全文获取类型
| 收费全文 | 929篇 |
| 免费 | 79篇 |
| 国内免费 | 40篇 |
专业分类
| 林业 | 80篇 |
| 农学 | 23篇 |
| 基础科学 | 1篇 |
| 293篇 | |
| 综合类 | 245篇 |
| 农作物 | 18篇 |
| 水产渔业 | 63篇 |
| 畜牧兽医 | 92篇 |
| 园艺 | 94篇 |
| 植物保护 | 139篇 |
出版年
| 2024年 | 4篇 |
| 2023年 | 15篇 |
| 2022年 | 45篇 |
| 2021年 | 31篇 |
| 2020年 | 38篇 |
| 2019年 | 66篇 |
| 2018年 | 37篇 |
| 2017年 | 37篇 |
| 2016年 | 42篇 |
| 2015年 | 40篇 |
| 2014年 | 44篇 |
| 2013年 | 50篇 |
| 2012年 | 44篇 |
| 2011年 | 55篇 |
| 2010年 | 62篇 |
| 2009年 | 60篇 |
| 2008年 | 60篇 |
| 2007年 | 62篇 |
| 2006年 | 59篇 |
| 2005年 | 42篇 |
| 2004年 | 21篇 |
| 2003年 | 24篇 |
| 2002年 | 8篇 |
| 2001年 | 12篇 |
| 2000年 | 12篇 |
| 1999年 | 8篇 |
| 1998年 | 11篇 |
| 1997年 | 2篇 |
| 1996年 | 9篇 |
| 1995年 | 3篇 |
| 1994年 | 5篇 |
| 1993年 | 3篇 |
| 1992年 | 5篇 |
| 1991年 | 4篇 |
| 1990年 | 4篇 |
| 1989年 | 8篇 |
| 1988年 | 7篇 |
| 1987年 | 4篇 |
| 1986年 | 1篇 |
| 1985年 | 1篇 |
| 1981年 | 1篇 |
| 1956年 | 2篇 |
排序方式： 共有1048条查询结果，搜索用时 31 毫秒
1.
2.
3.
Cynodon dactylon is a low-growing C4 weed that is highly sensitive to shading. This species shows severe biomass reductions and highly plastic morphological changes in response to resource availability. The objective of this study was to analyse the effect of shading on the spatial and biomass growth of C. dactylon . Two experiments were carried out exploring different environmental conditions. Six treatments were applied on C. dactylon patches. Daylight and 18%, 41%, 48%, 69% and 85% shading of daylight photosynthetic active radiation (PAR) were imposed in Expt 1 and 0%, 22%, 44%, 50%, 74% and 87% shading of daylight PAR in Expt 2. Patches were slightly elliptical in shape and grew 3.2 and 1.5 cm d−1 with 0% shading, on average, during the whole studied periods in Expts 1 and 2 respectively. Biomass was significantly reduced from 41% shading onwards in Expt 1 and from 50% shading in Expt 2. However, patch extension rate was only significantly reduced from 85% and 74% shading in Expts 1 and 2 respectively. Biomass partitioning was modified by shading in both experiments. A functional analysis of C. dactylon patches showed that the patch extension rate diminished linearly when patch biomass growth rate was lower than 1.66 g d−1 ; above this value, the extension rate remained constant. 相似文献
4.
The invasive ability of Cynodon dactylon is dependent on self dispersal and on cultivation practices. Tillage can seriously change patch biomass and spatial structure, spreading vegetative propagules of the weed. The objectives of this study were: (i) to quantify the effect on non‐inversion tillage on dispersal, establishment and colonization of C. dactylon and (ii) to propose a simple model considering soil cultivation effects and light availability on spatial growth of weed patches. Two experiments were carried out, exploring different soils and environmental conditions. Spatial distribution of vegetative units differed when tillage was conducted with different non‐inversion implements and could be described by simple functions. A minimum patch biomass seems necessary before vegetative structures are vulnerable to movement by cultivation. Only a small proportion of the biomass dispersed from original patches was able to establish. However, simulation showed that the area colonized by C. dactylon mostly increased by means of tillage dispersal, both with and without crop competition, in one growing cycle. It appears sensible to consider changing cultivation practices to reduce weed dispersal and to use crop competition for light to create unsuitable habitats limiting weed colonization. 相似文献
5.
草原灌丛化已成为全球性问题,也是生态学、畜牧学研究的热点问题。紧扣草原灌丛化过程、发生机理及其生态学后效等关键问题,介绍了国内外的最新研究进展。灌木入侵途径包括有意识引进入侵和自然入侵2类,入侵过程分为传播到达、定居建群和扩散入侵3个阶段。放牧、火烧、温室气体增加,干旱以及降水格局改变等均可导致草本向木本的转化,且机制复杂,主要有影响草本,木本植物盖度、水分利用效率、化感作用等。木本植物入侵打破了草原生态系统的稳定,表现出沃岛效应,影响动植物的分布以及生态系统的功能和服务。阐明草原“草本-木本”群落演替机制,可为我国研究草原灌丛化和管理提供理论借鉴。 相似文献
6.
The European mole (Talpa europaea L.) inhabits an underground tunnel system whereby the density, extent and condition of the subsurface tunnels are indicative of its activity. Currently, no survey method was able to reveal the spatial extent and condition of the mole's tunnel network. Frequency-domain ground penetrating radar (GPR) was evaluated on its potential to image these shallow tunnel systems. This technique allows for a non-invasive, high-resolution mapping of the subsurface. We examined the effectiveness of this GPR system for delineating the mole's tunnel network. The integration of different depth slices allowed a detailed overview of the tunnel system. Automatic feature recognition on these GPR images was proven valuable for the detection and representation of the mole tunnels. The GPR survey proved successful in mapping the mole's tunnel network, which facilitates the interpretation and characterization of the mole's living environment. This can be linked to the occurrence of earthworms, as the principle food source of the moles, which regulate important ecosystem processes within the soil. This offers new perspectives for the understanding of the mole's habitat. 相似文献
7.
Pheromones are communication chemicals and regulatory signals used by animals and represent unique tools for organisms to mediate behaviors and make “decisions” to maximize their fitness. Phenotypic plasticity refers to the innate capacity of a species to tolerate a greater breadth of environmental conditions across which it adapts to improve its survival, reproduction, and fitness. The pinewood nematode, Bursaphelenchus xylophilus, an invasive nematode species, was accidentally introduced from North America into Japan, China, and Europe; however, few studies have investigated its pheromones and phenotypic plasticity as a natural model. Here, we demonstrated a novel phenomenon, in which nematodes under the condition of pheromone presence triggered increased reproduction in invasive strains (JP1, JP2, CN1, CN2, EU1, and EU2), while it simultaneously decreased reproduction in native strains (US1 and US2). The bidirectional effect on fecundity, mediated by presence/absence of pheromones, is henceforth termed pheromone-regulative reproductive plasticity (PRRP). We further found that synthetic ascaroside asc-C5 (ascr#9), the major pheromone component, plays a leading role in PRRP and identified 2 candidate receptor genes, Bxydaf-38 and Bxysrd-10, involved in perceiving asc-C5. These results suggest that plasticity of reproductive responses to pheromones in pinewood nematode may increase its fitness in novel environments following introduction. This opens up a new perspective for invasion biology and presents a novel strategy of invasion, suggesting that pheromones, in addition to their traditional roles in chemical signaling, can influence the reproductive phenotype among native and invasive isolates. In addition, this novel mechanism could broadly explain, through comparative studies of native and invasive populations of animals, a potential underlying factor behind of the success of other biological invasions. 相似文献
8.
9.
The nontarget effects of fresh and used motor oil were studied in a soil test system involving such criteria as earthworm survival, response of soil dehydrogenase and urease, and nitrification. When earthworms were exposed to motor oil-contaminated soil for 4 weeks, the observed median lethal concentrations (LC50) were 40.33 and 3.88 g kg−1 soil for fresh and used oil, respectively. Only fresh motor oil application increased earthworms' body weight even at the higher dose of 19 g kg−1 soil. Gas chromatography/mass spectrometry revealed that used motor oil contained more of aromatic hydrocarbons and heavy metals than fresh oil. This disparity in the chemical composition might be the factor responsible for the significant toxicity of used motor oil towards earthworms. Activities of soil dehydrogenase and urease were significantly enhanced in presence of both the motor oils, while there was a significant inhibition in nitrification by the used motor oil even at a low concentration of 0.2 g kg−1 soil. This study clearly demonstrated that earthworm survival and nitrification could serve as suitable indices to assess motor oil pollution in soil. 相似文献
10.