全文获取类型
收费全文 | 22938篇 |
免费 | 1244篇 |
国内免费 | 2853篇 |
专业分类
林业 | 1453篇 |
农学 | 1958篇 |
基础科学 | 5824篇 |
5017篇 | |
综合类 | 8137篇 |
农作物 | 826篇 |
水产渔业 | 740篇 |
畜牧兽医 | 1344篇 |
园艺 | 355篇 |
植物保护 | 1381篇 |
出版年
2024年 | 134篇 |
2023年 | 461篇 |
2022年 | 783篇 |
2021年 | 825篇 |
2020年 | 875篇 |
2019年 | 967篇 |
2018年 | 660篇 |
2017年 | 990篇 |
2016年 | 1277篇 |
2015年 | 1020篇 |
2014年 | 1229篇 |
2013年 | 1458篇 |
2012年 | 1786篇 |
2011年 | 1703篇 |
2010年 | 1348篇 |
2009年 | 1331篇 |
2008年 | 1184篇 |
2007年 | 1397篇 |
2006年 | 1220篇 |
2005年 | 966篇 |
2004年 | 747篇 |
2003年 | 717篇 |
2002年 | 541篇 |
2001年 | 499篇 |
2000年 | 410篇 |
1999年 | 349篇 |
1998年 | 331篇 |
1997年 | 295篇 |
1996年 | 249篇 |
1995年 | 236篇 |
1994年 | 212篇 |
1993年 | 187篇 |
1992年 | 162篇 |
1991年 | 123篇 |
1990年 | 109篇 |
1989年 | 71篇 |
1988年 | 80篇 |
1987年 | 42篇 |
1986年 | 14篇 |
1985年 | 8篇 |
1984年 | 11篇 |
1983年 | 3篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1979年 | 5篇 |
1978年 | 4篇 |
1977年 | 3篇 |
1962年 | 1篇 |
1956年 | 1篇 |
1955年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 281 毫秒
61.
杨雨行 《北京林业大学学报》1995,(1)
依据航片判读、森林资源调查资料和水文数据,利用流域自身对比法,得出森林变化对河川径流泥沙的定量关系。文中还阐明本方法的技术关键——消除降雨量不同对水沙的影响.该方法具有适合我国国情、费小效宏、精度较高、简捷易行等优点,值得推广提倡。 相似文献
62.
大型Sou的总脂和脂肪酸组成随食物的变化及营养强化的实施而发生显著变化。酵母组总脂为2.5%,明显高于海水小球藻组(1.65%)和有机肥组(1.5%)。经鱼油强化或海水小球藻二次培养后脂类含量明显升高。三组中酵母组的MUFA含量是高为55%。主要是C16:1ω9(23%),而PUFA最低(16.6%),其中EPA(2.7%)、DHA(0.8%)等ω3HUFA含量均很低;与酵母组相比,海水小球藻组和有机肥组的MUFA含量显著下降,PUFA含量显著升高,均为30%-33%左右,三组中海水小球藻组EPA含量最高(14%),其次是有机肥组(5.9%)和酵母组(2.7%);有机肥组DHA含量最高,达11.7%,而酵母组和海水小球藻组DHA含量均低,仅为0.7%-0.8%。经鱼油强化后,酵母组的MUFA含量显著下降,PUFA显著升高,EPA由2.7%提高到10%左右,DHA由0.8%以7%;酵母组经海水小球藻二次培养后MUFA显著降低,PUFA相应升高,EPA由2.7%提高到9.7%,而DHA含量无变化。 相似文献
63.
细菌耐药拮抗剂的研究 总被引:6,自引:1,他引:5
本文介绍了具有拮抗细菌耐药性作用的物质的研究进展情况,包括灭活酶抑制剂、药物渗透促进剂、外输泵抑制剂、细菌生物被膜抑制剂、抗菌药物增强剂、耐药质粒消除剂等。 相似文献
64.
65.
66.
Trends in NE Atlantic landings (southern Portugal): identifying the relative importance of fisheries and environmental variables 总被引:7,自引:1,他引:6
KARIM ERZINI 《Fisheries Oceanography》2005,14(3):195-209
Time series of commercial landings from the Algarve (southern Portugal) from 1982 to 1999 were analyzed using min/max autocorrelation factor analysis (MAFA) and dynamic factor analysis (DFA). These techniques were used to identify trends and explore the relationships between the response variables (annual landings of 12 species) and explanatory variables [sea surface temperature, rainfall, an upwelling index, Guadiana river (south‐east Portugal) flow, the North Atlantic oscillation, the number of licensed fishing vessels and the number of commercial fishermen]. Landings were more highly correlated with non‐lagged environmental variables and in particular with Guadiana river flow. Both techniques gave coherent results, with the most important trend being a steady decline over time. A DFA model with two explanatory variables (Guadiana river flow and number of fishermen) and three common trends (smoothing functions over time) gave good fits to 10 of the 12 species. Results of other models indicated that river flow is the more important explanatory variable in this model. Changes in the mean flow and discharge regime of the Guadiana river resulting from the construction of the Alqueva dam, completed in 2002, are therefore likely to have a significant and deleterious impact on Algarve fisheries landings. 相似文献
67.
C. E. Cornelius B. A. Myers M. L. Bruss J. W. George 《Veterinary research communications》1989,13(5):395-401
Fasted Brazilian squirrel monkeys (BrSMs) exhibited slightly higher serum bilirubin levels (0.30±0.05 mg/dl) than others in the fed state (0.13±0.01). The mean liver weight was 50% lower following a 22 h fast. The rate of bile flow was unaffected by fasting and averaged 13.8 l/min/kg and 47.5 l/min/100g liver in six BrSMs. No significant difference in mean bilirubin excretion/min was observed on a body weight basis following fasting. When the mean rate of bilirubin excretion was calculated as a function of liver weight, a two-fold higher rate was present in fasted monkeys, but only at the p=0.06 level of statistical significance. From data collected in this and earlier studies, it would appear that BrSMs represent the best animals studied to date to serve as experimental controls in comparative studies with Bovilian squirrel monkeys which exhibit a Gilbert-like syndrome. 相似文献
68.
对在山东的济南、平度、莱西、烟台种植的几种陆地棉(Gossypium hirsutumL.)核雄性不育材料的育性表现进行了研究,结果表明:洞A不育系的衍生材料MB、MA在不同气候条件下有育性的变化。在济南高度不育,可作为不育系。在其他几个地区表现不同程度的可育。其中MB在平度有相当好的育性,能正常结铃。而双隐性核不育材料59A、62A未能观察到育性变化。本文就核不育材料在棉花杂交制种中的应用前景及需深入研究的问题作了探讨。 相似文献
69.
酿酒葡萄成熟特性的研究 总被引:1,自引:0,他引:1
葡萄浆果成熟过程中,存在着多方面的变化。只有含糖量、含酸量的变化最有规律,最能代表葡萄品种的成熟特性。含糖量的变化与logistic曲线、含酸量的变化与对数曲线拟合性最好。不同品种成熟特性的差异,主要表现为含糖量上升速率与含酸量下降速率的不同。气象因素能明显影响葡萄的成熟特性,最终造成同一品种在不同地区成熟度的差异。 相似文献
70.
Land cover data for landscape ecological studies are frequently obtained by field survey. In the United Kingdom, temporally separated field surveys have been used to identify the locations and magnitudes of recent changes in land cover. However, such map data contain errors which may seriously hinder the identification of land cover change and the extent and locations of rare landscape features. This paper investigates the extent of the differences between two sets of maps derived from field surveys within the Northumberland National Park in 1991 and 1992. The method used in each survey was the Phase 1 approach of the Nature Conservancy Council of Great Britain. Differences between maps were greatest for the land cover types with the smallest areas. Overall spatial correspondence between maps was found to be only 44.4%. A maximum of 14.4% of the total area surveyed was found to have undergone genuine land cover change. The remaining discrepancies, equivalent to 41.2% of the total survey area, were attributed primarily to differences of land cover interpretation between surveyors (classification error). Differences in boundary locations (positional error) were also noted, but were found to be a relatively minor source of error. The implications for the detection of land cover change and habitat mapping are discussed. 相似文献