了丰富的条件,且大部分的寺观园林是与山地相结合而修建的山地型寺观园林.笔者将渝东南寺观园林与山体的结合方式分为山顶型、山腰型、山麓型,认为寺观周边环境主要有寺观置于旅游开发古镇之中和置于山林自然景区之中两种.渝东南山林景区中寺观规模大小不一,建筑布局也各有千秋,现存的寺观园林的建筑布局主要有井院式布局、线性布局和综合布局方式.研究渝东南寺观园林的环境特征,旨在为充实地方寺观园林研究,补充巴渝园林特点做出贡献. 相似文献
105.
杉木对不同岩性土体的生长效应研究 总被引:2,自引:0,他引:2
用实验生态学方法 ,在不同岩性土体上进行了杉木栽植的研究 ,10a试验结果表明 :杉木的高、径生长以及生物量在不同岩性所发育的土壤上的差异是很显著的。其中以玄武岩与石英砂岩发育的土体上的杉木生长最好 ,依次为长石石英砂岩与变余砂岩 ,而以煤系砂页岩和第四纪红色粘土上的杉木生长最差。不同岩性土体上杉木的生物量与其树高、胸径生长相关 ,反映在杉木生长好的岩性土体上的杉木生物量为高 ,但随着杉木的生长势降低 ,其地下部分的生物量占全株总生物量的比例增大。该研究为南方山区杉木立地分类中应重视岩性因子提供了依据 相似文献
106.
简单介绍了目前的应用很多的两种网络安全技术,并结合两种技术实现可靠的网络安全。 相似文献
107.
108.
对700个母蛾用N.b碳素凝集反应法进行检微,碳检检出率为95.45%,对检的镜检检出率为22.72%.碳、镜检检出率的比值是4.2倍(即碳检有微数>镜检有微数);碳检发生的阳性反应样品,再用显微镜复检有微吻合率达100%,准确性大,误判率为零.检出微孢灵敏度高,最高N.b浓度是7.5×10~6个/ml,最低N.b浓度是2.5×10~3个/ml,可视为碳检700个母蛾批的检微浓度范围,以上数据证实用N.b致敏碳素检查母蛾微孢是可行的. 相似文献
109.
柳杉木材密度测定研究 总被引:4,自引:0,他引:4
设定了长岭岗林场23年生柳杉人工林木材密度指标,分析立地条件对柳杉木材密度的影响,同时讨论了改善木材密度的方法。结果显示;立地条件对柳杉全干密度的影响差异显著,全干密度的差异主要是由土层厚度和土壤肥力的差异所导致。薄土陡坡及山脊上林木的气干密度和基本密度高于其它3种类型的密度指标。 相似文献
110.
本文介绍在航空象片上进行树高和冠幅的量测,通过其比值以判读森林立地指数。这种方法不仅可以提高立地指数测定的工作效率,而且可以与立地制图相结合,充分地发挥航空遥感信息的潜力。 相似文献