首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   1090篇
  免费   74篇
  国内免费   3篇
林业   96篇
农学   49篇
基础科学   6篇
  223篇
综合类   159篇
农作物   37篇
水产渔业   89篇
畜牧兽医   392篇
园艺   21篇
植物保护   95篇
  2023年   13篇
  2022年   7篇
  2021年   26篇
  2020年   18篇
  2019年   22篇
  2018年   29篇
  2017年   45篇
  2016年   54篇
  2015年   21篇
  2014年   32篇
  2013年   44篇
  2012年   78篇
  2011年   80篇
  2010年   41篇
  2009年   33篇
  2008年   85篇
  2007年   72篇
  2006年   54篇
  2005年   51篇
  2004年   44篇
  2003年   58篇
  2002年   41篇
  2001年   12篇
  2000年   15篇
  1999年   9篇
  1998年   10篇
  1995年   8篇
  1994年   4篇
  1993年   4篇
  1991年   7篇
  1990年   4篇
  1989年   10篇
  1988年   7篇
  1986年   8篇
  1985年   5篇
  1984年   6篇
  1983年   5篇
  1982年   7篇
  1979年   5篇
  1978年   3篇
  1977年   6篇
  1976年   4篇
  1975年   6篇
  1973年   5篇
  1972年   3篇
  1971年   5篇
  1970年   4篇
  1969年   6篇
  1967年   4篇
  1964年   4篇
排序方式: 共有1167条查询结果,搜索用时 15 毫秒
991.
Objective: The purpose of this study was to examine a population of miniature schnauzers exhibiting syncope as their primary presenting complaint and to correlate their electrocardiographic (ECG) findings with the criteria for the diagnosis of sick sinus syndrome (SSS) reported in the human and veterinary literature. Design: Retrospective study. Patients: Case records of 31 miniature schnauzers with syncope during the period 1996–2000 were reviewed. Measurements and results: This study supported two previous studies finding that females were more predisposed to ECG abnormalities associated with SSS than males. It was also noted that females were slightly older (10.1 years) when compared to males (8.9 years) at the time of diagnosis. A heart murmur was present in 61% of the dogs and this was most likely secondary to mitral valve endocardiosis and regurgitation. The primary rhythm disturbance seen was sinus arrest/sinoatrial block in 48.5% of the cases; sinus bradycardia was the second most common arrhythmia noted. Conclusion: In miniature schnauzers with syncope, SSS should be considered as a primary etiology if abnormal ECG findings are noted.  相似文献   
992.
In this project we optimized a minimal inhibitory concentration testing protocol for Francisella noatunensis orientalis. Thirty-three F. noatunensis orientalis isolates recovered from different fish species and locations were tested, and Escherichia coli ATCC 25922 was used as a quality control reference strain. A modified cation-adjusted Mueller Hinton broth supplemented with 2% IsoVitalex and 0.1% glucose (MMH) was tested at a pH of 6.4 ± 0.1, 7.1 ± 0.1, and 7.3 ± 0.1. Growth curves generated for F. noatunensis orientalis indicated that MMH at a pH of 6.4 ± 0.1 provided optimal growth. There were no significant differences in the growth curves obtained from isolates recovered from different fish species or from fresh or marine water. The pH of 6.4 ± 0.1 in the MMH media interfered with the inhibitory properties of the potentiated sulfonamides (ormetoprim-sulfadimethoxine and trimethoprim-sulfamethoxazole) when using the E. coli ATCC reference strain. Minimal inhibitory concentrations of eight antimicrobials (gentamicin, enrofloxacin, ampicillin, oxytetracycline, erythromycin, florfenicol, flumequine, and oxolinic acid) were similar for all F. noatunensis orientalis isolates. The in vitro susceptibility data provided here can provide a baseline for monitoring the development of antimicrobial resistance among F. noatunensis orientalis isolates, as well as provide valuable data in the development of potential therapeutics.

Received October 27, 2015; accepted April 13, 2016 Published online August 2, 2016  相似文献   

993.
994.
995.
This work addresses the driving factors responsible for patterns in the detritivore macrofaunal communities of a managed beechwood chronosequence (28 to 197 years old, Normandy, France). We investigated the variation patterns of density, biomass and diversities of detritivore macrofauna throughout this rotation. Multivariate analyses were carried out to identify the main covariation patterns between species and some properties of their physical environment, and to describe the main ecological gradients constraining the macro-invertebrate community assembly. A total of 6 earthworm, 6 woodlouse and 7 millipede species were found in the whole data set. Density, biomass and diversity were profoundly influenced by forest ageing, mainly because of variation in humic epipedon spatial variability. Three groups of species were identified according to their environmental requirements. Some hypotheses regarding the external (related to management practices) or internal (related to inter-specific interactions) assembly rules behind species assemblages are proposed, an approach which has rarely been used in soil ecology. Finally, the impact of forestry practices on soil functioning through their impact on detritivore macro-invertebrate communities is discussed.  相似文献   
996.
Estimating root system biomass from breast-height diameters   总被引:1,自引:0,他引:1  
Drexhage  Michael; Colin  Francis 《Forestry》2001,74(5):491-497
  相似文献   
997.
The importance of the ecological functions of farm forests in France calls for a better understanding of the social systems influencing forest management. The traditional ‘house-centred system’ involved a sharing of activities in farm forestry between fathers and their sons. Retired farmers were traditionally managers of the forests while their sons dealt with farming activities. The evolution of this relationship since World War 2 has been investigated with an ethnologic approach in two villages in south-western France where the traditional ‘house-centred system’ constrains the social rules. This system has led to a lifetime status for fathers as head of the farm and of the household. Before WW2, sons played a secondary role in relation to their fathers who took strategic decisions. After WW2, sons became more involved in the forest work and decisions which led to a more frequent use of agricultural methods in forests. This created differences of opinion between fathers and sons, with the fathers considering their sons’ management too intensive. The social consequence was that the fathers felt that their role as forest managers and as head of the farm was reduced. In future, farm forest management might become more dependent on agricultural activities, with the traditional social systems losing importance.  相似文献   
998.
This paper analyses a local innovation in response to intense soil degradation in olive orchards of north‐west Syria. Farmers developed a practice consisting of quarrying red clayey soil in valley bottoms and applying this soil to hillslope olive orchards with heavily degraded calcareous soils. A biophysical, economic and social analysis of the practice of soil application identified the opportunities and risks of this innovative soil management technique. On the basis of a pairwise comparison of nine adjacent treated and nontreated orchard plots, soil applications were found to increase soil depth by 36%, soil water availability by 28% and total available soil nutrients: potassium (+45%), nitrogen (+12%) and phosphorus (+6%). Olive yield increased by about 40%. A cost–benefit analysis found this practice to be economically viable within a large geographical area, and farmers scored the practice higher than alternative methods. A socio‐economic analysis revealed its widespread adoption among different farmer types. The positive results of soil applications at the farm level explain its fast adoption. However, potential risks – including the further depletion of soil resources and the transfer of soil‐borne diseases – limit the long‐term sustainability of this locally developed practice. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   
999.

Key message

Ozone, one of the major atmospheric pollutants, alters tree growth, mainly by decreasing carbon assimilation and allocation to stems and roots. To date, the mechanisms of O3 impact at the cellular level have been investigated mainly on young trees grown in controlled or semi-controlled conditions. In the context of climate change, it is necessary to introduce a valuable defence parameter in the models that currently predict O3 impact on mature trees and the carbon sequestration capacity of forest ecosystems.

Context

Air pollution is an important factor that affects negatively forest ecosystems. Among oxidative air pollutants, ozone is considered as the most toxic in terms of impact on vegetation.

Aims

This paper focuses on the negative impacts of ozone on trees in controlled conditions or in their natural environment. The current knowledge of the responses at cell level is presented and ways to improve their use for ozone risk assessment of forest stands are discussed.

Methods

Information was collected from original papers or reviews, providing an overview of the research conducted over the last 60 years.

Results

The negative effects of ozone on carbon assimilation and tree biomass production were reviewed and discussed, with a focus on effects on cell processes implied in cell defence, including stomatal regulation, detoxification, signalling, and biosynthesis of wood compound.

Conclusion

In the context of increasing significance of O3 flux approach, this review intends to shed light into the black box of defence processes, which are playing a crucial part within the effective O3 dose modelling. Today, it is recognized that tropospheric ozone inhibits tree growth and its role on the future carbon sink of the forest ecosystem is discussed along with the combination of other environmental factors like elevated temperature, water, and nitrogen supply, likely to be modified in the context of climate change.
  相似文献   
1000.
The Salicaceae and Betulaceae are important plant families within riparian ecosystems throughout the Northern hemisphere, and cuttings of species within these families are often used in riparian restoration projects due to their capacity for vegetative (asexual) propagation and rapid growth. Riparian zones can experience a wide range of hydrological conditions and establishing cuttings may be subjected to substantially varying levels of hydric stress from inundation and drought. Tolerance to such stress can vary notably between species, and the absence of knowledge of species-specific responses to inundation and drought has resulted in high mortality in many riparian restoration projects. In this paper, the survival and growth responses of Salix elaeagnos (Salicaceae), Populus nigra (Salicaceae) and Alnus incana (Betulaceae) cuttings in relation to varying rates of water table decline and periods of inundation were investigated using rhizopods during a 69-day greenhouse experiment, in substrates of contrasting sediments (sand versus gravel).

Each species responded differently to the experimental treatments. S. elaeagnos demonstrated an ability to establish in stable, declining and inundated conditions by the production of relatively substantial root and shoot biomass in all treatments, along with elongated roots when experiencing water table decline. P. nigra showed high mortality and poor root and shoot production in inundation treatments, suggesting that this species is intolerant to reduced oxygen conditions. A. incana displayed complete mortality in all gravel-based treatments and high mortality or poor growth in the sand-based inundation treatments. All species showed preferences for stable conditions without inundation, along with slow water table decline for S. elaeagnos. Fast rates of water table decline were detrimental to all species. Overall, sand was a much more effective substrate than gravel for supporting cutting establishment. These results suggest that riparian restoration schemes utilizing S. elaeagnos cuttings may experience greater success than those using the other species investigated. P. nigra cuttings may establish more easily if situated where inundation does not occur often or for prolonged periods, and A. incana cuttings should be planted only where very stable conditions dominate. The study highlights the inter-species variability in the survival and growth response of riparian trees to differing hydrological conditions.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号