全文获取类型
收费全文 | 108785篇 |
免费 | 5504篇 |
国内免费 | 5365篇 |
专业分类
林业 | 8313篇 |
农学 | 9873篇 |
基础科学 | 4884篇 |
14736篇 | |
综合类 | 24140篇 |
农作物 | 4561篇 |
水产渔业 | 4568篇 |
畜牧兽医 | 39368篇 |
园艺 | 2201篇 |
植物保护 | 7010篇 |
出版年
2022年 | 1412篇 |
2021年 | 2327篇 |
2020年 | 2086篇 |
2019年 | 2267篇 |
2018年 | 2089篇 |
2017年 | 2548篇 |
2016年 | 2441篇 |
2015年 | 3010篇 |
2014年 | 2895篇 |
2013年 | 5107篇 |
2012年 | 4647篇 |
2011年 | 5275篇 |
2010年 | 4331篇 |
2009年 | 4120篇 |
2008年 | 4529篇 |
2007年 | 4497篇 |
2006年 | 4327篇 |
2005年 | 4166篇 |
2004年 | 2646篇 |
2003年 | 2494篇 |
2002年 | 2245篇 |
2001年 | 2614篇 |
2000年 | 2780篇 |
1999年 | 2792篇 |
1998年 | 1803篇 |
1997年 | 1610篇 |
1996年 | 1483篇 |
1995年 | 1505篇 |
1994年 | 1463篇 |
1993年 | 1339篇 |
1992年 | 1930篇 |
1991年 | 1949篇 |
1990年 | 1739篇 |
1989年 | 1652篇 |
1988年 | 1422篇 |
1987年 | 1320篇 |
1986年 | 1286篇 |
1985年 | 1268篇 |
1984年 | 1010篇 |
1983年 | 936篇 |
1979年 | 899篇 |
1978年 | 713篇 |
1976年 | 713篇 |
1975年 | 715篇 |
1974年 | 871篇 |
1973年 | 844篇 |
1972年 | 815篇 |
1971年 | 730篇 |
1970年 | 771篇 |
1969年 | 737篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
This research was conducted to determine the changes in the seasonal carbohydrates of annual shoots and growth characteristics of 10 years old ‘Trakya ?lkeren’ grape variety. Grapevines are grafted on 5BB and 5C rootstocks and they were grown in heavy clay soil conditions. In the study, changes of sugar, starch and total carbohydrates were determined from bud burst to dormancy. While sugar, starch and total carbohydrates showed significant differences (P < 0.01) there were no differences between rootstocks. Although sugar, starch and total carbohydrates of shoots decreased from bud burst to blooming, they increased thereafter until vegetative growth stopped. Total carbohydrates of annual shoots have been found to accumulate in period from blooming to harvest. Total and mean leaf area, shoot length and diameter, total chlorophyll and chlorophyll a/b ratio showed significantly variation among rootstocks and phenological stages (P < 0.01). Whereas the highest chlorophyll contents were found in the blooming, it was found at the lowest through to harvest. Overall mean and total leaf area, shoot length and shoot diameter, total chlorophyll content and chlorophyll a/b ratio were significantly higher on 5BB grafted vines. In the study, 5BB rootstocks are found suitable in terms of shoot carbohydrate accumulation and growth characteristics for ‘Trakya ?lkeren’ grape variety. 相似文献
992.
John B. Graham Joan I. Nassauer William S. Currie Herbert Ssegane M. Cristina Negri 《Landscape Ecology》2017,32(5):1023-1037
Context
Wild bee populations are currently under threat, which has led to recent efforts to increase pollinator habitat in North America. Simultaneously, U.S. federal energy policies are beginning to encourage perennial bioenergy cropping (PBC) systems, which have the potential to support native bees.Objectives
Our objective was to explore the potentially interactive effects of crop composition, total PBC area, and PBC patches in different landscape configurations.Methods
Using a spatially-explicit modeling approach, the Lonsdorf model, we simulated the impacts of three perennial bioenergy crops (PBC: willow, switchgrass, and prairie), three scenarios with different total PBC area (11.7, 23.5 and 28.8% of agricultural land converted to PBC) and two types of landscape configurations (PBC in clustered landscape patterns that represent realistic future configurations or in dispersed neutral landscape models) on a nest abundance index in an Illinois landscape.Results
Our modeling results suggest that crop composition and PBC area are particularly important for bee nest abundance, whereas landscape configuration is associated with bee nest abundance at the local scale but less so at the regional scale.Conclusions
Strategies to enhance wild bee habitat should therefore emphasize the crop composition and amount of PBC.993.
Krystina D. Mossop Nicholas P. Moran David G. Chapple Bob B. M. Wong 《Landscape Ecology》2017,32(5):1065-1078
Context
Dispersal has important fitness consequences for individuals, populations, and species. Despite growing theoretical insights into the evolution of dispersal, its behavioral underpinnings remain empirically understudied, limiting our understanding of the extent and impact of responses to landscape-level heterogeneity of environments, and increasing the risk of inferring species-level responses from biased population sampling.Objectives
We asked if predictable ecological variation among naturally fragmented arid waterbodies is correlated with disparate dispersal responses of populations of the desert goby Chlamydogobius eremius, which naturally inhabits two habitat “types” (permanent springs, ephemeral rivers), and different levels of hydrological connectivity (high and low) that potentially convey different costs and benefits of dispersal.Methods
To test for possible behavioral divergence between such populations, we experimentally compared the movement behaviors (correlates of emigration and exploration) of wild-caught fish. We used two biologically relevant spatial scales to test movement relevant to different stages of the dispersal process.Results
Behavior differed at both spatial scales, suggesting that alternative dispersal strategies enable desert gobies to exploit diverse habitat patches. However, while emigration was best predicted by the connectivity (flood risk) of fish habitats, exploration was linked to their habitat type (spring versus river).Conclusions
Our findings demonstrate that despite a complex picture of ecological variation, key landscape factors have an overarching effect on among-population variation in dispersal traits. Implications include the maintenance of within-species variation, potentially divergent evolutionary trajectories of naturally or anthropogenically isolated populations, and the direction of future experimental studies on the ecology and evolution of dispersal behavior.994.
为了分析当前国内区域间农业机械化生态服务功能的空间异质性变化特征,研究运用2014年的省际面板数据基于"压力-状态-响应"视角构建了农业机械化生态服务功能评价指标体系。评价结果表明:基于"压力-状态-响应"视角来看,当前我国农业机械化生态服务功能存在一定程度的空间异质性,且与各区域所处的经济发展阶段和农业经济发展速度有紧密的联系;农业机械化生态服务功能较好的省(市、区)主要分布在华北、华东和东北地区,而服务功能差的省(市、区)遍布华北、华南、青藏、西北及西南地区,且经济发展速度有快有慢。 相似文献
995.
The objective of this research was to survey the effects of starch quaternization and sulfosuccinylation on the adhesion of cold starch paste to raw cotton fibers for cotton warp sizing at low temperature. Acid-thinned cornstarch (ATS) was quaternized and then sulfosuccinylated to introduce 3-(trimethylammonium chloride)-2-hydroxypropyl and sulfosuccinate substituents onto its backbones. The electroneutrality of starch samples prepared was achieved by maintaining a constant mole ratio (5.3:1) of the two substituents. A series of electroneutral cornstarch (ECS) samples with different levels of the substituents were derived by altering the feed ratio of the modifying reagents to starch for determining desirable level of starch modification. Adverse influences of cotton wax and starch retrogradation on the adhesion of cold starch paste to raw cotton fibers were evaluated to illustrate the effectiveness of starch quaternization and sulfosuccinylation. It was found that the modification was able to alleviate the adverse influence of starch retrogradation and ameliorate the adhesion to the fibers at low temperature. Higher level of the modification led to less retrogradation and resulted in strong adhesion. Furthermore, the adverse influence of cotton wax on the adhesion could be eliminated after a pre-wetting treatment of raw cotton warps with hot water. The adhesion of ECS paste to raw cotton at 60 °C was statically the same as that of ATS at 95 °C when total DS of ECS was 0.0443 or higher. 相似文献
996.
B. 《European Journal of Forest Research》1866,10(1):415-429
Ohne Zusammenfassung 相似文献
997.
Wang Yangyang Liu Yidan Zhan Wenhao Niu Liumin Zou Xueyan Zhang Chaosheng Ruan Xinling 《Journal of Soils and Sediments》2020,20(1):91-98
Journal of Soils and Sediments - Cd-contaminated soil is a common environmental problem. Stabilization is an effective way of in situ remediation for Cd contamination in soils, but there is a lack... 相似文献
998.
Bai Junhong Yu Lu Ye Xiaofei Yu Zibo Guan Yanan Li Xiaowen Cui Baoshan Liu Xinhui 《Journal of Soils and Sediments》2020,20(1):513-523
Journal of Soils and Sediments - Sediment organic phosphorus (OP) mineralization plays an important role in phosphorus cycling in coastal salt marshes. However, information on OP mineralization... 相似文献
999.
Journal of Soils and Sediments - Soil bulk density and hydraulic conductivity are two of many key factors for vegetation establishment and the other way around. Variations in them may cause patchy... 相似文献
1000.
Journal of Soils and Sediments - The work aimed to (1) better understand how C rate and type affecting N2O emissions when combined application with different N forms in a strong ammonia oxidation... 相似文献