全文获取类型
收费全文 | 843篇 |
免费 | 40篇 |
国内免费 | 56篇 |
专业分类
林业 | 83篇 |
农学 | 62篇 |
基础科学 | 43篇 |
170篇 | |
综合类 | 373篇 |
农作物 | 33篇 |
水产渔业 | 22篇 |
畜牧兽医 | 120篇 |
园艺 | 19篇 |
植物保护 | 14篇 |
出版年
2024年 | 5篇 |
2023年 | 16篇 |
2022年 | 18篇 |
2021年 | 23篇 |
2020年 | 16篇 |
2019年 | 26篇 |
2018年 | 20篇 |
2017年 | 27篇 |
2016年 | 33篇 |
2015年 | 20篇 |
2014年 | 42篇 |
2013年 | 44篇 |
2012年 | 78篇 |
2011年 | 62篇 |
2010年 | 71篇 |
2009年 | 56篇 |
2008年 | 53篇 |
2007年 | 67篇 |
2006年 | 57篇 |
2005年 | 42篇 |
2004年 | 31篇 |
2003年 | 26篇 |
2002年 | 8篇 |
2001年 | 11篇 |
2000年 | 10篇 |
1999年 | 10篇 |
1998年 | 7篇 |
1997年 | 9篇 |
1996年 | 9篇 |
1995年 | 8篇 |
1994年 | 6篇 |
1993年 | 7篇 |
1992年 | 7篇 |
1991年 | 4篇 |
1990年 | 1篇 |
1989年 | 4篇 |
1988年 | 3篇 |
1987年 | 2篇 |
排序方式: 共有939条查询结果,搜索用时 203 毫秒
101.
对酸枣中的主要活性成分黄酮类、皂苷类、生物碱类物质、脂肪酸、氨基酸及矿物质等进行分析,并对其主要药理作用的研究进展进行综述,为酸枣仁的进一步开发利用提供依据。 相似文献
102.
农产品质量安全监管系统的功能和复杂性及其化解路径 总被引:5,自引:2,他引:3
农产品质量安全监管是一项复杂的系统工程。系统科学的基本原理与系统复杂性理论,为农产品质量安全监管系统工程建设提供了理论依据。农产品质量安全监管系统包括物理、事理和人理三类,实现农业经济发展和社会和谐稳定功能,呈现出系统要素、系统关系、系统规模、系统特征和系统演进的复杂性。从法制框架、追溯信息系统、技术支撑体系、多方联动机制,从定性到定量综合集成方法和整体最优原则等维度,提出推进我国农产品质量安全监管系统工程的关键环节和策略建议。 相似文献
103.
104.
《Scandinavian Journal of Forest Research》2012,27(6):470-488
The main objective of this case study was to explore the possible influence of forest management on the levels and distribution of biomass and carbon (C) in even-aged stands of Norway spruce [Picea abies (L.) Karst.] in Denmark. Data originated from a long-term thinning experiment and an adjacent spacing experiment at stand ages of 58 and 41 years, respectively. Biomass of 16 trees from different thinning and spacing treatments was measured or partly estimated, and soils were sampled for determination of C stocks. All trees in each plot were measured for stem diameter and some for total height, to allow for scaling-up results to stand-level estimates. For trees of similar size, foliage biomass tended to be higher in the spacing experiment, which was located on slightly more fertile land. Foliage biomass increased with increasing thinning grade, but the effect could not be separated from that of tree size. At stand level, foliage biomass tended to increase with increasing spacing as well as with increasing thinning grade. For branchwood, stems and roots (including below-ground stump), the biomass increased with increasing tree size and stand volume at tree and stand level, respectively, but no differences between stands, spacings or thinning grades were observed, apart from that expressed by tree size or stand volume. At stand level, C stocks of all biomass compartments decreased with increasing thinning grade, while the distribution between compartments was hardly influenced. The ratio between above-ground and stem biomass was about 1.21 at stand level, while the ratio between below- and above-ground biomass was about 0.17. Thinning influenced the C stock of the forest floor and mineral soil oppositely, resulting in no effect of thinning on total soil C. 相似文献
105.
《Scandinavian Journal of Forest Research》2012,27(3):312-322
Abstract We propose a conceptual (generic) allometric (power function) relationship between tree-size-related forest inventory attributes (e.g. biomass, volume, basal area, quadratic mean diameter, Lorey's height) and canopy height (CH) as estimated from first-return airborne light detection and ranging (LiDAR) pulses. A data-driven estimation of the parameters in the power function is complicated, so we recommend an alternative approximation obtained via a linearisation step. Only two predictors appear in the approximation: the mean CH and the variance of CHs within the spatial domain supported by field data. The proposed model eliminates an otherwise complex search for the best predictors amongst a large number of candidate LiDAR metrics. It also facilitates model comparisons and interpretation. Fit statistics estimated for volume, basal area, quadratic mean diameter and Lorey's height – using three separate datasets from Norway – were compelling. 相似文献
106.
Particle arrangement and internal porosity of coarse fragments affect water retention in stony soils
Caroline Andrade Pereira Rodrigo Pivoto Mulazzani Quirijn de Jong Van Lier Fabrício de Araújo Pedron Paulo Ivonir Gubiani 《European Journal of Soil Science》2023,74(3):e13382
Information about water retention in stony soils lags behind due to methodological difficulties. We applied a new strategy to measure the water retention in soils with coarse fragments (CFs) and to get insights into the effect of CFs porosity on water retention. Water retention at zero, 10, and 150 m suction, bulk density, and the mass fraction of six particle size classes were measured in undisturbed blocks from soils with variable CFs contents, originating from three parent materials. The results showed that some soils contain porous CFs (2–250 mm) with a water holding capacity as high as the fine fraction (<2 mm). The water held in the suction range of 1–150 m in a soil with porous CFs was twice as high as in soils with non-porous CFs. Multilinear regressions revealed that both the water retention capacity at 1 m suction and in the range 1–150 m were more dependent on bulk density than on the fraction of CFs and fine particles. In the soil with porous CFs, there was no correlation between their fraction and soil water retention. These results show that the bulk water retention capacity of soils with CFs is underestimated when not considering the internal porosity of the CFs. A better understanding of the effect of the porosity of CFs on bulk soil porosity and water retention is important to propose suitable pedotransfer functions and refine physically-based hydraulic functions for stony soils. 相似文献
107.
Based on complex variable theory in plane elasticity, the equivalent spring stiffness is derived from the analytical solution by means of simplifying shallow shield tunneling to semi-infinite plane with hole, which is applied to response displacement method. The performances of the analytical solution are evaluated and compared with that of the deep shield tunneling through a series of selected various tunnel depths and soil Poisson's ratios. The relation of compression and shear spring stiffness is discussed. Meanwhile, two factors affected soil spring stiffness, tunnel depth and soil Poisson's ratio, are also analyzed. It is shown that soil spring stiffness of shallow shield tunneling is different from that of deep shield tunneling, and the ratio of compression and shear spring stiffness varies along shallow shield tunneling. It is also found that the value and distribution law of soil spring stiffness of shallow shield tunneling greatly depend on tunnel depth and soil Poisson's ratio. 相似文献
108.
指出了农业是人类的衣食之源,生存之本,是国民经济其他部门独立和进一步发展的基础。农业问题是当前中国最大的经济问题,因为农业问题解决得好与不好,不仅关系到国民经济的发展,而且关系到我们在2020年能否实现全面建设小康社会的宏伟目标,能否推进社会主义现代化的建设。结合我国的实际情况分析我国农业的特点,利用简单的Excel工具,通过数据剖析我国农业的作用,最后得出结论:在推进工业化进程中,农业对国民经济的贡献虽然有所下降,但是它是发展工业化,走现代化道路的基石,它的基础性地位不可动摇。 相似文献
109.
M. A. Mojid A. B. M. Z. Hossain G. C. L. Wyseure 《Communications in Soil Science and Plant Analysis》2019,50(10):1199-1213
Reliable transport parameters of agrochemicals and soluble pollutants are crucial for modeling and management of soil and groundwater quality. This study investigated impacts of municipal wastewater on the transport parameters of five heavy metal/metalloid compounds (NaAsO2, Cd(NO3)2, Pb(NO3)2, Ni(NO3)2 & ZnCl2), two pesticides (cartap & carbendazim) and an inert salt (CaCl2) in four agricultural soils of Bangladesh. Solute-breakthrough concentrations were measured in repacked soil columns with time-domain reflectometry (TDR) both before and after wastewater treatment. Transport velocity (V), dispersion coefficient (D), dispersivity (λ) and retardation factor (R) of the solutes, and pertinent soil properties were determined. Wastewater reduced bulk density (γ) of the soils (from 1.32–1.37 g/cm3 to 1.26–1.35 g/cm3) by increasing organic carbon (OC) (from 0.37%–0.84% to 0.40–0.93%), increased pore-size distribution index (n) (by 0.02 unit) and reduced soil pH (from 6.32–7.45 to 5.92–6.46). D and λ decreased while V and R increased after wastewater treatment; D decreased and R increased linearly with decreasing bulk density. The correlations of V, D, and R with n improved significantly (p < 0.05) after wastewater treatment. The correlation between λ and OC improved markedly for Ca, Pb, Ni, and cartap. The observed indicative results have practical implications in developing pedo-transfer functions for solute-transport parameters using basic soil properties, which are subject to progressive modification due to agrochemicals application and wastewater irrigation. 相似文献
110.