全文获取类型
收费全文 | 48870篇 |
免费 | 2191篇 |
国内免费 | 5341篇 |
专业分类
林业 | 5919篇 |
农学 | 8067篇 |
基础科学 | 4595篇 |
8190篇 | |
综合类 | 13984篇 |
农作物 | 2483篇 |
水产渔业 | 1822篇 |
畜牧兽医 | 6522篇 |
园艺 | 1502篇 |
植物保护 | 3318篇 |
出版年
2024年 | 129篇 |
2023年 | 457篇 |
2022年 | 1276篇 |
2021年 | 1983篇 |
2020年 | 1719篇 |
2019年 | 1794篇 |
2018年 | 1229篇 |
2017年 | 1663篇 |
2016年 | 1661篇 |
2015年 | 2264篇 |
2014年 | 2075篇 |
2013年 | 2565篇 |
2012年 | 3006篇 |
2011年 | 3289篇 |
2010年 | 3078篇 |
2009年 | 2881篇 |
2008年 | 2672篇 |
2007年 | 2680篇 |
2006年 | 2617篇 |
2005年 | 2597篇 |
2004年 | 1157篇 |
2003年 | 964篇 |
2002年 | 788篇 |
2001年 | 823篇 |
2000年 | 1004篇 |
1999年 | 1337篇 |
1998年 | 1193篇 |
1997年 | 1010篇 |
1996年 | 940篇 |
1995年 | 867篇 |
1994年 | 866篇 |
1993年 | 770篇 |
1992年 | 710篇 |
1991年 | 582篇 |
1990年 | 485篇 |
1989年 | 390篇 |
1988年 | 260篇 |
1987年 | 187篇 |
1986年 | 92篇 |
1985年 | 74篇 |
1984年 | 51篇 |
1983年 | 38篇 |
1982年 | 37篇 |
1981年 | 21篇 |
1980年 | 25篇 |
1979年 | 10篇 |
1978年 | 12篇 |
1977年 | 9篇 |
1965年 | 9篇 |
1963年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 8 毫秒
991.
992.
993.
Vegetation recovery is a key measure to improve ecosystems in the Loess Plateau in China. To understand the evolution of soil
microorganisms in forest plantations in the hilly areas of the Loess Plateau, the soil microbial biomass, microbial respiration
and physical and chemical properties of the soil of Robinia pseudoacacia plantations were studied. In this study, eight forest soils of different age classes were used to study the evolution of
soil microbial biomass, while a farmland and a native forest community of Platycladus orientalis L. were chosen as controls. By measuring soil microbial biomass, metabolic quotient, and physical and chemical properties,
it can be concluded that soil quality was improved steadily after planting. Soil microbial biomass of C, N and P (SMBC, SMBN
and SMBP) increased significantly after 10 to 15 years of afforestation and vegetation recovery. A relatively stable state
of soil microbial biomass was maintained in near-mature or mature plantations. There was an increase of soil microbial biomass
appearing at the end of the mature stage. After 50 years of afforestation and vegetation recovery, compared with those in
farmland, the soil microbial biomass of C, N and P increased by 213%, 201% and 83% respectively, but only accounting for 51%,
55% and 61% of the increase in P. orientalis forest. Microbial soil respiration was enhanced in the early stages, and then weakened in the later stage after restoration,
which was different from the change of soil organic carbon. The metabolic quotient (qCO2) was significantly higher in the soils of the P. orientalis forest than that in farmland at the early restoration stage and then decreased rapidly. After 25 years of afforestation and
vegetation recovery, qCO2 in soils of the R. pseudoacacia forest was lower than that in the farmland soil, and reached a minimum after 50 years, which was close to that of the P. orientalis forest. A significant relationship was found among soil microbial biomass, qCO2 and physical and chemical properties and restoration duration. Therefore, we conclude that it is possible to artificially
improve the ecological environment and soil quality in the hilly area of the Loess Plateau; a long time, even more than 100
years, is needed to reach the climax of the present natural forest.
__________
Translated from Acta Ecologica Sinica, 2007, 27(3): 909–917 [译自: 生态学报] 相似文献
994.
Ye Tian Kikuo Haibara Hiroto Toda Fangjun Ding Yanhui Liu Dongsu Choi 《Journal of Forest Research》2008,13(4):205-214
Mountain closure, considered an effective and economic measure for natural restoration of degraded forest ecosystems, has
been widely carried out in the karst region of southwest China. The aim of this study was to evaluate microbial aspects of
soil quality after mountain closure by analyzing soil microbial biomass, basal respiration, metabolic quotient, and relationships
with basic chemical properties in Guizhou Province, a karst region of the upper Yangtze River. Soil quality was considered
poor from the low levels of microbial biomass carbon (MBC), nitrogen (MBN), and microbial quotient (MBC/total C and MBN/total
N), but high metabolic quotient (qCO2). Soil pH, showing marked variation from 4.1 to 7.9 in this karst region, was proved to significantly affect soil microbial
biomass and activity. Soil microbial biomass, microbial quotient, and soil basal respiration declined significantly with decreasing
soil pH, while qCO2 showed an apparently increasing, but not statistically significant, trend. The changes in microbial biomass and activity
following the change in soil pH could possibly be because of a change in soil microbial composition, and more detailed research
is necessary. Compared with soil pH, soil organic matter content was another, more important, factor that directly restricted
microbial growth because of the serious loss as a result of disturbance. As a practical application based on microbial aspects,
introduction of some N-fixing tree species may be an active and effective measure to improve soil fertility and thus to accelerate
restoration of the forest ecosystem in the karst region. 相似文献
995.
996.
小兴安岭典型泥炭沼泽土壤水分物理性质研究 总被引:2,自引:0,他引:2
对小兴安岭5类典型泥炭沼泽土壤水分物理性质进行对比研究,结果表明:落叶松-杜香-泥炭藓沼泽土壤容重最小,非毛管孔隙最高,持水性最好;漂筏苔草沼泽和油桦-笃斯越桔-修氏苔草沼泽次之,修氏苔草沼泽和白桦-油桦-小叶章沼泽相对较差.落叶松-杜香-泥炭藓沼泽0~40 cm平均土壤容重为0.06g·cm-3,非毛管孔隙31.69%,总孔隙度高迭87.19%,平均土壤质量含水量、最大持水量、毛管持水量和最小持水量分别为1130.1%、1586.57%、1007.71%和882.70%,是对照林地的56.6倍、26.4倍、24.9倍和25.5倍.在5类泥炭沼泽中,白桦-油桦-小叶章沼泽0~40 cm平均土壤容重最高(0.74 g·cm-3),总孔隙度最低(62.02%),土壤各持水指标均最小,但其持水指标均显著高于对照林地. 相似文献
997.
998.
基于知识仓库的虚拟产品开发技术整合社会资源,利用web技术在不实际生产产品实物的情况下,利用模块化思想采用网络化并行设计,在产品设计初期就考虑到后续工艺和加工需求以及用户可能的附加要求,可缩短新产品开发周期。本文基于知识仓库,针对虚拟产品开发技术,分析虚拟产品技术的国内外研究现状,阐述虚拟产品开发技术的原理以及相关技术,构造虚拟产品开发的设计流程,并且基于Delphi8.0网络环境下构建基于知识的虚拟产品创新设计制造个人工作空间子系统。 相似文献
999.
伊春市发展旅游经济的意义是有效地保护生态环境、充分发掘非林资源、拓宽产业链、增加就业门路 ,促进经济发展。伊春旅游经济目前存在的问题主要有管理体制尚未理顺、旅游形象不突出 ,旅游项目开发不充分、旅游产品档次低等。对此提出旅游经济开发与保护并重、产品多样化与重点开发相结合等原则及对策。 相似文献
1000.
在设计和运行应用程序时,一般都希望应用程序能够发挥最佳性能。本文介绍了在Visual Foxpro中提高程序性能的一些技巧。 相似文献