全文获取类型
收费全文 | 18564篇 |
免费 | 717篇 |
国内免费 | 1756篇 |
专业分类
林业 | 1806篇 |
农学 | 1842篇 |
基础科学 | 1744篇 |
2568篇 | |
综合类 | 9063篇 |
农作物 | 826篇 |
水产渔业 | 418篇 |
畜牧兽医 | 1640篇 |
园艺 | 480篇 |
植物保护 | 650篇 |
出版年
2024年 | 117篇 |
2023年 | 306篇 |
2022年 | 506篇 |
2021年 | 598篇 |
2020年 | 566篇 |
2019年 | 687篇 |
2018年 | 376篇 |
2017年 | 725篇 |
2016年 | 942篇 |
2015年 | 817篇 |
2014年 | 1048篇 |
2013年 | 1094篇 |
2012年 | 1519篇 |
2011年 | 1449篇 |
2010年 | 1154篇 |
2009年 | 1122篇 |
2008年 | 1042篇 |
2007年 | 1044篇 |
2006年 | 973篇 |
2005年 | 742篇 |
2004年 | 539篇 |
2003年 | 501篇 |
2002年 | 400篇 |
2001年 | 359篇 |
2000年 | 324篇 |
1999年 | 274篇 |
1998年 | 266篇 |
1997年 | 224篇 |
1996年 | 209篇 |
1995年 | 176篇 |
1994年 | 148篇 |
1993年 | 153篇 |
1992年 | 142篇 |
1991年 | 116篇 |
1990年 | 101篇 |
1989年 | 104篇 |
1988年 | 83篇 |
1987年 | 43篇 |
1986年 | 26篇 |
1985年 | 6篇 |
1984年 | 3篇 |
1983年 | 1篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1979年 | 2篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1976年 | 2篇 |
1956年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
以图书馆搬迁为契机重塑高校图书馆整体形象 总被引:1,自引:1,他引:0
马天玉 《农业图书情报学刊》2006,18(2):112-115
文章结合我馆搬迁的实际工作,论述了以科学规划为指导的图书馆搬迁推动了高校图书馆服务方式扣管理模式的改进,更新思雏方法,树立以人为本的服务理念,重新塑造高校图书馆的职业道德形象、学术形象、优质服务形象、内部环境形象等整体形象。 相似文献
82.
83.
星载高光谱仪器的光谱通道以及光谱分辨率和信噪比等核心参数设置直接影响土壤有机碳定量反演精度。本研究开展了卫星载荷光谱分辨率、信噪比、光谱特征波段对不同土壤类型有机碳反演影响的研究,提出了基于大气传输模型、光谱分辨率分析模型、信噪比分析模型、特征波段的提取分析模型以及偏最小二乘回归反演模型的面向不同土壤类型有机碳监测的高光谱卫星“地面–大气–仪器–观测–反演”全链路仿真分析方法,实现了土壤类型、大气效应、仪器特性参数、反演方法的耦合影响分析。结果表明:①3种类型土壤有机碳反演的最佳光谱分辨率均在10~20 nm。②不同土壤类型对观测的信噪比需求不同。对于Phaeozem的有机碳监测,较另外两种土壤有更高的信噪比需求。③在不同特征波段提取分析方法下所需的最佳光谱分辨率和信噪比一致。不同类型土壤光谱数据提取出的特征波段不同,其中反演效果最佳的土壤类型为Chernozem,特征波段数为26个,R2=0.826 5,RMSE=3.438 9 g/kg。④反演模型与仪器特性参数无耦合关系,同一类型土壤不同反演算法的最佳光谱分辨率和信噪比需求一致。⑤Chernozem有机碳最佳反演参数需求为光谱分辨率15 nm,信噪比大于506.66,特征波段提取数为26个;Kastanozem有机碳最佳反演参数需求为光谱分辨率17 nm,信噪比大于331.42,特征波段提取数为22个;Phaeozem有机碳最佳反演参数需求为光谱分辨率15 nm,信噪比大于432.51,特征波段提取数为19个。 相似文献
84.
85.
86.
为了解猕猴桃属植物间的亲缘关系,以41份不同猕猴桃花粉为供试材料,采用扫描电镜对猕猴桃花粉进行形态学观察和分析,选取6个性状指标进行测量,根据观测结果对供试材料进行聚类分析。结果表明:供试花粉均为单粒花粉,均以单粒、等极和辐射对称形式存在。极轴长度范围在21.50~33.40μm之间,赤道轴长度范围在9.87~16.24μm之间,萌发沟长度范围在17.53~29.08μm之间,萌发沟脊宽度范围在0.47~2.03μm之间,花粉粒大小(极轴×赤道轴))范围在218.68~543.18μm2之间,极轴长度与赤道轴长度比值范围为1.36~2.26,花粉形状为超长球形或长球形,萌发孔类型均为三孔沟,极面观为三裂圆形,均为N3P4C5型。外壁纹饰为波纹状、颗粒状、疣状三种类型,占样品总数的比例分别为63.41%、19.51%、17.08%。在遗传距离为7.5时,供试材料可以分为3个类群,在遗传距离为1.5时,供试材料可以分为6个类群。 相似文献
87.
S. J. Kerley 《Biology and Fertility of Soils》2000,32(2):94-101
The effects of a limed soil upon root and shoot growth of white lupin (Lupinus albus L.) were investigated using soil tubes and pots. After 75 days in the soil tubes, the combined taproot and lateral root dry
weight in limed soil (2.5% CaO w/w) was significantly less than in neutral pH soil (by 57%). However, the dry weight and numbers
of cluster roots remained comparable between the treatments, demonstrating for the first time that the cluster roots respond
differently to the rest of the root system. Cluster roots accounted for 17% of the total root biomass in neutral soil, increasing
significantly to over 30% in limed soil. When grown for 43 days in pots containing soil with different additions of lime (0.5–2.5%
CaO w/w), soil citrate concentrations were higher than in the neutral pH soil treatment in all except the 2.5% lime treatment,
in which they were lower. In both experiments, shoot dry weights were lower in the presence of the limed soil compared with
those in the neutral pH soil. Although a reduction in shoot dry weight was not apparent at 21 days in the limed-soil tubes,
the initiation of fewer mainstem leaf primordia indicated a slower shoot development than occurred in the neutral soil. Plants
grown in the limed-soil tubes showed leaf yellowing and some chlorosis within 9 days. At the final harvest, the shoot phosphorus
and manganese concentrations were significantly lower in plants grown in limed soil compared with those in the neutral pH
soil, whereas the concentration of calcium was higher.
Received: 11 October 1999 相似文献
88.
Summary Pot experiments were carried out to study the influence of bulk density (D
b), soil water tension (pF) and presence of plants (spring wheat) on denitrification in a low-humus Bt-horizon of a udalf. Pots of only 5-cm depth were found to be most suitable for the experiments when using the acetylene inhibition method. Almost homogeneous soil compaction between 1.1 and 1.6g soil cm–3 was achieved by a Proctor tamper. Water tensions were adjusted by means of ceramic plates on which negative pressure was applied. No denitrification was detected in unplanted pots. With planted pots and increasing bulk density denitrification increased more in pots with 14-day-old plants than in pots with 7-day-old plants. With 14-day-old plants N2O emission pot–1 increased steadily from 2 mol at D
b 1.1 to 8 mol at D
b 1.6, when soil moisture was adjusted to pF 1.5, although root growth was impaired by higher bulk density. From an experiment with different bulk densities and water tensions it could be deduced that the air-filled porosity ultimately determined the rate of denitrification. When low water tension was applied for a longer period, water tension had an overriding effect on total denitrification. Denitrification intensity, however, i.e. the amount of N2O g–1 root fresh weight, was highest when low water tension was accompanied by high bulk density. The results suggest that the increase in denitrification intensity at oxygen stress is partly due to higher root exudation. 相似文献
89.
为揭示滇西地区土壤环境质量状况,以陇川县为例,参照《绿色食品产地环境技术条件》,对耕地土壤pH值、有机质、全氮、速效磷、速效钾营养元素含量及重金属元素铅、镉、汞、砷、铬、铜总量进行测定分析。采用标准对比法、单因子污染指数法和Nemero综合污染指数法,对其土壤环境质量进行了评价。结果表明,研究区土壤pH值介于3.49~7.45之间,均值为5.05,土壤偏酸;各营养元素平均值中有机质、全氮、速效磷含量为优良,速效钾含量为中等,有机质、全氮、速效磷、速效钾含量分别有64.17%、59.34%、75.33%、71.66%的耕地面积达到生产绿色食品土壤肥力要求,分别有35.83%、40.66%、24.67%、28.34%的面积土壤肥力不足;采用单因子污染指数法对土壤重金属评价,砷、铬、铜处于清洁状态;有20.27%、2.03%、4.73%的样本铅、镉、汞单因子污染指数大于1,所代表的面积受到铅、镉、汞轻度污染,镉有1.35%样本单因子污染指数大于2,所代表的面积受到镉中度污染;综合污染指数评价,4.79%样本达到轻度污染。 相似文献
90.
Shinsuke Yamanaka Ikuo Nakamura Hirokazu Nakai Yo-Ichiro Sato 《Genetic Resources and Crop Evolution》2003,50(5):529-538
The direct ancestor of rice (Oryza sativa L.) is believed to be AA genome wild relatives of rice in Asia. However, the AA genome wild relatives involve both annual and perennial forms. The distribution of the retrotransposon p-SINE1-r2, a short interspersed nuclear element (SINE) at the waxy locus was analyzed in diverse accessions of the AA genome wild relatives of rice (O. rufipogon sensu lato). Most annual wild rice accessions had this retrotransposon, while most perennial types lacked this element, contradicting results to the previous studies. Results presented here suggest that O. sativa has dual origin that lead to indica-japonica differentiation. Results suggest the indica line of rice varieties evolved from the annual genepool of AA genome and the japonica varieties from the perennial genepool of AA genome wild rice. 相似文献